TS-2026-009: Insecure argument handling in Tailscale SSH permitted root access

https://tailscale.com/security-bulletins

by jervant • 8 days ago

216 149 8 days ago

tptacek• 8 days ago

This is such a venerable and ancient class of bugs, going at least as far back as AIX 3. Glad to see they're still makin' 'em like they used to.

(If you had SSH access to a host in your Tailscale ACL, you could log in as `-i` and get a root login.)

iririririr2• 8 days ago

you can also add parameters to env vars in some popular cloud providers for the same effect.

RossBencina• 8 days ago

I'm somewhat alarmed that the context that this bug was running in was capable of root login. Is there a reason that an SSH login process would, by default, have enough capabilities to facilitate direct root login?

yjftsjthsd-h• 8 days ago

If it runs as your user and can only log in as you, then I wouldn't expect it to be able to become root. But if it can log you in as different users, I would expect that 1. it needs to run from root, and 2. it can log in as root.

anilakar• 8 days ago

We did Tailscale-like SSH reverse tunnels at scale first in 2013 and the main issue has always been that there are no good libraries. Bash scripting around the OpenSSH binaries is pretty much the only way to go.

There's Paramiko, but Python is still a huge liability in memory-constrained systems.

amluto• 8 days ago

Making a useful multi-uid-capable daemon that can’t become root but is not so nerfed that no one uses it is nontrivial. If nothing else, what policy would you use? Why do you think that no other uid is equivalent to root?

unprovable• 8 days ago

It's so old skool, it's almost new again... almost... Still waiting for that third (or fourth?!) wave of XXE (or similar) bugs.

e40• 8 days ago

So, giving access via tailscale but using OpenSSH is safe, right?

lugoues• 8 days ago

Yes, this only involves their wrapper that is managed by ACL rules.

iririririr2• 8 days ago

as much as handing control to a remote third part is, yes.

cevn2• 8 days ago

Good point. I self host headscale but it also has the ssh feature, probably also insecure.

kbumsik• 8 days ago

Why own numbering instead of CVE?

vngzs• 8 days ago

It lets organizations (Tailscale) control the timing and narrative around the disclosure more directly. Organizations sometimes avoid the bureaucracy of going through CVE Numbering Authorities by self-publishing. Often a CVE assignment follows self-disclosure, especially when there's pressure to interoperate with vuln-scanning/compliance tooling

bigfatkitten• 8 days ago

And sometimes it’s just impossible to get a CVE number in a reasonable amount of time, or indeed at all.

wereHamster• 8 days ago

Some reasons why an org might want to become their own CNA: https://daniel.haxx.se/blog/2024/01/16/curl-is-a-cna/

Retr0id2• 8 days ago

To be clear creating your own custom IDs isn't the same thing as being a CNA, but some of the motivations are the same.

cyberax5• 8 days ago

"Tailscale SSH now rejects usernames with leading dashes."

Really? That's the fix?

A proper fix is to use "--" to separate arguments.

sedatk2• 8 days ago

Their fix just future-proofs it in case the same bug gets reintroduced.

cyberax5• 8 days ago

This is just a dirty fix. It adds weird restrictions and masks issues.

Refactoring external invocations to use safe argument handling is a better way to fix it. Along with tests that exercise weird names.

turbert2• 8 days ago

A correct implementation would be to just call glibc directly, this seems like a hasty fix to get the patch out the door. The history of vulns from bad shell escaping is as old as bash, whenever possible you probably shouldn't be mixing code and data, especially in a security critical application like this.

valleyer• 8 days ago

A proper fix is not to shell out to a command at all; use getpwnam(3) or similar.

cyphar4• 7 days ago

They actually do use "os/user".Lookup() as a fallback if fetching via getent(1) fails, so they might be using NSS modules to dynamically define users and then not linking their binaries with glibc (musl and the opt-in pure-Go implementation of "os/user".Lookup() do not support NSS).

Still, requesting the information from systemd-userdb would be a better way of doing this if it's really necessary.

