Never mind telnetd. Tier 1 transit providers doing port filtering is EXTREMELY alarming. They have partitioned the Internet, and in a way that automatic routing (BGP) can't get around.
> Tier 1 transit providers doing port filtering is EXTREMELY alarming.
I was admining a small ISP when blaster and its variants hit. Port filtering 139 and the rest was the easiest way to deal with it, and almost over night most of the ISPs blocked it, and we were better for it. There was a time when if you'd put a fresh XP install on the Internet you'd get 5-10 minutes until it would get restarted.
I guess if you're really an admin that needs telnet, you can move it to another port and go around it? Surely you'd tunnel that "old box that needs to stay alive" if that's the usecase? Is there anyone seriously running default telnet on 23 and is really affected by this filtering?
I do not know what is more critical: the risk of censorship or stand by while hospitals, banking, nuclear power plants and other systems become compromised and go down with people dying because of it. These decision makers not only have powers but also have a responsibility
This feels more akin to discovering an alarming weakness in the concrete used to build those hospitals, banks and nuclear power plants – and society responding by grounding all flights to make sure people can't get to, and thus overstress, the floors of those hospitals, banks and nuclear power plants.
not to mention, filtering on udp vs tcp, which makes using anything else impossible. Not that I have one, but it's just a bit in a field, why filter on it?
The most interesting thing here isn't the CVE - it's the invisible coordination. A backbone provider acted on advance knowledge of a critical flaw, implemented filtering at scale, and the rest of us didn't notice until GreyNoise's data showed the drop. The vulnerability got patched at the network layer before it ever reached the application layer. This is what mature security ecosystems look like - the boring, quiet fixes that happen before the press release.
What an amazing bug. I probably spent my first 10 years on the internet just using telnet. They were wild times. You could log ethernet traffic and see passwords. Towards the end of those we started to have a few more single-user machines, but the vast majority were old school many many user machines, where "root" was thought to be tightly restricted (of course, even then, in practice it wasn't if you were in the know).
It's hilarious, especially given that I have memories of similar rlogin vulnerabilities -- various unixes being vulnerable to rlogin -l '-froot' in the 90s.
I never sent root over telnet, but I spent too much vacation time browsing the web via lynx on my school AIX account from a library near my parents' home, because it had a telnet client in addition to the card catalogue program on the otherwise locked down desktop. It was just a more innocent time: you didn't assume your traffic was being logged six ways to Sunday. With telnet access to my AIX account, I could do all the internet things, like mail (pine) and the web (lynx) and irc, from a convenient command line anywhere in the world.
So Telnet as a client is not dead though, right? A long time ago, I used to use the Telnet client to talk to SMTP servers (on port 25) and send spoofed emails to friends for fun.
With port blocking widening in scope, I’ve long believed that we would one day have every service and protocol listening on port 443. Since all other ports are being knocked off in the name of security, we’ll end up having one port that makes port based filtering useless.
I've never really understood why it's a thing to use a telnet client for transmitting text on a socket for purposes other than telnet. My understanding is that telnet is a proper protocol with escape sequences/etc, and even that HTTP/SMTP/etc require things like \r\n for line breaks. Are these protocols just... close enough that it's not a problem in practice for text data?
Because for a long time, on most computers, the telnet client was the closest thing to an "open a tcp socket to this ip/port and connect the i/o from it to stdin/stdout" application you can get without installing something or coding it up yourself.
These days we have netcat/socat and others, but they're not reliably installed, while telnet used to be generally available because telnetting to another machine was more common.
These days, the answer would be to use a netcat variant. In the past, telnet was the best we could be confident would be there.
If it's alright to be pedantic, anyone with programming knowledge can do the same without these tools. What these offer is tried and tested secure code for client side needs, clear options and you don't need to hand roll code for.
None of this affects the use of telnet the client program nor the ability to run a telnetd on your own host (but do be sure it's patched!).
What's happened is that global routing on the internet (or big chunks of it, it's not really clear) has started blocking telnet's default port to protect presumably-unpatched/unpatchable dinosaur systems from automated attack. So you can no longer (probably) rely on getting to a SMTP server to deliver that spoofed email unless you can do it from its own local environment.
You would still be able to use the telnet client to connect to an SMTP server on TCP port 25, just not port 23, right? I don't think that part changed here.
It's... not super clear from the article whether this is a port block or a stateful protocol thing. But yes, you're probably right and SMTP spoofing is probably safe for now.
(Remember hearing about this a long time ago (from some searching I think it was in 1999 via Slashdot) and verified some instance of it still exists/works.)
213.136.8.188 appears to not respond to telnet from any ISP I attempt to connect to it on, I wonder if its just not bound to port 23 on IPv4 or the ISP is filtering port 23. IPv6 works fine to connect.
Telnet is used in legacy, IoT, embedded, and low-level industrial hardware. It's also intentionally enabled on devices where automation was written for telnet and it wasn't easy to switch to ssh.
If you investigate most commercial uses of ssh, the security is disabled or ignored. Nobody verifies host keys, and with automation where hosts cycle, you basically have to disable verification as there's no easy way around the host keys constantly changing. Without host key verification, there's kinda no point to the rest.
Even assuming the host keys were verified, the popular ssh conventions are to use either long-lived static keys (and almost nobody puts a password on theirs), or a password. Very few people use SSH with 2FA, and almost no-one uses ephemeral keys (OIDC) or certificates (which many people screw up).
So in terms of how people actually use it, SSH is one of the least secure transport methods. You'd be much more secure by using telnet over an HTTPS websocket with OAuth for login.
How do you automate, for example, "HTTPS over websocket with OAuth", without providing some kind of hard-coded, static or otherwise persistent authentication credentials to the calling system in some form (either certificate based auth, OAuth credentials, etc.)?
The problem with IoT and embedded secrets isn't really a solved problem, from what I can tell. I'm not sure that OAuth exactly solves the problem here. Though all your comments about SSH (especially host verification) holds true.
Just honestly trying to understand the possible solution space to the IoT problem and automated (non-human) authorization.
Unless you manage to leak your private host/client SSH keys, this is close to being as secure as it gets.
I'd say that HTTPS (or TLS in general) is more problematic, since you need to trust numerous root CAs in machine/browser store. Sure, you can use certificate pinning, but that has the same issues as SSH host key verification.
The known_hosts file is verification of host keys. It's not verification of a host cert, which is a different thing. Most sshd instances are running on ad hoc hardware without the ability to associate them with someone a cert authority would be willing to authenticate.
Basically people running services that need cert-based authentication are already using TLS (or if they're using sshd they've locked it down appropriately). SSH is for your workstation and your RPi and whatnot.
SSH certs aren't TLS certs. Totally different format. All SSH CAs are private, you run your own CA to issue certs to devices you want to allow to connect to your server.
> IMHO we need a good telnet replacement that sends signed data. Most people interpret signatures as allowed under FCC rules, just not encryption.
I know from bitter experience that IPsec is a “now you have two problems” kind of solution, but the Authentication Header is a thing and is supported by most (all?) implementations. Ham radio operators probably don’t have much use for the actual features of telnet compared to plain netcat, do they? (It’s mostly terminal feature negotiation and such.)
TIL that IPsec can be used without encryption. That should work pretty well.
Telnet is mostly used for auth and straightforward terminal/BBS access in my experience. There are some other alternatives like HamSSH but I don’t think it’s that common.
What I meant in my remark about Telnet is that, if you just want is a bidirectional byte pipe to e.g. run a terminal over, then you just need TCP or anything else providing the same abstraction, like TLS-over-TCP or TCP-over-IPsec; whether you then choose to run a getty on that terminal is not for the network to care. (I don’t believe you can get netcat to drive a PTY, so you’ll need e.g. socat. And of course if you want cryptographic authentication then you don’t need or want a getty.)
Telnet, on the other hand, is quite a bit fancier than that and has a fairly involved feature negotiation mechanism for terminal connections that is not entirely in line with the prevalent DEC tradition. As admittedly one of the funkiest examples of what you can do with it, there is for instance a mode[1] where the client is asked to emulate a terminal of the IBM 3270 lineage. (To a practicioner of the aforementioned DEC tradition, those feel like the marsupials of terminals: everything is functionally there, but primitive and derived are occasionally flipped and some features are oddly weak or misdesigned due to a lack of competition.) So if you do actually use Telnet the protocol, by all means, I’ll be delighted to learn what you do with it (partly why I asked in the first place). But if you just need a pipe, then TCP is enough, and netcat or socat make fine ad-hoc clients.
Probably one of the reasons this bug survived so long is that it isn't used much for priveleged access any more, but so you can play a moo or play you an ASCII movie, as people below you are replying.
MUDs were my introduction to telnet- I grew up a university kid and had access to Wesleyan's minicomputer EAGLE.WESLEYAN.EDU running OpenVMS. I used it to telnet to CMU's TinyMUD and later other TinyMUDs around the country. I recall OpenVMS's telnet had a problem with newlines/carriage returns so all the text was staircased, so I ended up learning C and writing a MUD client. I still habitually use telnet today even if netcat and many other tools have replaced it.
All of that was foundational for my career and I still look back fondly on the technology of the time, which tended to be fairly "open" to exploration by curious-minded teenagers.
I've always used ssh to connect to it. And it's true that their port 23 is still open at last check. If you cannot reach port 23, and you irrationally hate ssh, you may use 14321 as an alternate.
