Security advisory: OpenSSH CVE-2024-6387 “regreSSHion” – update your servers ASAP

Announcements Security
8

That’s so useful! It should have a separate announcement post :smiley:

9

Am I the only one who thinks that such a delay in non-small channels is unacceptable? It doesn’t prevent things from breaking anyway

10

You can use the -small channel if it works for you; you’ll just have to build whatever you use that isn’t already built, and deal with lower test coverage (e.g. Fix SSH in initrd by r-vdp · Pull Request #323796 · NixOS/nixpkgs · GitHub wasn’t caught by the -small channel, which could be pretty bad if you rely on it). Building the many thousands of additional packages that the full channel contains takes time.

I do wish the channel bump cycle was quicker, but it’s a hard problem. I think using the -small channel and including whatever additional tests are relevant to your system in its build is best for servers; it took less than 4 hours from the surprise release announcement to a fix being in nixos-unstable-small and nixos-24.05-small, which I think is pretty good.

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11

In addition, you can totally use multiple channels and just use a non-small channel for whatever packages you depend on that take longer to build and you don’t want to build downstream for some reason. There can still be CVEs that affect lower level libraries where this doesn’t help, but someone ultimately has to run those builds, and upstream can only prioritize so much.

Even without non-small channels, for your typical headless server, the number of packages that need to be built downstream in addition to what is in the *-small channels should be quite doable even on a small build host.

If nothing else, deliberately keeping that number small is probably a good exercise in figuring out what your system closure contents actually are - I realized today that I depend on two different postgres versions, for example.

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12

Is it really already in nixos-24.05-small, and how can I test if the currently running version is patched?
When I use nixos-24.05-small and update the system, sshd still says:

# sshd -V
OpenSSH_9.7p1, OpenSSL 3.0.14 4 Jun 2024
1 Like
13

Yes, it is. We used a minimal patch from OpenSSH upstream on stable branches rather than upgrading to a version with potentially‐breaking changes, so it’s expected that the version number doesn’t change.

Maybe we should have tweaked the derivation name or something so it’s easy to tell if you have the fix. Other people probably have smarter ideas than I do for checking you have the right version; there’s nix derivation show at a pinch, and you can check the nixos-version --revision output against channels, etc.

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14

Thanks for your hard work. :heartpulse:

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15

Then just set your openssh package manually to the new one?

services.openssh.package = (builtins.getFlake "github:nixos/nixpkgs/7f993cdf26ccef564eabf31fdb40d140821e12bc?narHash=sha256-pY0wosAgcr9W4vmGML0T3BVhQiGuKoozCbs2t+Je1zc=").legacyPackages.x86_64-linux.openssh

You could even override the derivation yourself like pkgs.openssh.override { patches = [ thePatch.patch ]; }

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16

The fix is in stable (24.05) now \o/

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17

now-ish it reached nixos-unstable. nixos-24.05 was ~11 hours ago. All three -small channels were covered long ago, and the three “nixpkgs*” channels have also updated.

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18

Thank you! https://nixpk.gs/pr-tracker.html?pr=323753 seems to be dying.

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19

And we’re done. Phew! The fixed packages are now in all of the unstable, 24.05, and 23.11 channels. If you haven’t already updated, you should do so now (in fact, you might as well do it again anyway, just in case you’re not on the channel or revision you thought you were). You can and should remove any workarounds and mitigations when you update. If you’re on 23.11, please take the opportunity to upgrade to 24.05 as soon as possible; it was out of official support by the time all this happened, and the next security vulnerability probably won’t get a fix backported.

Thank you for all the kind words, but I didn’t make this happen alone – shout‐outs are also due to the others who helped get this done, including ari (no Discourse account I think), @qyliss, @vcunat, @hexa, @R-VdP, @ElvishJerricco, @leona, @tgerbet, @j-k, and of course the OpenSSH developers. (I’m probably forgetting people, too.)