EDIT: Yeah, some older comments confirm that was their reasoning. https://github.com/tailscale/tailscale/commit/1fc1077052dfa7...

catlifeonmars3• 8 days ago

“--“ doesn’t work on all versions of getent.

A better fix is to call “getent passwd” with no user controlled arguments and then parse the resulting list. This gets rid of the input sanitization problem entirely.

cyberax5• 8 days ago

Are there any actual systems that can run Tailscale and that have faulty getent?

tryauuum• 8 days ago

that's stupid. getent passwd will fetch the user list from the network depending on setup. Have fun fetching 40000 records from the network every call

doublepg23• 8 days ago

I’m a heavy Tailscale user, so I do trust them quite a bit, but I never used the Tailscale SSH feature. I feel like OpenSSH’s security record is pretty unbeatable, not sure why I’d swap over for such a security-sensitive tool.

jdiff2• 8 days ago

I've used it before to access my tailnet machines through a browser on a machine I can't download software on.

Shorel• 8 days ago

I just don't use stranger's machines to access my personal stuff. Possibly compromised stranger's machines. I don't see the benefit about that, as I have more laptops than I need.

isatty• 8 days ago

Convenience for the most part but in general, I agree. I like having it as an option.

bakies• 8 days ago

Yeah pretty much just use tailscale as a vpn.. do one thing as they say.

dgacmu• 8 days ago

I used it for a bunch of remote monitor boxes to have a way of centrally managing ssh access to things that were often on- and off-line. It was simple and convenient and access was easily revocable.

OJFord3• 8 days ago

The SSH vulnerability here only applies if the attacker is already on the network. It violates your Tailscale ACLs, but it's not arbitrary external root ssh access. Arguably that's a more secure starting point than vanilla ssh to publicly accessible machine.

semi-extrinsic• 8 days ago

OTOH, if you run vanilla ssh on a publicly accessible machine where only port 22 is open, sshd only allows publickey-based authentication and the only accepted key types are FIDO2/U2F hardware-backed keys, it's probably more secure again (less attack surface).

fodkodrasz• 8 days ago

With a plain VPN like WireGuard when they get access to your network, they don't have plain ssh, not to mention root ssh access to hosts. This is a serious issue.

theshrike792• 5 days ago

The advantage of tailscale ssh is that the machine doesn't need to have the ssh port open publicly.

modeless• 8 days ago

Tailscale SSH has caused me other problems in the past because it takes over port 22. I'm not a fan.

LeoPanthera• 8 days ago

That is what it's supposed to do, though. It's not a secret.

newdee• 8 days ago

It takes over port 22 on the Tailscale interface only. Only had problems with this when I’ve wanted to hit a host’s non-Tailscale ssh service via Tailscale. Otherwise it’s been great for me

drnick1• 8 days ago

I'll stick to my 100% self-hosted Wireguard setup, thank you very much.

m_mueller2• 8 days ago

Why not tailscale plus head scale for self hosting?

diarrhea2• 8 days ago

I do not understand this rebuttal.

I also run self-hosted Wireguard. Initially on a Debian box, nowadays it is integrated into my router (admittedly, this is closed source). For around 6 years at this point.

The whole thing could not be easier and simpler. It has never randomly broken on me. It is fast. It is free. There is no middle man, no vendor.

I never understood the popularity of Tailscale, though that is on me. I'm sure it is a great product, I just never tried it, do not seem the target audience.

What confuses me is the often accompanying, sometimes aggressive anti-selfhosting stance in these sorts of threads. I do not see this in other topics, e.g. someone mentioning they run Jellyfin isn't met with "why not Plex?". Where does that come from? We are on HackerNews, not ProductShillNews, aren't we? I guess self hosting Wireguard is too boring to warrant any further discussion? The VPN equivalent of a Toyota Corolla.

apexalpha• 8 days ago

I also tested tailscale, headscale and netbird and found all these drain your battery on mobile.

Others report no issues but I had massive drain on iOS even with only 4 connections open.