Ah, not really. We are on a non-standard port (9000). I just meant some folks use the telnet client to connect, and we do negotiate some telnet options. I use tintin++ these days but I think most of our players are still using decades old zMUD versions to connect!
Wouldn't attention to getenv() calls yield more benefit? Such calls are where input typically isn't parsed--because parsing is "hard"--becoming targets for exploit.
The present fix is to sanitize user input. Does it cover all cases?
On the bright side that CVE seems like pretty great news for the hardware hacking community hoping to get root on embedded devices which have open telnetd.
When I was an intern for some reason they issued me a voip phone for my desk. One day I got bored and figured out I could telnet into it. Nothing interesting but it was still a fun moment for me!
A very very long time ago as an intern I was working on a perl cgi script and I would often test it with telnet. I was used to messing around with hayes commands so manually typing in HTTP commands seemed like a natural extension of that.
That's crazy. This is core business critical software but they just YOLO critical changes without any automated tests? this PR would be insta-rejected in the small SAAS shop I work at.
Culture has changed a lot since the 20th century and older projects can have antiquated norms around things like testing. I was just listening to a recent podcast talking about how worrisome it is that OpenSSL has a casual culture about testing[1] and was reminded about how normal that used to be. I think in the case of telnetd you also have the problem that it’s been deprecated for multiple decades so I’d bet that they struggle even more than average to find maintainer time.
Even with automated tests you'd need to think of this exploit right? Perhaps fuzzing would have got it. The mailing lists says they proved it successful on
If you think you can do better you're welcome to do better. I say this without a hint of sarcasm. This is how open source works. It's a do–ocracy, not a democracy. Whoever makes a telnet server gets to decide how the telnet server works and how much testing it gets before release.
Maybe the lesson here is to stop letting the GNU folks do things, if this is what they do. This is only one example of craziness coming out of the GNU camp.
Or, flip the responsibility to what it has always been understood to be, when using open source software from random volunteers (some being bad actors) on the internet for anything remotely critical: audit the source.
GNU doesn’t provide labor, only organizational tools like mailing lists and whatnot. The projects that GNU supports are still run by individual volunteers. If you want it done better then please volunteer so that you can be the one doing it better.
Any business that has a telnet daemon able to be reached by an unauthenticated user is negligent. Just the fact that everything is in the clear is reason enough to never use it outside of protected networks.
> Someone upstream of a significant chunk of the internet’s transit infrastructure apparently decided telnet traffic isn’t worth carrying anymore. That’s probably the right call.
Does this impact traffic for MUDs at all? I know several MUDs operate on nonstandard Telnet ports, but many still allow connection on port 23. Does this block end-to-end Telnet traffic, or does it only block attempts to access Telnet services on the backbone relays themselves?
MUDs use plaintext TCP protocols that are accessible to a wide range of clients.
The Telnet protocol is well-defined and not completely plaintext. There are in-band signaling methods and negotiations. Telnet is defined to live on 23/tcp as an IANA well-known, privileged, reserved port.
MUDs do none of this. You can usually connect to a MUD using a Telnet client, but most players hate the experience and often deride this method in favor of a dedicated, programmable client.
The fact that MUDs inhabit higher 4-digit ports is an artifact from their beginnings as unprivileged, user-run servers without a standardized protocol or an assigned “well-known port” presence. If you want your MUD to be particularly inaccessible, you could certainly run on port 23 now!
As a MUD enthusiast of two decades, this is not accurate. Where are you getting this information?
Most MUDs implement RFC 854, and a number of non-standard Telnet option subnegotiation protocols have been adopted for compression (MCCP2), transmission of unrendered data (ATCP, GMCP, ZMP), and even a mechanism for enabling marking up the normal content using XML-style tags (MXP). These telopts build on the subnegotiation facility in standard Telnet, whose designers knew that the base protocol would be insufficient for many needs; there are a great number of IANA-controlled and standardized telopt codes that demonstrate this, and the MUD community has developed extensions using that same mechanism.
> You can usually connect to a MUD using a Telnet client, but most players hate the experience and often deride this method in favor of a dedicated, programmable client.
I think you are confusing "telnet" the program with "telnet" the protocol. I am speaking here of the protocol, defined at base in RFC 854, for which "telnet" the program is but one particularly common implementation. You look at any of those "dedicated, programmable clients" and they will contain an implementation of RFC 854, probably also an implementation of RFC 1143 (which nails down the rules of subnegotiation in order to prevent negotiation loops), and an implementation of the RFCs for several standard telopts as well as non-standardized MUD community telopts. I can speak for the behavior of MUSHclient in especial regard here, though I am also familiar with the underlying Telnet nature of Mudlet, ZMud, and CMUD, not to mention my very own custom-made prototype client for which I very much needed to implement Telnet as described above.
Yes, perhaps we should define “MUD” and your incomplete experience of “most”.
As a MUD enthusiast for 37 years, I learned to program in C and Unix through TinyMUD, MUCK, and MUSH derived servers. From the beginning, none of these codebases implemted Telnet. There was nothing but a raw transparent TCP connection. In fact, I facilitated the introduction of a grand innovation: the "port concentrator" system which multiplexed TCP connections. Unix processes had a hard rlimit of 64 file descriptors, which crimped our style as an emerging MMORPG. The multiplexer increased this to 4096, for the biggest games of the era.
You mention MUSHclient, and I do not know about later revisions of the TinyMUSH server, but I can assure you that every MUSH I found from Larry Foard on, was not implementing Telnet. (I was privileged to help Larry "test" new features as I red-teamed his server with bizarre edge cases!)
Likewise, after I handed off TinyMUCK 2.3 to the furries, it was not doing the Telnet protocol. When we backported stuff to MUCK 1.x, it was not doing Telnet. I wrote a bonkers Perl program to read MUCK databases and sort of implement the game. No Telnet there. I've got to wonder whether the Ubermud or MOO guys had folded it in; they were close collaborators with us, back in the day.
Now as for the Diku, LP, and other “combat” type games, I’ve no idea. Perhaps they did. We never cared. I was aware that some of them had a pesky “prompt” that violated the line-mode assumptions of conventional clients and needed workarounds.
telnet(1), the program, was historically the only program that implemented the protocol. If you use Tinyfugue or Tinywar or tinymud.el, they are not, and no, I am not confused, because I was giving an example of why the Telnet-implementation, the program, the client, was so inadequate for playing on MUD servers.
It wouldn’t have been difficult to retrofit the Telnet RFC 854 into any MUD server, but none of us wizards had any use for it, seeing that our clients were mature and capable of much more processing without it.
If modern MUD servers have mostly implemented Telnet, then that is cool, but what surprises me is that it is mandatory, and your clients don’t seem to interoperate without it? That is a strange reversal!
The modern MUSH forks do generally support telnet, but yes -- as a 29 year old who's been pathologically obsessed with "MUD archeology" off and on, I'll confirm -- historically, most MUDs did not do any sort of Telnet negotiation.
Further, most older clients did not anticipate any kind of Telnet negotiation from the server, and will print garbage to the screen if connecting to modern MUSHes that do. (I've tested tinywar, vt, and that one VMS client...)
MUCKs never, to my knowledge, implemented telnet, though. They barely support ANSI escapes, nevermind Telnet. :-)
> [...] no, I am not confused, because I was giving an example of why the Telnet-implementation, the program, the client, was so inadequate for playing on MUD servers.
Then this is at the heart of our disconnect, because the post of mine that you originally replied to --- as well as, unless I drastically misread, the original article under discussion --- was concerned with traffic on port 23, the Telnet protocol port, and not with any particular implementation communicating on that port. The concern of my original comment was that this might affect MUDs that operate on port 23. Perhaps you can understand my confusion when you reply stating categorically that most MUDs do not use "Telnet" (meaning the program), when that wasn't really what was at concern (and therefore implied that my question had no basis).
It is a true fact that many MUDs operate on port 23. Many do not, but you can skim a MUD aggregator like MudConnect [0] to see that it is quite common. Aardwolf, Discworld MUD, and the IRE games --- which consistently topped TopMudSites (when that aggregator was still running, anyway) all operate on 23, potentially in addition to an unreserved port.
> what surprises me is that it is mandatory, and your clients don’t seem to interoperate without it? That is a strange reversal!
All telopts are disabled by default, per Telnet RFC; the only things you must absolutely parse under the RFC are the standard complement of NVT commands (such as IAC GA "Go Ahead"), even if they are otherwise implemented as no-ops.
Any input stream with the high bit clear is treated as pure data -- with the incidental exception of bare `\r`, which must always be followed either by `\n` or by `\0`; but Postel's Law has turned that into more of a guideline. So as long as the standard NVT encoding is assumed (which is just 7-bit ASCII) and the NVT core escape sequences are avoided, a modern Telnet-based MUD client can interoperate with a plaintext MUD server without issue. (As you know, this is also why people get away with using `telnet` (the program) to access HTTP and SMTP services instead of using something like netcat.)
Some MUD clients will eagerly send IAC DO / IAC WILL subnegotiations, but general practice is to let the server offer first -- probably precisely to ensure compatibility with MUDs that don't implement Telnet subnegotiations.
> Now as for the Diku, LP, and other “combat” type games, I’ve no idea
Diku-family MUDs are certainly the ones I have the most experience with. I understand LP MUDs also generally have Telnet support; or at least, I recall seeing a patch for them that MUD owners often sought to apply to their games.
