Filtering one port is not censorship. Not even close.
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?
This may seem like a good idea, and frankly is likely a net-positive thing, but it is literally the definition of "ISP decides what apps its customers can and cannot use."
I share the concern and don't really like it either.
Anyway, just wild seeing this:
> telnet -l 'root -f' server.test
or
> USER='-f root' telnet -a server.test
Survive 11 years.
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.
As are many other tools. But the ones above are basically far better direct telnet alternatives.
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.
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.
But that's 23 and smtp is 25.
(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)
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.)
Connection failed
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.
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.
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.
PCI DSS, HIPAA, and ISO 27001 each either highly recommend or enforce this.
I wouldn't use a jumphost without it.
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.
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.)
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.
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.
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.
The problem is the auth is plain text too and you're open to having your credentials stolen.
I really should update it to allow more secure options
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.
How can I get access?
It seems to use a different telnetd (busybox?), because from what I can tell it's not prone to this error.
and also the pattern:
> The most interesting thing here isn't the CVE… This is what mature security…
> The most interesting finding isn't that hyperbolic growth… This is Kuhnian paradigm…
both comments in the last 24h
https://codeberg.org/inetutils/inetutils/commit/fa3245ac8c28...
One of the changes is:
- getterminaltype (char *user_name, size_t len)
+ getterminaltype (char *uname, size_t len)
* telnetd/utility.c (getterminaltype): Change the
name `user_name' to `uname', as the former shadows a precious
and global variable name.The present fix is to sanitize user input. Does it cover all cases?
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!
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.
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!
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. :-)
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...
MOOs do.
Since Telnet is totally plain text that would absolutely be easy to do right?
That said in this day and age, servers on the public network really ought to use SSH.
openssl s_client -connect www.yahoo.com:443Who?
Where's the commit?
Apparently the owners of that website don't like my choice of user agent, and have decided to punish me accordingly.
1. https://securitycryptographywhatever.com/2026/02/01/python-c...
- 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.
Which one?
In this case the hero's name is apparently Simon Josefsson (maintainer).
Ah, someone beat me to it!
Do you mean that it's intentional? Why do you think so?
> giant security flaw
Checks out.
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.
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.
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.
Ubuntu does not include it by default (starting 16.04?). Most most distros don't.
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.
* 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.
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.
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.
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.
https://i.imgur.com/tZoTWu6.png
Still seeing a sizable number of open ports but it's on the decline.
ps.
telnet SDF.org
just works...
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.
"Not X, not Y, not Z" is a common LLM tic, and there's a few more like it in there.
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
(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.)
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.
btw if you want a quick telnet client, and an old python happens to be installed, you can use `python -m telnetlib IP`
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 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.
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.
Sshing as a regular user and then sudo to root works 95% of the time…
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.
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.
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.
>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.
Yes, but potentially any login. See the problem? If you compromise the gatekeeper, you are now the keymaster. Or whatever :)
In these terms, telnet has been dead for a long while, but it's extinct now.
[1] nc (1) - arbitrary TCP and UDP connections and listens
[2] socat (1) - Multipurpose relay (SOcket CAT)
The modern replacement for telnet used in the "probe a port" fashion is nc/netcat.
> 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.
GGP has a good eye.
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.
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.)
$ telnet smtp.example.co.uk 25
HELO me
MAIL FROM: gerdesj@example2.co.uk
RCPT TO: gerdesj@example.co.uk
DATA
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.
Never heard about https://jetmore.org/john/code/swaks/, thanks for the tip.
I find myself using curl telnet://server:port too often these days because telnet and nc don’t get installed.
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?