Native wireguard is unnoticeable.

dizhn• 7 days ago

I found Netbird to be easier to implement. Among its peers it seems to be the most complete open source solution. It IS a bit fast moving though so there might be issues. And they just added an Agent network thing that nobody asked for but it's par for the course these days.

doctorpangloss3• 8 days ago

haha self hosted wireguard, an opportunity to find out AllowedIPs: 0.0.0.0/0 does the opposite of what you think it will do

hdgvhicv2• 8 days ago

Allow any (ip4) traffic to enter the tunnel, install a route in the default routing table to make that happen (well the second depends on the client)

Does it do the opposite of that?

sva_2• 8 days ago

If you don't care to invest half an hour into learning some basics of how computer networking and in particular CIDR notation and subnet masks work, maybe it is not for you.

petee• 7 days ago

Is this what your referring to? https://utcc.utoronto.ca/~cks/space/blog/linux/WireGuardAllo...

nicman232• 8 days ago

i really dislike that there is no way to do dhcp for new clients and that i have to manually define peers in each "exit node"

gear54rus• 8 days ago

Because wireguard to tailscale is like git to git GUIs. It's solid base but never should be used separately without a proper wrapper if one wants to keep one's sanity.

luciana1u• 8 days ago

tailscale ssh: replacing a 25-year-old battle-tested codebase with a startup's Go rewrite and then acting surprised when it has bugs

mintflow3• 8 days ago

>> We would like to thank Anthropic and Ada Logics for reporting this issue.

it seems anthropic also use tailscale or it's just being discovered by the mythos model?

tristanj• 8 days ago

I presume Ada Logics has access to Anthropic's Mythos model via Project Glasswing, and Ada Logics discovered this exploit during their vulnerability research.

mintflow3• 8 days ago

pure logic error, the undergoing tailscale rust rewrite can't help this too:)

mintflow3• 8 days ago

that said, the limit impact perhaps is only affected multiple users in tailnet if the ACL is not configure correctly

As single tailnet+single user, perhaps it's just okay

bestouff2• 8 days ago

If it used one of the standard arguments-handling crates (e.g. "clap") there's no way it can happen.

pferde3• 8 days ago

If it used one of the standard system APIs for looking up user accounts (e.g. getpwnam(3)), there's no way it can happen.

This is incredibly bad engineering, on level of a SQL injection, in 21st century. Something a highschool student experimenting with scripting could come up with, but not a supposedly professional software company.

farfatched• 8 days ago

Sadly, yet another path to root via Tailscale.

If their scope grows, and they run so much as root, it won't be their last.

eviks3• 8 days ago

Tailscale SSH now rejects usernames with leading dashes.

Is the proper fix not restricting users not possible in these poorly designed ancient systems?

Similarly re another issue: why not just fix the permission issues instead of restricting users?

Tailscale now disallows the use of UIDs or numeric-only usernames via SSH to avoid this ambiguity

pferde3• 8 days ago

The proper fix would be to not use getent CLI tool in their logic, but instead use proper system APIs for looking up user account entries, like one of earlier comments here already mentions. This is shocking amateur hour!

My guess is they hastily threw together something hacky in early development, and forgot to replace it with a real, safe solution later.

rfoo• 8 days ago

The issue here is there is NO single system API for looking up user account entries on Linux.

It's implemented in libc. So you need to link to libc. Tailscale is a Go binary, and they probably prefer it to be statically-linked. glibc NSS implementation also REQUIRES you to load `.so` so you just can't emulate it in Go.

Then, "link to libc". Which libc? glibc? musl?

sam_lowry_4• 8 days ago

There is a whole class of security issues where fixes are worse that the issues themselves. Case in point, the OpenSSH itself that sends 100 packets on each keystroke to avoid timing attacks.

fragmede3• 8 days ago

Why is that worse than the issue itself? If someone could figure out, say, my root password via an ssh timing attack, that seems bad. Sending 100 packets for each keystroke to protect against this seems cheap in the face of that.

eviks3• 7 days ago

The example doesn't seem worse, but more importantly - are these issues part of the class (and why)?

s_ting7652• 8 days ago

I don't see the point of publishing a security bulletin if you are not going to timely push the fix to artifacts on all affected platforms. Tailscale needs to do better on their release process, docker hub shows last update was 8 days ago.

dijit• 8 days ago

so you can disable or mitigate it…

dust-jacket3• 8 days ago

I don't mind having a bulletin so much as the claim that it's fixed in 1.98.9 or newer, when that release doesn't appear to exist yet. Feels pretty weird practice to advise upgrading to a non-existent version.

s_ting7652• 8 days ago

1.98.9 has already been tagged since bulletin was published (don't know why they chose on github to tag but not release).