Wouldn't that imply that >80% of all monitored telnet sessions were exploit attempts for the specific CVE in question? Even with the scale of modern botnets, that seems unrealistic for a single vuln that was undisclosed at the time.
For about 15 years beginning in 2003 I had some VPSs with CrystalTech/NewTek. I noticed right away that they had blocked all port 23 traffic in/out of their edge.
I asked them about it and they said it was a security measure. Apparently they used telnet for managing their routers.
It turned out that they did not have very good security anyway.
An RCE in GNU's telnetd has no relationship to the sunsetting of telnet. Something could equally likely happen with SSH (but not really because the OpenBSD folks are paranoid by nature).
Apple removing the telnet client from OS X was a stupid move. How can you call yourself UNIX and not have a telnet client? It's like removing grep or ed.
To actually pass the certification test suite on a real system, Apple sometimes needs to apply special configurations (e.g., disabling System Integrity Protection (SIP), using case-sensitive filesystem, enabling certain legacy services, etc.).
telnet(1) is not required by POSIX (nor is nc or ssh required!)
Ironically, telnet(1) did not begin as a "Unix" utility but an ARPANET protocol suite program. It was available cross-platform. It is unclear whether all editions of Unix included a client, but BSD for sure was the point where telnet and TCP/IP became essential integrations for the systems.
Apple still includes uucp for some unknown reason.
The saving disk space argument makes no sense because telnet was one of the smaller binaries in /usr/bin.
Telnet continues to be widely used for select use cases and being told we're naughty by not including it feels punitive and just adds extra steps. What are you supposed to do, trash a $1m piece of industrial equipment because Apple wants to remind you Telnet is insecure?
New devices are still being released with Telnet where SSH is impractical or unnecessary.
There are many things I want to say in reply to this. So I’ll bullet point them:
* yes, do not buy equipment that has acquired so much tech debt that it still requires telnet.
* there are a million telnet clients out in the world. And ones far better than the default OS one. Apple not shipping one standard is not the end of the world or really anything more than a mild inconvenience for the small handful of people who need actual “Telnet” as opposed to Netcat or socat, both of which are far better than base Telnet.
Ubuntu and derivates removing telnet from the default install, along with other basic tools like traceroute etc, was one of the driving factors toward me creating my own distro. I'm sick of basic stuff being omitted because somebody just decided it's not needed anymore.
Because I go long periods of time without internet access, and I don't want to have to "sudo apt install" a fucking thing, ever. Especially not a tiny utility that is all of 172k in size, that I might need for something. Understand?
I want EVERYTHING that I might use installed AT ALL TIMES, FROM DAY ONE, so that I can IMMEDIATELY USE IT when required.
This is only one of many reasons why I abandoned the giant dumpster fire that is mainstream Linux. I do not agree with their idiotic philosophy, on practically every level.
You've now discovered that there are sections of God's Green Earth that you never knew existed! One of many benefits of stepping outside the Matrix for a moment.
Since Tier 1 transit providers have now blocked telnet (port 23), this means the death of watching ASCII Star Wars with `telnet towel.blinkenlights.nl`
However, if you still long for nostalgia, I was able to access it over IPv6 using a VPN based in the Netherlands:
telnet 2001:7b8:666:ffff::1:42
I'm sure the port 23 telnet blocking will be coming to IPv6 soon though.
Well, I mean, the first part is a song by Don McLean called American Pie. You might know that, unsure that everyone will pick it out though.
One of the most famous play choices at karaoke bars these days too. I think because the song is a long story, of sorts? But it's a terribly long song and I will leave to take a smoke break anytime it gets chosen. You're going to be there for a good 10 minutes before it concludes.
So maybe the AI prompt was something like, "take CVE-2026-24061 and compose a song lyric in the style of American Pie by Don Mclean". I wonder if you would get similar results with that prompt.
The rest of it seems to be substantially edited by an LLM too, or at least it's composed much like LLM outputs often are these days: “not a gradual decline, not scanner attrition, not a data pipeline problem, but a step function.”
"Not X, not Y, not Z" is a common LLM tic, and there's a few more like it in there.
I mean, that's fair. I guess I just wanted to put my old man hat on. The song is a tribute to an era of lost innocence. Which I think is quite apropos to the current situation surrounding telnet. Vestiges of the days of the early internet continue to disappear, almost like an endangered species. Old/obsolete protocols, like telnet, are pined for by old guys like me.
Am I the only one who feels like it isn't the responsibility of backbone ISPs to filter traffic like this? In the case of a DDoS situation I could get behind it, but in this case I feel as though it's not Cogent's problem if I want to use telnet from a device on Charter's network to a Vultr VPS, even if it may be ill-advised.
(Of course, the article only speculates that this traffic filtering is what's going on; there isn't any hard proof, but it feels plausible to me.)
Not the parent poster, but I also still use telnet. For me it's "Ancient", I have a few retired SPARC and PA-RISC boxes that run their period appropriate OSes as a hobby. Telnet/rlogin is the more reliable method to get into them remotely (just over the LAN).
They're on a LAN behind a NAT Router/Firewall, and I don't always keep them powered up (I'm not that insane) so I really don't have a concern for them.
Some of the more modern/high-performance examples I have run NetBSD with modern sshd and modern ciphers, but you can tell it's a bit of a workout for them.
There is something poetic about telnet finally going quiet. It was never meant to survive this long, yet it outlasted entire generations of 'secure replacements' simply because it was simple and it worked. The real lesson is that protocols do not die when better alternatives arrive; they die when the last device running them gets unplugged.
I think it would be better suited to use the terms we use for natural languages. A natural language is dead when the last person who learned it as first language dies and are extinct when there is noone that would speak it at all.
In these terms, telnet has been dead for a long while, but it's extinct now.
The design of telnet and ssh where you have a daemon running as root is bad security that as shown here is a liability, a ticking time bomb ready to give attackers root.
Oldschool telnetd didn’t actually run as root; rather, it just set up a PTY for the incoming socket to talk to, and then fork-exec’ed a /bin/login subprocess to live inside that pty. /bin/login is setuid-root, so it’s “where the security lived.”
I think we all collectively decided that that was a bad idea at some point — probably because /bin/login was never designed under the assumption that it would have to deal with arbitrary binary network traffic being thrown at it (it really only expects keyboard input.) So we switched to doing auth directly in our network daemons, since at least then “people who are aware the code is network-facing” would be maintaining it.
I think a proper architecture would not even have a root account. The server would just expose an authenticated endpoint that allows for configuration and updates to be pushed for it.
SSH should not become a different user; it should call something like `/bin/login` which uses PAM for authentication and is capable of starting user sessions.
That still needs a way to change users, and OpenSSH already has privilege separation. That hardens the process somewhat to reduce the amount of code running in the process which can change the uid for a session but fundamentally something needs permission to call setuid() or the equivalent.
Congratulations, you've created a server that lets people have shells running as the user running telnetd.
You presumably want them to run as any (non root) user. The capability you need for that, to impersonate arbitrary (non-root) users on the system, is pretty damn close to being root.
I'm not sure that you need root because of the port - I think login itself needs to run as root, otherwise it cant login to anything other than the account its running under.
Those are already unprivileged operations, but how does it start the initial process in that terminal with the correct privileges for a different user?
Any breach of the daemon will still give access to a system that can approve/deny user logins. Breaching the daemon therefore allows permission escalation, because you can simply jump to an account. Chain with any local vuln of your choice to completely own the box.
It doesn't matter what user it is running as.
If this was so easy to deal with, someone would have done it. Instead, we get endless HN comments about people that act like they can do better but never submit a PR.
Breaching the daemon only allows for the attacker to get access to the login. User accounts should still be secured requiring authentication.
>If this was so easy to deal with, someone would have done it.
Sadly this is not the case. There is a lot of inertia towards solutions like ssh or sudo. It may be easy to delete them, but actually getting such a changed accepted is no trivial task.
1. TELNET is an IETF-standard protocol defined by RFCs.
2. Telnet is a well-known port assigned by the IANA (tcp/23).
3. telnet is a client program, originated on Unix, available on many systems, and likely from a quite homogeneous codebase.
4. telnetd is a server program, also originated on Unix for the purpose of implementing Telnet protocol as a login server. Also a homogeneous codebase or two.
TFA is about items 2 and 4, and 1/3 are completely unrelated.
IIRC, the only traffic that was monitored and detected here is the scanning. The vulnerability scanners that try and detect, for better or worse, what someone's running on port 23, fingerprint it, and figure out if it's a vulnerability.
Interestingly, filtering port 23 only mitigates the CVE by happenstance. It is merely by convention that telnetd runs on port 23, so that people can use it to log in remotely. There is no constraint that requires port 23. Any other service could usurp 23/tcp for itself if the admin decrees it. So, filtering port 23 is an effective mitigation for the defaults of someone running a vulnerable server on the standard port. But it is not a panacea, and it doesn't prevent anyone from using the telnetd server, or the telnet client, except for port 23.
But it also prevents you from offering any service on port 23/tcp, lest it be filtered. You wouldn't want to run a web server, sshd, a MUD, or anything else, because your connectivity would be negatively impacted for this reason. (The common experience is that a lot of Windows SMB/NetBIOS ports are blocked, and SMTP and port 80, on a lot of consumer ISPs, although this is contrasting the ISP situation to Tier-1 transit carriers now.)
I'm not sure I understand how this argument refutes the claim that this isn't about telnetd. There'd be no reason to respond to the vulnerability in the way they did if the vulnerability in telnetd hadn't existed and been exploited -- and the proof is that nobody ever did until now.