1.98.9 version exists! That's not the question. It should already have been made available for Linux distros assuming this resource from Tailscale is accurate https://pkgs.tailscale.com/stable/?v=1.98.9

Edit: Their changelog also mentions the version: https://tailscale.com/changelog#all

jcarrano• 8 days ago

usernames were passed as arguments to getent(1) to retrieve the corresponding passwd entry

Always try to use actual API/system calls (in this case getpwnam) instead of calling sub-processes.

traceroute6611• 8 days ago

At $work we use Tailscale mostly because we were running into too many random issues with NAT with our standard DIY Wireguard setup, especially when people were working from hotels and other places with half-ass network setups.

But we don't trust Tailscale. Just look at the thousands of unresolved Github issues, many of which are actually quite important/useful but have been ignored for months and years.

We very much fear at $work that there are vulnerabilities in the Tailscale product awaiting discovery. Especially as, AFAIK, Tailscale have never had a formal security audit on their software.

So we install it on hardened bastion hosts in an old-school "jump host" model. So people can still get access to where they need to be, but we don't need to install Tailscale's unaudited shit on every single server / vm / etc.

And we only use Tailscale as a glorified VPN, we don't use their hundreds of extra random features like this SSH one.

In terms of SSH, we use old-school OpenSSH and SSH certificates. Its really not that difficult and its really not expensive, you can do offline signing with Yubikeys, no need for expensive HSMs.

fragmede3• 8 days ago

Tailscale contracts with cybersecurity firm Latacora to conduct traditional assessments, advisory services, design reviews, auditing and testing.

https://tailscale.com/security

traceroute6611• 8 days ago

Tailscale contracts with cybersecurity firm Latacora to conduct

And these published audits of the `tailscale` software are where ?

Even half-serious VPN providers like Mullvad publish in public their regular security audits of their app and infrastructure.

There is zero reason Tailscale cannot do the same.

And frankly, given the nature of this vulnerability, "insecure argument handling" I'm not entirely sure it has been audited ? Or if it has, they should be looking for a new auditor ASAP !

evan_a_a• 8 days ago

We very much fear at $work that there are vulnerabilities in the Tailscale product awaiting discovery

Spoiler alert: This applies to all software. This is why security preaches defense in depth.

traceroute6611• 8 days ago

Spoiler alert: This applies to all software. This is why security preaches defense in depth.

Thank you Mr/Mrs Pedant.

However this is a VPN product I am talking about which has an integral part in the defence in depth of which you speak.

Therefore it is only right and proper that I, and others, should be able to trust them to a greater degree. Never 100% of course, as you say. But if you are selling me a security product then you should be able to demonstrate you have put some damn effort into securing it.

"insecure argument handling" in a security product is not a good look. It stinks of sloppy coding practices followed up by a lack of security auditing.

ivlad• 8 days ago

Genuine question, if you use bastion hosts, why do you need Tailscale? Why not to expose tcp/22 to the internet and allow public key authentication only (or, certificate based one, if you prefer fancy)? OpenSSH security track record seems to be better than that of Tailscale.

traceroute6611• 8 days ago

Genuine question, if you use bastion hosts, why do you need Tailscale? Why not to expose tcp/22

Fair question. The short answer is my wording was a little off.

We do have pure SSH bastion hosts. Those are OpenBSD-based with a few tweaks, but basically they are locked-down and heavily monitored to within an inch of their life.