...except that port 23 seems to now be filtered across the internet at large, leading to a huge drop-off in telnet traffic over the course of days if not hours. I think it's safe to say that even if you patch telnetd, being able to use telnet over the internet is not possible in many places (including Canada, according to the data).
The difference between "telnet" the program and "telnet" the protocol is especially important in this discussion, I think.
A more "proper" tool for that is netcat -- I doubt SMTP supports the Telnet option negotiations subsystem. (I also doubt SMTP servers can interpret the full suite of Network Virtual Terminal (NVT) commands that the Telnet protocol supports.) There's clearly enough similarity between the two protocols that if you're just using it to transfer plaintext it will probably work out fine, but they are distinct protocols.
I used telnet(1) as a generic TCP text client for many years before switching to GNU/BSD netcat. Nowadays, netcat is more prominent then telnet, and telnet had its corner cases with control characters.
You want nc (usually with -v) or socat. telnet is muscle memory for a lot of people (myself included sometimes) but it's a strictly inferior choice these days for poking arbitrary plaintext services.
Am I the only one who finds this suspicious ? About Telnetd “…The vulnerable code was introduced in a 2015 commit and sat undiscovered for nearly 11 years.”
Okay, it is really weird. This was not an exploit difficult to pull off, or discover. It is such an elementary error that any script kiddie could have leveraged it anywhere, once it was understood.
Is there proof or evidence that it was never exploited in all of 10 years and remained as a latent zero-day?
The only saving grace I would propose, is that since telnetd has been aggressively deprecated once ssh became popular, and encryption became ubiquitous, and remote exploits became commonplace, and Starbucks WiFi was routinely surveilled, that telnetd simply wasn't running anywhere, anymore.
We have commenters saying that embedded systems and IoT used telnet servers. But were they running an actual GNU telnetd or just a management interface that answered on port 23/tcp? Commenters are citing statistics of "open port 23", but that means nothing in terms of this CVE, if it ain't GNU telnetd. Cisco has literally always used port 23 for management. Other routers and network devices use port 23 without telnetd.
How popular was GNU telnetd to be running on a system and exposed to the Internet? This article pertains to all the port-scanners running everywhere, so surely someone with a Shodan account can make a survey and tell us: who was still exposing GNU telnetd in 2026?
Not the original commenter, but I noticed it too. I guess it's hard since AI is trained on human content, so presumably humans write like this too, but a few that stood out to me:
> Five entire countries vanished from GreyNoise telnet data: Zimbabwe, Ukraine, Canada, Poland, and Egypt. Not reduced — zero.
> An attacker sends -f root as the username value, and login(1) obediently skips authentication, handing over a root shell. No credentials required. No user interaction.
> The GreyNoise Global Observation Grid recorded a sudden, sustained collapse in global telnet traffic — not a gradual decline, not scanner attrition, not a data pipeline problem, but a step function. One hour, ~74,000 sessions. The next, ~22,000.
> That kind of step function — propagating within a single hour window — reads as a configuration change on routing infrastructure, not behavioral drift in scanning populations.
(and I'm not just pointing these out because of the em dashes)
GPTZero (which is just another AI model that can have similar flaws and is definitely not infallible, but is at least another data point) rates my excerpts as 78% chance AI written, 22% chance of AI-human mix.
To me at least, the article still seems to be majority human-written, though.
Also, one of the authors is "Orbie", which looks like an AI name, and if you go and read through some of the recent posts, all of the posts with that author feel very LLM-y and bland, and the posts without that author are much more normal.
Never mind telnetd. Tier 1 transit providers doing port filtering is EXTREMELY alarming. They have partitioned the Internet, and in a way that automatic routing (BGP) can't get around.
> Tier 1 transit providers doing port filtering is EXTREMELY alarming.
I was admining a small ISP when blaster and its variants hit. Port filtering 139 and the rest was the easiest way to deal with it, and almost over night most of the ISPs blocked it, and we were better for it. There was a time when if you'd put a fresh XP install on the Internet you'd get 5-10 minutes until it would get restarted.
I guess if you're really an admin that needs telnet, you can move it to another port and go around it? Surely you'd tunnel that "old box that needs to stay alive" if that's the usecase? Is there anyone seriously running default telnet on 23 and is really affected by this filtering?
I do not know what is more critical: the risk of censorship or stand by while hospitals, banking, nuclear power plants and other systems become compromised and go down with people dying because of it. These decision makers not only have powers but also have a responsibility
This feels more akin to discovering an alarming weakness in the concrete used to build those hospitals, banks and nuclear power plants – and society responding by grounding all flights to make sure people can't get to, and thus overstress, the floors of those hospitals, banks and nuclear power plants.
So basically the same as censorship because that is the exact same thing blocking ports does.
not to mention, filtering on udp vs tcp, which makes using anything else impossible. Not that I have one, but it's just a bit in a field, why filter on it?
The most interesting thing here isn't the CVE - it's the invisible coordination. A backbone provider acted on advance knowledge of a critical flaw, implemented filtering at scale, and the rest of us didn't notice until GreyNoise's data showed the drop. The vulnerability got patched at the network layer before it ever reached the application layer. This is what mature security ecosystems look like - the boring, quiet fixes that happen before the press release.
Stop spamming AI slop
?
What an amazing bug. I probably spent my first 10 years on the internet just using telnet. They were wild times. You could log ethernet traffic and see passwords. Towards the end of those we started to have a few more single-user machines, but the vast majority were old school many many user machines, where "root" was thought to be tightly restricted (of course, even then, in practice it wasn't if you were in the know).
Anyway, just wild seeing this:
> telnet -l 'root -f' server.test
or
> USER='-f root' telnet -a server.test
Survive 11 years.
The more I work in software, the more amazed I am that anything works at all. There's likely so much low hanging fruit out there
It's hilarious, especially given that I have memories of similar rlogin vulnerabilities -- various unixes being vulnerable to rlogin -l '-froot' in the 90s.
I never sent root over telnet, but I spent too much vacation time browsing the web via lynx on my school AIX account from a library near my parents' home, because it had a telnet client in addition to the card catalogue program on the otherwise locked down desktop. It was just a more innocent time: you didn't assume your traffic was being logged six ways to Sunday. With telnet access to my AIX account, I could do all the internet things, like mail (pine) and the web (lynx) and irc, from a convenient command line anywhere in the world.
So Telnet as a client is not dead though, right? A long time ago, I used to use the Telnet client to talk to SMTP servers (on port 25) and send spoofed emails to friends for fun.
With port blocking widening in scope, I’ve long believed that we would one day have every service and protocol listening on port 443. Since all other ports are being knocked off in the name of security, we’ll end up having one port that makes port based filtering useless.
netcat, socat and openssl s_client are all available for general manual connection testing.
As are many other tools. But the ones above are basically far better direct telnet alternatives.
I've never really understood why it's a thing to use a telnet client for transmitting text on a socket for purposes other than telnet. My understanding is that telnet is a proper protocol with escape sequences/etc, and even that HTTP/SMTP/etc require things like \r\n for line breaks. Are these protocols just... close enough that it's not a problem in practice for text data?
Because for a long time, on most computers, the telnet client was the closest thing to an "open a tcp socket to this ip/port and connect the i/o from it to stdin/stdout" application you can get without installing something or coding it up yourself.
These days we have netcat/socat and others, but they're not reliably installed, while telnet used to be generally available because telnetting to another machine was more common.
These days, the answer would be to use a netcat variant. In the past, telnet was the best we could be confident would be there.
Because it's there.
If it's alright to be pedantic, anyone with programming knowledge can do the same without these tools. What these offer is tried and tested secure code for client side needs, clear options and you don't need to hand roll code for.
who's hand rolling code anymore these days though?
None of this affects the use of telnet the client program nor the ability to run a telnetd on your own host (but do be sure it's patched!).
What's happened is that global routing on the internet (or big chunks of it, it's not really clear) has started blocking telnet's default port to protect presumably-unpatched/unpatchable dinosaur systems from automated attack. So you can no longer (probably) rely on getting to a SMTP server to deliver that spoofed email unless you can do it from its own local environment.
> started blocking telnet's default port
But that's 23 and smtp is 25.
SMTP has and is almost blocked everywhere to dissuade spam.
You would still be able to use the telnet client to connect to an SMTP server on TCP port 25, just not port 23, right? I don't think that part changed here.
It's... not super clear from the article whether this is a port block or a stateful protocol thing. But yes, you're probably right and SMTP spoofing is probably safe for now.
Why are people still using telnet across the internet in this century? Was this _all_ attack traffic?
(OK, I know one ancient talker that uses it - but on a very non-standard port so a port 23 block wouldn't be relevant)
To watch Star Wars in ASCII.
telnet towel.blinkenlights.nl https://www.youtube.com/watch?v=Mhcf6tc2jeQ
(Remember hearing about this a long time ago (from some searching I think it was in 1999 via Slashdot) and verified some instance of it still exists/works.)
It might be the telnet filtering in action. The host responds to ping but I get nothing back on TCP/23, not even a reset.
It's still working over IPv6
213.136.8.188 appears to not respond to telnet from any ISP I attempt to connect to it on, I wonder if its just not bound to port 23 on IPv4 or the ISP is filtering port 23. IPv6 works fine to connect.
~~IIRC the blinkenlights telnet movies have been offline for a few years already.~~
I connected a few times today to the IPv4 one. Have had no problems myself.