The reason Tailscale is there is mostly so the non-techies can access intranet portals and such-like.

But Tailscale does have a useful party-trick for the techies too, in that you can do DNS-suffix based routing (without needing to use Tailscale's DNS).

So they can automagically hop via a Tailscale bastion host to "*.$region.rds.amazonaws.com" or whatever. You just specify the DNS suffix and anything on that wildcard will work. It therefore enables us to further harden access to that stuff to a source IP of the Tailscale bastion hosts (in addition to the usual security stuff, of course).

Tailscale takes care of the automagic load-balancing/re-routing to the Tailscale bastion hosts, so we just have a bunch installed in random geographically-dispersed places and as long as >0 are up and running the Techies will always have a route to where they need to be.

Obviously the above is over-simplified summary before the pedants start picking it apart. ;)

Retr0id2• 8 days ago

I've been planning a similarly "paranoid" (but apparently not that paranoid) Tailscale setup, for the same reasons.

Another concern I have is whether a compromise of Tailscale's own infra could let an attacker just add itself to my network. Apparently the "Tailnet Lock" feature mitigates this, but it is off by default. If I was an APT, compromising Tailscale would be priority number 1!

traceroute6611• 8 days ago

Another concern I have is whether a compromise of Tailscale's own infra could let an attacker just add itself to my network. Apparently the "Tailnet Lock" feature mitigates this, but it is off by default.

Yeah, we use "tailnet lock" to sort of cover that. AFAIK its the only option available.

I say "sort of" because "tailnet lock" is a bit half-assed in its design and implementation.

For example, you cannot sign new nodes from mobile devices (e.g. iOS). And as we know, locking down iOS is easier than a full desktop machine. So that's a bit of a missed opportunity. And as usual, there's a Tailscale Github issue open for it for years ...

And in practice, the "tailnet lock" addition is done through the `tailscale` software on the desktop/laptop. Its click a button or run a CLI without further authentication/authorisation required. So basically anyone could do it. And the keys are, of course, open to exfiltration, you can't use a Yubikey or anything like that. And you can't require multi-signer. Complete joke really.

Also if you use "tailnet lock" with the Tailscale Mullvad integration, its a one-way street. You can sign Mullvad nodes so they can be used as exit nodes, but it is impossible to revoke signatures from obsolete/replaced Mullvad nodes (with tailscale clients, you simply delete them from the org which means they have to re-auth and re-join under a new ID ... but you can't do that with the Mullvad integration, so the 'ghost' exit node could easily return). Your only option is to remove the signing node that signed the Mullvad nodes and start again from scratch. Another joke.

So, yeah, "better than nothing" is the way I would summarise "tailscale lock".

pedrocr• 7 days ago

If I was an APT, compromising Tailscale would be priority number 1!

Crowdstrike would be at a much higher priority. They already have a cloud connected root console to all the machines in entire corporations. Compromising your network is small potatoes compared to that.

Bnjoroge• 7 days ago

Did you try Headscale? https://github.com/juanfont/headscale or netbird? The latter has been great for me.

traceroute6611• 7 days ago

Did you try Headscale? https://github.com/juanfont/headscale or netbird?

Am aware of them but IIRC they are both unaudited which kind of brings us back to square one ? We would still end up running them at arms-length as we do with Tailscale at the moment.

Isn't Headscale server-side only ?

Also a bit strange that "Tailscale vs Netbird" doesn't feature more prominently on their "Compare Netbird" page. It is hidden behind "Load More". ;D

xyzzy_plugh2• 7 days ago

we don't trust Tailscale. Just look at the thousands of unresolved Github issues, many of which are actually quite important/useful but have been ignored for months and years.

This is a poor measure of quality. I've spent considerable time knee-deep in these issues in particular and the vast, vast majority of them are feature requests, bug reports awaiting more information from the submitter, or bug reports that cannot easily be reproduced (likely conflicts with other software). On the whole, Tailscale does an excellent job of "just working" in practically every possible environment. They do an incredible job thriving in the diverse ecosystem of software networking and their work will naturally never be remotely done. There will always be gaps, it's the nature of the beast. I'm not aware of any other product that does a better job here.