Telnet is used in legacy, IoT, embedded, and low-level industrial hardware. It's also intentionally enabled on devices where automation was written for telnet and it wasn't easy to switch to ssh.
If you investigate most commercial uses of ssh, the security is disabled or ignored. Nobody verifies host keys, and with automation where hosts cycle, you basically have to disable verification as there's no easy way around the host keys constantly changing. Without host key verification, there's kinda no point to the rest.
Even assuming the host keys were verified, the popular ssh conventions are to use either long-lived static keys (and almost nobody puts a password on theirs), or a password. Very few people use SSH with 2FA, and almost no-one uses ephemeral keys (OIDC) or certificates (which many people screw up).
So in terms of how people actually use it, SSH is one of the least secure transport methods. You'd be much more secure by using telnet over an HTTPS websocket with OAuth for login.
How do you automate, for example, "HTTPS over websocket with OAuth", without providing some kind of hard-coded, static or otherwise persistent authentication credentials to the calling system in some form (either certificate based auth, OAuth credentials, etc.)?
The problem with IoT and embedded secrets isn't really a solved problem, from what I can tell. I'm not sure that OAuth exactly solves the problem here. Though all your comments about SSH (especially host verification) holds true.
Just honestly trying to understand the possible solution space to the IoT problem and automated (non-human) authorization.
The manufacturer should at least supply certificates, and it could be up to you to ignore or use. It's not much but it's something.
Unless you manage to leak your private host/client SSH keys, this is close to being as secure as it gets.
I'd say that HTTPS (or TLS in general) is more problematic, since you need to trust numerous root CAs in machine/browser store. Sure, you can use certificate pinning, but that has the same issues as SSH host key verification.
> Very few people use SSH with 2FA.
PCI DSS, HIPAA, and ISO 27001 each either highly recommend or enforce this.
I wouldn't use a jumphost without it.
> Nobody verifies host keys,
The known_hosts file is verification of host keys. It's not verification of a host cert, which is a different thing. Most sshd instances are running on ad hoc hardware without the ability to associate them with someone a cert authority would be willing to authenticate.
Basically people running services that need cert-based authentication are already using TLS (or if they're using sshd they've locked it down appropriately). SSH is for your workstation and your RPi and whatnot.
SSH certs aren't TLS certs. Totally different format. All SSH CAs are private, you run your own CA to issue certs to devices you want to allow to connect to your server.
Hams use it over packet radio sometimes since encryption is forbidden on the amateur bands.
IMHO we need a good telnet replacement that sends signed data. Most people interpret signatures as allowed under FCC rules, just not encryption.
> IMHO we need a good telnet replacement that sends signed data. Most people interpret signatures as allowed under FCC rules, just not encryption.
I know from bitter experience that IPsec is a “now you have two problems” kind of solution, but the Authentication Header is a thing and is supported by most (all?) implementations. Ham radio operators probably don’t have much use for the actual features of telnet compared to plain netcat, do they? (It’s mostly terminal feature negotiation and such.)
TIL that IPsec can be used without encryption. That should work pretty well.
Telnet is mostly used for auth and straightforward terminal/BBS access in my experience. There are some other alternatives like HamSSH but I don’t think it’s that common.
What I meant in my remark about Telnet is that, if you just want is a bidirectional byte pipe to e.g. run a terminal over, then you just need TCP or anything else providing the same abstraction, like TLS-over-TCP or TCP-over-IPsec; whether you then choose to run a getty on that terminal is not for the network to care. (I don’t believe you can get netcat to drive a PTY, so you’ll need e.g. socat. And of course if you want cryptographic authentication then you don’t need or want a getty.)
Telnet, on the other hand, is quite a bit fancier than that and has a fairly involved feature negotiation mechanism for terminal connections that is not entirely in line with the prevalent DEC tradition. As admittedly one of the funkiest examples of what you can do with it, there is for instance a mode[1] where the client is asked to emulate a terminal of the IBM 3270 lineage. (To a practicioner of the aforementioned DEC tradition, those feel like the marsupials of terminals: everything is functionally there, but primitive and derived are occasionally flipped and some features are oddly weak or misdesigned due to a lack of competition.) So if you do actually use Telnet the protocol, by all means, I’ll be delighted to learn what you do with it (partly why I asked in the first place). But if you just need a pipe, then TCP is enough, and netcat or socat make fine ad-hoc clients.
[1] https://tools.ietf.org/html/rfc6270
One? All the talkers still use it and all the MUDs/MOOs etc. far out number the talkers.
N.U.T.S. 3.3.3 4eva! There was a NUTS 4, but about a decade too late.
As I understand it, greynoise is monitoring scanner traffic, so yes this would all be scans or attacks
Probably one of the reasons this bug survived so long is that it isn't used much for priveleged access any more, but so you can play a moo or play you an ASCII movie, as people below you are replying.
telnet lambda.moo.mud.org 8888
MUDs were my introduction to telnet- I grew up a university kid and had access to Wesleyan's minicomputer EAGLE.WESLEYAN.EDU running OpenVMS. I used it to telnet to CMU's TinyMUD and later other TinyMUDs around the country. I recall OpenVMS's telnet had a problem with newlines/carriage returns so all the text was staircased, so I ended up learning C and writing a MUD client. I still habitually use telnet today even if netcat and many other tools have replaced it.
All of that was foundational for my career and I still look back fondly on the technology of the time, which tended to be fairly "open" to exploration by curious-minded teenagers.
nethack.alt.org still maintains a telnet server!
I've always used ssh to connect to it. And it's true that their port 23 is still open at last check. If you cannot reach port 23, and you irrationally hate ssh, you may use 14321 as an alternate.
https://www.alt.org/nethack/
Aardwolf works well from my work laptop. And I don’t care if someone sees what I’m doing
Do you care if they steal your account though and drop all your inventory?
The problem is the auth is plain text too and you're open to having your credentials stolen.
TBH, I don’t care if someone drop all my inventory and delete my account. If I would care about it then I would obviously not use telnet.
I run a DikuMUD that users connect to using Telnet
I really should update it to allow more secure options
> that users connect to using Telnet
Not anymore ;)
Seriously though: did you notice any spikes up or down?
If you'd run it on a non-standard port, anyone can still connect with netcat, socat, etc etc.
Ah, not really. We are on a non-standard port (9000). I just meant some folks use the telnet client to connect, and we do negotiate some telnet options. I use tintin++ these days but I think most of our players are still using decades old zMUD versions to connect!
I always preferred gmud, but zmud has all the bells and whistles. All I needed was ANSI color, aliases, triggers, and command history.
How can I get access?
Or with telnet (the client)
The CVE referenced is caused by this commit:
https://codeberg.org/inetutils/inetutils/commit/fa3245ac8c28...
One of the changes is:
What is the reason for a rename these days? If I saw that in a code review I’d immediately get annoyed (and probably pay more attention)From ChangeLog:
Congratulations! Now you've got yourself a precious and global(ly exploitable) vulnerability...
Wouldn't attention to getenv() calls yield more benefit? Such calls are where input typically isn't parsed--because parsing is "hard"--becoming targets for exploit.
The present fix is to sanitize user input. Does it cover all cases?
On the bright side that CVE seems like pretty great news for the hardware hacking community hoping to get root on embedded devices which have open telnetd.
When I was an intern for some reason they issued me a voip phone for my desk. One day I got bored and figured out I could telnet into it. Nothing interesting but it was still a fun moment for me!
A very very long time ago as an intern I was working on a perl cgi script and I would often test it with telnet. I was used to messing around with hayes commands so manually typing in HTTP commands seemed like a natural extension of that.
If you miss that and long for the olden days, you can still do it today with OpenSSL’s sclient:
I did this too, lol
So eleven years ago someone put a backdoor in the Telnet daemon.
Who?
Where's the commit?
https://codeberg.org/inetutils/inetutils/commit/fa3245ac8c28...
That link goes to a page full of random garbage. No commits there to be seen.
Apparently the owners of that website don't like my choice of user agent, and have decided to punish me accordingly.
That's crazy. This is core business critical software but they just YOLO critical changes without any automated tests? this PR would be insta-rejected in the small SAAS shop I work at.
Culture has changed a lot since the 20th century and older projects can have antiquated norms around things like testing. I was just listening to a recent podcast talking about how worrisome it is that OpenSSL has a casual culture about testing[1] and was reminded about how normal that used to be. I think in the case of telnetd you also have the problem that it’s been deprecated for multiple decades so I’d bet that they struggle even more than average to find maintainer time.
1. https://securitycryptographywhatever.com/2026/02/01/python-c...
Even with automated tests you'd need to think of this exploit right? Perhaps fuzzing would have got it. The mailing lists says they proved it successful on
- OpenIndiana
- FreeBSD
- Debian GNU/Linux
So not complete YOLO.
See https://lists.gnu.org/archive/html/bug-inetutils/2015-03/msg...
FWIW, a well known LLM agent, when I asked for a review of the patch, did suggest it was dodgy but didn't pick up the severity of how dodgy it was.
> a well known LLM agent
Which one?
Not GP, but my local Ministral 3 14B and GPT-OSS 20B didn't catch anything unless I gave some hints.
He says 'well known' so I assume Claude or GPT, I just don't get why he's being coy.
I thought by not naming it wouldn't shift the focus to the particular model, but it did the opposite. It was gpt-5.3-codex in medium mode.