If you've spent any time dealing with enterprise software you'd know that there is an infinite firehose of these sort of issues. We're lucky that Tailscale keeps these public. Many other vendors track these sort of issues privately.

If there are particular issues which jeopardize the security posture of Tailscale deployments that have been open for a significant amount of time, my clients and I would love to know. Please share!

We very much fear at $work that there are vulnerabilities in the Tailscale product awaiting discovery. Especially as, AFAIK, Tailscale have never had a formal security audit on their software.

I can't take this seriously. If you were a customer you could, you know, ask them? Or inspect their SOC2 documents?

I absolutely guarantee they undergo regular formal security audits. There's no question.

So we install it on hardened bastion hosts in an old-school "jump host" model. So people can still get access to where they need to be, but we don't need to install Tailscale's unaudited shit on every single server / vm / etc.

Bastion hosts are a terrible model in 2026. I can't take this approach seriously.

we don't need to install Tailscale's unaudited shit on every single server / vm / etc.

You never need to do this. Simply create an exit node into your subnet, and everything you want becomes routable.

It sounds to me like your architecture struggles with a zero-trust network approach if you view this as a blocker. I've got some slots available if you need a consultation!

In terms of SSH, we use old-school OpenSSH and SSH certificates. Its really not that difficult and its really not expensive, you can do offline signing with Yubikeys, no need for expensive HSMs.

It's expensive in terms of engineer-hours, especially compared to Tailscale. It's also easy to get wrong, and end up with the same vulnerability as TFA, or worse.

But my main issue is that if you see this as equivalent it's telling me that you're not really the target audience. Your set-up is far more vulnerable than the out-of-the-box experience you get with Tailscale. If you don't mind, then I'm happy for you.

Lastly, I don't work for Tailscale and I'm not affiliated with them in any way beyond being a happy user that has solved a lot of problems very easily with their product. I highly recommend it to practically everyone. It's great.

I'm having a hard time taking your comment seriously. It just seems like non-constructive FUD.

traceroute6611• 7 days ago

I absolutely guarantee they undergo regular formal security audits. There's no question.

Well, they clearly don't if they have an "insecure argument handling" vulnerability.

As others here have said already here, its an "venerable and ancient class of bugs".

Its the sort of thing that should be picked up by modern defensive programming that includes fuzz testing.

And it is CERTAINLY the sort of thing that should be flagged by any competent security audit. "insecure argument handling" is bread-and-butter for security auditors.

I can't take this seriously. If you were a customer you could, you know, ask them? Or inspect their SOC2 documents?

SOC2, ISO27001 and all that shit is not the same thing.

As I said, anyone serious who is proud of having had their software audited as clean would publish their reports in public. Nothing to hide. And it strengthens your case with customers.

It can always be a suitably redacted management summary written by the auditor. That's what everyone else does.

zie3• 7 days ago

And we only use Tailscale as a glorified VPN, we don't use their hundreds of extra random features like this SSH one.

Same, you are not alone. The Tailscale VPN stuff just works. None of their competitors can claim this that I'm aware of. We tried several other products and none of them were as reliable and 'just worked'. I imagine they spend a lot of time just keeping that stuff working.

eightys3v3n2• 7 days ago

I haven't had problems with NetBird on Android and Linux. I have also used tailscale and found it comparable.

formerly_proven3• 7 days ago

Tailscale on iOS has always been highly unreliable for me, while the regular Wireguard app just works.

TacticalCoder• 7 days ago

I'll fix this entire HN thread for you all and you'll thank me later:

    s/bug/backdoor/g

aftbit• 7 days ago

It appears that tailscale ssh is off by default? How can I validate that? When I try to log in, I just get:

    No ED25519 host key is known for $host and you have requested strict checking.

dcow• 7 days ago

Why does the page only show up to 008? Did TS remove the notice?

Edit: my bad, it's between 008 and 007 ?=

TS-2026-009: Insecure argument handling in Tailscale SSH permitted root access

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