If you think you can do better you're welcome to do better. I say this without a hint of sarcasm. This is how open source works. It's a do–ocracy, not a democracy. Whoever makes a telnet server gets to decide how the telnet server works and how much testing it gets before release.
Maybe the lesson here is to stop letting the GNU folks do things, if this is what they do. This is only one example of craziness coming out of the GNU camp.
Or, flip the responsibility to what it has always been understood to be, when using open source software from random volunteers (some being bad actors) on the internet for anything remotely critical: audit the source.
GNU doesn’t provide labor, only organizational tools like mailing lists and whatnot. The projects that GNU supports are still run by individual volunteers. If you want it done better then please volunteer so that you can be the one doing it better.
I am the one doing it better. GNU software is slowly being deprecated on my system, starting with glibc.
Any business that has a telnet daemon able to be reached by an unauthenticated user is negligent. Just the fact that everything is in the clear is reason enough to never use it outside of protected networks.
unless it doesn’t matter if it’s evesdropped
Traffic could be tampered as well.
Sometimes that doesn't matter either. That is the valid use case of a plain-text protocol like telnet: doesn't matter.
Most 90’s era software had zero tests. Nobody gave it a second thought.
There's a famous XKCD about this: https://xkcd.com/2347/
In this case the hero's name is apparently Simon Josefsson (maintainer).
I feel like we should just start saying 2347. Everyone knows what you mean.
https://xkcd.com/2347/
Ah, someone beat me to it!
Telnet's cleartext and always has been. A backdoor seems like overkill.
You still have to know the password or snoop on someone typing the password. But with this vuln, you don't. You can just get root instantly.
> backdoor
Do you mean that it's intentional? Why do you think so?
It wasn't a backdoor, just a very serious security bug. Congrats on jumping straight to conspiracy and paranoia, though.
It's only a conspiracy and paranoia if it's wrong. 11 years ago was 2015.
> GNU organization
> giant security flaw
Checks out.
> Someone upstream of a significant chunk of the internet’s transit infrastructure apparently decided telnet traffic isn’t worth carrying anymore. That’s probably the right call.
Does this impact traffic for MUDs at all? I know several MUDs operate on nonstandard Telnet ports, but many still allow connection on port 23. Does this block end-to-end Telnet traffic, or does it only block attempts to access Telnet services on the backbone relays themselves?
Most MUDs do not use Telnet.
MUDs use plaintext TCP protocols that are accessible to a wide range of clients.
The Telnet protocol is well-defined and not completely plaintext. There are in-band signaling methods and negotiations. Telnet is defined to live on 23/tcp as an IANA well-known, privileged, reserved port.
MUDs do none of this. You can usually connect to a MUD using a Telnet client, but most players hate the experience and often deride this method in favor of a dedicated, programmable client.
The fact that MUDs inhabit higher 4-digit ports is an artifact from their beginnings as unprivileged, user-run servers without a standardized protocol or an assigned “well-known port” presence. If you want your MUD to be particularly inaccessible, you could certainly run on port 23 now!
As a MUD enthusiast of two decades, this is not accurate. Where are you getting this information?
Most MUDs implement RFC 854, and a number of non-standard Telnet option subnegotiation protocols have been adopted for compression (MCCP2), transmission of unrendered data (ATCP, GMCP, ZMP), and even a mechanism for enabling marking up the normal content using XML-style tags (MXP). These telopts build on the subnegotiation facility in standard Telnet, whose designers knew that the base protocol would be insufficient for many needs; there are a great number of IANA-controlled and standardized telopt codes that demonstrate this, and the MUD community has developed extensions using that same mechanism.
> You can usually connect to a MUD using a Telnet client, but most players hate the experience and often deride this method in favor of a dedicated, programmable client.
I think you are confusing "telnet" the program with "telnet" the protocol. I am speaking here of the protocol, defined at base in RFC 854, for which "telnet" the program is but one particularly common implementation. You look at any of those "dedicated, programmable clients" and they will contain an implementation of RFC 854, probably also an implementation of RFC 1143 (which nails down the rules of subnegotiation in order to prevent negotiation loops), and an implementation of the RFCs for several standard telopts as well as non-standardized MUD community telopts. I can speak for the behavior of MUSHclient in especial regard here, though I am also familiar with the underlying Telnet nature of Mudlet, ZMud, and CMUD, not to mention my very own custom-made prototype client for which I very much needed to implement Telnet as described above.
Yes, perhaps we should define “MUD” and your incomplete experience of “most”.
As a MUD enthusiast for 37 years, I learned to program in C and Unix through TinyMUD, MUCK, and MUSH derived servers. From the beginning, none of these codebases implemted Telnet. There was nothing but a raw transparent TCP connection. In fact, I facilitated the introduction of a grand innovation: the "port concentrator" system which multiplexed TCP connections. Unix processes had a hard rlimit of 64 file descriptors, which crimped our style as an emerging MMORPG. The multiplexer increased this to 4096, for the biggest games of the era.
You mention MUSHclient, and I do not know about later revisions of the TinyMUSH server, but I can assure you that every MUSH I found from Larry Foard on, was not implementing Telnet. (I was privileged to help Larry "test" new features as I red-teamed his server with bizarre edge cases!)
Likewise, after I handed off TinyMUCK 2.3 to the furries, it was not doing the Telnet protocol. When we backported stuff to MUCK 1.x, it was not doing Telnet. I wrote a bonkers Perl program to read MUCK databases and sort of implement the game. No Telnet there. I've got to wonder whether the Ubermud or MOO guys had folded it in; they were close collaborators with us, back in the day.
Now as for the Diku, LP, and other “combat” type games, I’ve no idea. Perhaps they did. We never cared. I was aware that some of them had a pesky “prompt” that violated the line-mode assumptions of conventional clients and needed workarounds.
telnet(1), the program, was historically the only program that implemented the protocol. If you use Tinyfugue or Tinywar or tinymud.el, they are not, and no, I am not confused, because I was giving an example of why the Telnet-implementation, the program, the client, was so inadequate for playing on MUD servers.
It wouldn’t have been difficult to retrofit the Telnet RFC 854 into any MUD server, but none of us wizards had any use for it, seeing that our clients were mature and capable of much more processing without it.
If modern MUD servers have mostly implemented Telnet, then that is cool, but what surprises me is that it is mandatory, and your clients don’t seem to interoperate without it? That is a strange reversal!
The modern MUSH forks do generally support telnet, but yes -- as a 29 year old who's been pathologically obsessed with "MUD archeology" off and on, I'll confirm -- historically, most MUDs did not do any sort of Telnet negotiation.
Further, most older clients did not anticipate any kind of Telnet negotiation from the server, and will print garbage to the screen if connecting to modern MUSHes that do. (I've tested tinywar, vt, and that one VMS client...)
MUCKs never, to my knowledge, implemented telnet, though. They barely support ANSI escapes, nevermind Telnet. :-)
> [...] no, I am not confused, because I was giving an example of why the Telnet-implementation, the program, the client, was so inadequate for playing on MUD servers.
Then this is at the heart of our disconnect, because the post of mine that you originally replied to --- as well as, unless I drastically misread, the original article under discussion --- was concerned with traffic on port 23, the Telnet protocol port, and not with any particular implementation communicating on that port. The concern of my original comment was that this might affect MUDs that operate on port 23. Perhaps you can understand my confusion when you reply stating categorically that most MUDs do not use "Telnet" (meaning the program), when that wasn't really what was at concern (and therefore implied that my question had no basis).
It is a true fact that many MUDs operate on port 23. Many do not, but you can skim a MUD aggregator like MudConnect [0] to see that it is quite common. Aardwolf, Discworld MUD, and the IRE games --- which consistently topped TopMudSites (when that aggregator was still running, anyway) all operate on 23, potentially in addition to an unreserved port.
> what surprises me is that it is mandatory, and your clients don’t seem to interoperate without it? That is a strange reversal!
All telopts are disabled by default, per Telnet RFC; the only things you must absolutely parse under the RFC are the standard complement of NVT commands (such as IAC GA "Go Ahead"), even if they are otherwise implemented as no-ops.
Any input stream with the high bit clear is treated as pure data -- with the incidental exception of bare `\r`, which must always be followed either by `\n` or by `\0`; but Postel's Law has turned that into more of a guideline. So as long as the standard NVT encoding is assumed (which is just 7-bit ASCII) and the NVT core escape sequences are avoided, a modern Telnet-based MUD client can interoperate with a plaintext MUD server without issue. (As you know, this is also why people get away with using `telnet` (the program) to access HTTP and SMTP services instead of using something like netcat.)
Some MUD clients will eagerly send IAC DO / IAC WILL subnegotiations, but general practice is to let the server offer first -- probably precisely to ensure compatibility with MUDs that don't implement Telnet subnegotiations.
> Now as for the Diku, LP, and other “combat” type games, I’ve no idea
Diku-family MUDs are certainly the ones I have the most experience with. I understand LP MUDs also generally have Telnet support; or at least, I recall seeing a patch for them that MUD owners often sought to apply to their games.
[0]: https://www.mudconnect.com/cgi-bin/search.cgi?mode=tmc_bigli...
> MUDs do none of this.
MOOs do.
It wasn’t clear from the article but I assumed they were filtering for the attack specifically.
Since Telnet is totally plain text that would absolutely be easy to do right?
Wouldn't that imply that >80% of all monitored telnet sessions were exploit attempts for the specific CVE in question? Even with the scale of modern botnets, that seems unrealistic for a single vuln that was undisclosed at the time.
I have a hard time thinking it’s popular enough these days that attacks, attempts at attacks, or just command and control couldn’t be the main use.
Not at interconnect speeds
It seems like they are doing a port based block similar to how residential lines often have their SMTP ports shut off.
That said in this day and age, servers on the public network really ought to use SSH.
The scope of this CVE and the response to it are genuinely wild.
It's crazy to think that some dude is singlehandedly responsible for ultimately ending the telnet era in such a definitive way.
One for the history books.
>some dude is singlehandedly responsible
Well, one person to put up the PR and one dude to approve it - back in 2015. It isn't the security researcher's fault.
For about 15 years beginning in 2003 I had some VPSs with CrystalTech/NewTek. I noticed right away that they had blocked all port 23 traffic in/out of their edge.
I asked them about it and they said it was a security measure. Apparently they used telnet for managing their routers.
It turned out that they did not have very good security anyway.
https://krebsonsecurity.com/2018/02/domain-theft-strands-tho...
I switched to A2 hosting shortly after the above incident, but I dumped them when they did not keep up to date on their Ubuntu LTS OS options.
I've been running on AWS for the past eight years. It costs more, but it's been extraordinarily reliable.
A2 and AWS do not restrict port 23.
An RCE in GNU's telnetd has no relationship to the sunsetting of telnet. Something could equally likely happen with SSH (but not really because the OpenBSD folks are paranoid by nature).
Apple removing the telnet client from OS X was a stupid move. How can you call yourself UNIX and not have a telnet client? It's like removing grep or ed.
Thats what the mystery exceptions for the Open Group macOS UNIX certification was for!
telnet has never been in POSIX though.
It's a joke and a poke that macOS isn't certified UNIX as shipped.
https://www.opengroup.org//openbrand/register/
To actually pass the certification test suite on a real system, Apple sometimes needs to apply special configurations (e.g., disabling System Integrity Protection (SIP), using case-sensitive filesystem, enabling certain legacy services, etc.).
telnet(1) is not required by POSIX (nor is nc or ssh required!)
Ironically, telnet(1) did not begin as a "Unix" utility but an ARPANET protocol suite program. It was available cross-platform. It is unclear whether all editions of Unix included a client, but BSD for sure was the point where telnet and TCP/IP became essential integrations for the systems.
There's always nc hostname 23 unless you need authentication
There's no UNIX requirement for telnet.
Ubuntu does not include it by default (starting 16.04?). Most most distros don't.
Two wrongs don't make a right.
Apple still includes uucp for some unknown reason.
The saving disk space argument makes no sense because telnet was one of the smaller binaries in /usr/bin.
Telnet continues to be widely used for select use cases and being told we're naughty by not including it feels punitive and just adds extra steps. What are you supposed to do, trash a $1m piece of industrial equipment because Apple wants to remind you Telnet is insecure?
New devices are still being released with Telnet where SSH is impractical or unnecessary.
There are many things I want to say in reply to this. So I’ll bullet point them:
* yes, do not buy equipment that has acquired so much tech debt that it still requires telnet.
* there are a million telnet clients out in the world. And ones far better than the default OS one. Apple not shipping one standard is not the end of the world or really anything more than a mild inconvenience for the small handful of people who need actual “Telnet” as opposed to Netcat or socat, both of which are far better than base Telnet.
> yes, do not buy equipment that has acquired so much tech debt that it still requires telnet.
No, you already own this capital equipment. It's the laptops running macOS that are ephemeral and disposable.
I don't care for excuses or workarounds; why did they do it?
It was an explicit decision whilst leaving a lot more—arguably more useless—garbage in.
Every OS that removed telnet did so for a symbolic reason, not because it was helpful technically.
It seems rather typical for Apple. The removal of the headphone jack obsoleted thousands of consumer devices.
Ubuntu and derivates removing telnet from the default install, along with other basic tools like traceroute etc, was one of the driving factors toward me creating my own distro. I'm sick of basic stuff being omitted because somebody just decided it's not needed anymore.
How on god’s green earth is `sudo apt install telnet` sufficiently challenging to be a driving factor to creating your own distro??
Because I go long periods of time without internet access, and I don't want to have to "sudo apt install" a fucking thing, ever. Especially not a tiny utility that is all of 172k in size, that I might need for something. Understand?
I want EVERYTHING that I might use installed AT ALL TIMES, FROM DAY ONE, so that I can IMMEDIATELY USE IT when required.
This is only one of many reasons why I abandoned the giant dumpster fire that is mainstream Linux. I do not agree with their idiotic philosophy, on practically every level.
You've now discovered that there are sections of God's Green Earth that you never knew existed! One of many benefits of stepping outside the Matrix for a moment.
Port 23 has decreased significantly over the past decade:
https://i.imgur.com/tZoTWu6.png
Still seeing a sizable number of open ports but it's on the decline.
It's nice to not see C being blamed for once! ... Just good old lack of reasoning (which is most C's codebase downfall, agreeably).
It's also not DNS!
Or Windows, for a change!
Since Tier 1 transit providers have now blocked telnet (port 23), this means the death of watching ASCII Star Wars with `telnet towel.blinkenlights.nl`
However, if you still long for nostalgia, I was able to access it over IPv6 using a VPN based in the Netherlands:
I'm sure the port 23 telnet blocking will be coming to IPv6 soon though.Stranger article. I wasn't able to get the main point of this article. Strangely written, but hey - I'm nob native by any means.
ps.
telnet SDF.org
just works...
it was just ai written thats why.. unexpectedly so from greynoise.
Well, I mean, the first part is a song by Don McLean called American Pie. You might know that, unsure that everyone will pick it out though.
One of the most famous play choices at karaoke bars these days too. I think because the song is a long story, of sorts? But it's a terribly long song and I will leave to take a smoke break anytime it gets chosen. You're going to be there for a good 10 minutes before it concludes.
So maybe the AI prompt was something like, "take CVE-2026-24061 and compose a song lyric in the style of American Pie by Don Mclean". I wonder if you would get similar results with that prompt.
The rest of it seems to be substantially edited by an LLM too, or at least it's composed much like LLM outputs often are these days: “not a gradual decline, not scanner attrition, not a data pipeline problem, but a step function.”
"Not X, not Y, not Z" is a common LLM tic, and there's a few more like it in there.
I mean, that's fair. I guess I just wanted to put my old man hat on. The song is a tribute to an era of lost innocence. Which I think is quite apropos to the current situation surrounding telnet. Vestiges of the days of the early internet continue to disappear, almost like an endangered species. Old/obsolete protocols, like telnet, are pined for by old guys like me.
More likely a specific botnet had it's c2 or telnet scanning report endpoint go down / get nulled on Jan 14th.
Am I the only one who feels like it isn't the responsibility of backbone ISPs to filter traffic like this? In the case of a DDoS situation I could get behind it, but in this case I feel as though it's not Cogent's problem if I want to use telnet from a device on Charter's network to a Vultr VPS, even if it may be ill-advised.
(Of course, the article only speculates that this traffic filtering is what's going on; there isn't any hard proof, but it feels plausible to me.)
I still used telnet today (had to). Unsure of the patching here. But its definitely locked down to a subset of internal use only.
Embedded? Ancient? What sort of systems are you telnetting into?
Not the parent poster, but I also still use telnet. For me it's "Ancient", I have a few retired SPARC and PA-RISC boxes that run their period appropriate OSes as a hobby. Telnet/rlogin is the more reliable method to get into them remotely (just over the LAN).
They're on a LAN behind a NAT Router/Firewall, and I don't always keep them powered up (I'm not that insane) so I really don't have a concern for them.
Some of the more modern/high-performance examples I have run NetBSD with modern sshd and modern ciphers, but you can tell it's a bit of a workout for them.
77k hosts with port 23 open https://www.shodan.io/search?query=telnet
There is something poetic about telnet finally going quiet. It was never meant to survive this long, yet it outlasted entire generations of 'secure replacements' simply because it was simple and it worked. The real lesson is that protocols do not die when better alternatives arrive; they die when the last device running them gets unplugged.
I think it would be better suited to use the terms we use for natural languages. A natural language is dead when the last person who learned it as first language dies and are extinct when there is noone that would speak it at all.
In these terms, telnet has been dead for a long while, but it's extinct now.
The design of telnet and ssh where you have a daemon running as root is bad security that as shown here is a liability, a ticking time bomb ready to give attackers root.
Oldschool telnetd didn’t actually run as root; rather, it just set up a PTY for the incoming socket to talk to, and then fork-exec’ed a /bin/login subprocess to live inside that pty. /bin/login is setuid-root, so it’s “where the security lived.”
I think we all collectively decided that that was a bad idea at some point — probably because /bin/login was never designed under the assumption that it would have to deal with arbitrary binary network traffic being thrown at it (it really only expects keyboard input.) So we switched to doing auth directly in our network daemons, since at least then “people who are aware the code is network-facing” would be maintaining it.
What do you think proper architecture would be, given that ssh needs a capability to let root logins?
I suppose it could be via a proper PAM module, which is widely supported.
Too bad the first PAM RFC was published about the same time the first be version of ssh was released.
> ssh needs a capability to let root logins
One can disable root login via SSH in /etc/ssh/sshd_config. sshd also drops root priviledges once it's running IIRC.
I use use sudo or doas as a regular user once logged in.
I think a proper architecture would not even have a root account. The server would just expose an authenticated endpoint that allows for configuration and updates to be pushed for it.
You are thinking 20 years ahead. In 1995 most servers were still pets, not cattle.
Does ssh need to allow root logins?
Sshing as a regular user and then sudo to root works 95% of the time…
How does SSH become an arbitrary user without effective root?
SSH should not become a different user; it should call something like `/bin/login` which uses PAM for authentication and is capable of starting user sessions.
OpenSSH has been moving quite quickly in the direction of multiple, privilege separated processes, each also heavily sandboxed with pledge and unveil
Literally how else is a remote login daemon supposed to work though?
1. Start with root to bind the port below 1024.
2. give up root because you don't need it any further.
3. Only accept non-root logins
4. when a user creates a session, if they need root within the session they can obtain it via sudo or su.
That still needs a way to change users, and OpenSSH already has privilege separation. That hardens the process somewhat to reduce the amount of code running in the process which can change the uid for a session but fundamentally something needs permission to call setuid() or the equivalent.
Yes, but changing users is a function of the shell (or maybe more specifically /usr/bin/login), not the SSH daemon.
Congratulations, you've created a server that lets people have shells running as the user running telnetd.
You presumably want them to run as any (non root) user. The capability you need for that, to impersonate arbitrary (non-root) users on the system, is pretty damn close to being root.
Well obviously each user just needs to run their own telnet daemon, on their own port of course.
I'm not sure that you need root because of the port - I think login itself needs to run as root, otherwise it cant login to anything other than the account its running under.
You still need to have privileges to become the userid of the user logging in. Openssh does do privsep, but you still need a privileged daemon.
The remote daemon has its own account and is given a privilege that allows it to connect a network socket to a pseudo terminal.
Those are already unprivileged operations, but how does it start the initial process in that terminal with the correct privileges for a different user?
The kernel could authenticate the user before starting it.
How does it do that?
There are many ways from passkeys to SAML. Though for complex methods we may want a dedicated services outside the kernel.
Any breach of the daemon will still give access to a system that can approve/deny user logins. Breaching the daemon therefore allows permission escalation, because you can simply jump to an account. Chain with any local vuln of your choice to completely own the box.
It doesn't matter what user it is running as.
If this was so easy to deal with, someone would have done it. Instead, we get endless HN comments about people that act like they can do better but never submit a PR.
Breaching the daemon only allows for the attacker to get access to the login. User accounts should still be secured requiring authentication.
>If this was so easy to deal with, someone would have done it.
Sadly this is not the case. There is a lot of inertia towards solutions like ssh or sudo. It may be easy to delete them, but actually getting such a changed accepted is no trivial task.
> Breaching the daemon only allows for the attacker to get access to the login
Yes, but potentially any login. See the problem? If you compromise the gatekeeper, you are now the keymaster. Or whatever :)
I'll admit it is still problematic. But at least there is only 1 gatekeeper instead of 2.
How is that better?
Between you and me telnet is not dead. Sometimes I use it to probe a port to verify it is working.
You might wanna use netcat for that instead [1]. Or, for example, socat [2]. Netcat has been around for a long, long time now.
[1] nc (1) - arbitrary TCP and UDP connections and listens
[2] socat (1) - Multipurpose relay (SOcket CAT)
That's not really telnet. Yeah, it's using the same client, but the server and underlying protocol are what's relevant here.
The modern replacement for telnet used in the "probe a port" fashion is nc/netcat.
It's more like telnetd died rather than telnet died.
btw if you want a quick telnet client, and an old python happens to be installed, you can use `python -m telnetlib IP`
I used to telnet into my POP3 account and check email by protocol. Shucks.
Your dial up ever die when you where checking your email? My ISP didn't allow for leave copy on server, I remember times I lost emails to this.
This is about Telnetd. Not telnet itself.
IIRC, the only traffic that was monitored and detected here is the scanning. The vulnerability scanners that try and detect, for better or worse, what someone's running on port 23, fingerprint it, and figure out if it's a vulnerability.
Interestingly, filtering port 23 only mitigates the CVE by happenstance. It is merely by convention that telnetd runs on port 23, so that people can use it to log in remotely. There is no constraint that requires port 23. Any other service could usurp 23/tcp for itself if the admin decrees it. So, filtering port 23 is an effective mitigation for the defaults of someone running a vulnerable server on the standard port. But it is not a panacea, and it doesn't prevent anyone from using the telnetd server, or the telnet client, except for port 23.
But it also prevents you from offering any service on port 23/tcp, lest it be filtered. You wouldn't want to run a web server, sshd, a MUD, or anything else, because your connectivity would be negatively impacted for this reason. (The common experience is that a lot of Windows SMB/NetBIOS ports are blocked, and SMTP and port 80, on a lot of consumer ISPs, although this is contrasting the ISP situation to Tier-1 transit carriers now.)
I'm not sure I understand how this argument refutes the claim that this isn't about telnetd. There'd be no reason to respond to the vulnerability in the way they did if the vulnerability in telnetd hadn't existed and been exploited -- and the proof is that nobody ever did until now.
...except that port 23 seems to now be filtered across the internet at large, leading to a huge drop-off in telnet traffic over the course of days if not hours. I think it's safe to say that even if you patch telnetd, being able to use telnet over the internet is not possible in many places (including Canada, according to the data).
I wonder if simply moving to a nearby port would work. I assume only port TCP/23 is filtered instead of filtering the telnet protocol itself.
telnet isn't just for ... telnet.
.. or you can use SWAKS! For some odd reason telnet is becoming rare as an installed binary.The difference between "telnet" the program and "telnet" the protocol is especially important in this discussion, I think.
A more "proper" tool for that is netcat -- I doubt SMTP supports the Telnet option negotiations subsystem. (I also doubt SMTP servers can interpret the full suite of Network Virtual Terminal (NVT) commands that the Telnet protocol supports.) There's clearly enough similarity between the two protocols that if you're just using it to transfer plaintext it will probably work out fine, but they are distinct protocols.
I used telnet(1) as a generic TCP text client for many years before switching to GNU/BSD netcat. Nowadays, netcat is more prominent then telnet, and telnet had its corner cases with control characters.
Never heard about https://jetmore.org/john/code/swaks/, thanks for the tip.
You want nc (usually with -v) or socat. telnet is muscle memory for a lot of people (myself included sometimes) but it's a strictly inferior choice these days for poking arbitrary plaintext services.
As long as it works, it doesn’t really matter for a quick test.
I find myself using curl telnet://server:port too often these days because telnet and nc don’t get installed.
I discovered swaks recently, god I love that tool
Am I the only one who finds this suspicious ? About Telnetd “…The vulnerable code was introduced in a 2015 commit and sat undiscovered for nearly 11 years.”
Okay, it is really weird. This was not an exploit difficult to pull off, or discover. It is such an elementary error that any script kiddie could have leveraged it anywhere, once it was understood.
Is there proof or evidence that it was never exploited in all of 10 years and remained as a latent zero-day?
The only saving grace I would propose, is that since telnetd has been aggressively deprecated once ssh became popular, and encryption became ubiquitous, and remote exploits became commonplace, and Starbucks WiFi was routinely surveilled, that telnetd simply wasn't running anywhere, anymore.
We have commenters saying that embedded systems and IoT used telnet servers. But were they running an actual GNU telnetd or just a management interface that answered on port 23/tcp? Commenters are citing statistics of "open port 23", but that means nothing in terms of this CVE, if it ain't GNU telnetd. Cisco has literally always used port 23 for management. Other routers and network devices use port 23 without telnetd.
How popular was GNU telnetd to be running on a system and exposed to the Internet? This article pertains to all the port-scanners running everywhere, so surely someone with a Shodan account can make a survey and tell us: who was still exposing GNU telnetd in 2026?
Why would somebody read something that somebody couldn't be bothered to write? This article is AI slop.
What stood out as AI written? It felt like a well-written article by an SME to me.
Not the original commenter, but I noticed it too. I guess it's hard since AI is trained on human content, so presumably humans write like this too, but a few that stood out to me:
> Five entire countries vanished from GreyNoise telnet data: Zimbabwe, Ukraine, Canada, Poland, and Egypt. Not reduced — zero.
> An attacker sends -f root as the username value, and login(1) obediently skips authentication, handing over a root shell. No credentials required. No user interaction.
> The GreyNoise Global Observation Grid recorded a sudden, sustained collapse in global telnet traffic — not a gradual decline, not scanner attrition, not a data pipeline problem, but a step function. One hour, ~74,000 sessions. The next, ~22,000.
> That kind of step function — propagating within a single hour window — reads as a configuration change on routing infrastructure, not behavioral drift in scanning populations.
(and I'm not just pointing these out because of the em dashes)
GPTZero (which is just another AI model that can have similar flaws and is definitely not infallible, but is at least another data point) rates my excerpts as 78% chance AI written, 22% chance of AI-human mix.
To me at least, the article still seems to be majority human-written, though.
Also, one of the authors is "Orbie", which looks like an AI name, and if you go and read through some of the recent posts, all of the posts with that author feel very LLM-y and bland, and the posts without that author are much more normal.
GGP has a good eye.
Who actually uses the tectia ssh client instead of openssh?
telnet + shijack = good times
port 22 2FA
Your cookie banner is very inconvenient and made me leave your website and not read the article
The pattern points toward one or more North American Tier 1 transit providers implementing port 23 filtering
Someone attempted to compromise my home router last week using CHARGEN. Can you imagine!
Attempted to compromise, or just port scanned?