Here’s a timeline for the curious:

  • T+0: OpenSSH 9.8p1 is announced with the first public disclosure of this bug.
  • T+25m: Upstream posts minimal patches for previous versions. The 9.8p1 release notes are posted in the security triage channel.
  • T+35m: #323753 is opened to bump unstable to 9.8p1.
  • T+50m: #323753 is merged.
  • T+56m: #323761 is opened to apply the minimal patches to 24.05.
  • T+1h2m: #323761 is merged.
  • T+1h10m: #323765 is opened to apply the minimal patches to 23.11.
  • T+1h19m: #323768 is opened to patch openssh_{hpn,gssapi} on unstable.
  • T+1h29m: #323765 is merged.
  • T+2h56m: #323768 is merged.
  • T+3h18m: The openssh bump reaches nixos-unstable-small and the first users start getting a fix through updates.
  • T+4h: The openssh patch reaches nixos-24.05-small. (It reaches nixos-23.11-small around the same time, but I don’t have the exact time.)
  • T+4h2m: This advisory is posted.
  • T+4h30m: Backports of #323768 to 24.05 and 23.11 are merged.
  • T+4h34m: #323796 is opened to fix SSH in initrd on unstable, which would have blocked the large channel bumps and potentially broken people’s setups. Stable versions are not affected due to the use of minimal patches rather than a full OpenSSH upgrade.
  • T+5h29m: #323796 is merged.
  • T+8h9m: The openssh bump reaches nixpkgs-unstable.
  • T+16h43m: The openssh and openssh_{hpn,gssapi} patches reach nixos-24.05. The first users on the large channels start to get fixes.
  • T+1d2h56m: The openssh bump, openssh_{hpn,gssapi} patches, and initrd SSH fix reach nixos-unstable.
  • T+1d6h23m: The openssh_{hpn,gssapi} patches reach nixos-23.11-small.
  • T+1d8h4m: The openssh and openssh_{hpn,gssapi} patches reach nixos-23.11.
  • T+1d11h25m: The openssh_{hpn,gssapi} patches and the initrd SSH fix reach nixpkgs-unstable. (Hopefully you’re not using nixpkgs-unstable (as opposed to nixos-unstable) on a NixOS machine anyway, though.)

(I’m not sure when the openssh_{hpn,gssapi} fixes reached nixos-{unstable,24.05}-small, or when the initrd SSH fix reached nixos-unstable-small, but this is probably already more exhaustive than anyone cares about.)

This is my first time doing this kind of thing; lessons learned:

  • Our ability to get security updates that don’t cause mass rebuilds into the small channels is pretty good! A four‐hour timespan from the issue being publicly announced to shipping binaries to users isn’t the fastest release a distro managed, but I believe we beat some distros that were in on the embargo. If you’re running a server and are worried about getting security fixes quickly, then using the small channel (and potentially making your system builds include the NixOS tests that aren’t part of the small channel but seem relevant to your server) is a good idea. Unfortunately this doesn’t do as much for fixes that have to go via staging.

  • Large channels are slow to update and it’s hard to fix this. It’s probably good to include information about how users on large channels can pull in security updates on the small ones. On the other hand, the large channel tests are useful; they uncovered 9.8p1 breaking SSH in initrd on unstable. Someone who was relying on that to be able to boot their remote server might have had a nasty surprise if they tried to get ahead of the channel bump.

  • It’s important for people to have an easy way to check whether their system is vulnerable. I don’t know whether this just means having appropriate Nix‐fu commands ready or whether there’s more that should be done as part of the fix PRs themselves to surface it better.

  • Monitoring the oss-security list and posting important stuff to the triage channel is probably a reasonably high‐impact practice. (Though it’s possible or even likely that people are already doing this; 25 minutes is a very quick turnaround from a mailing list message to it being raised on Matrix anyway.)

  • Hydra requires more active management than you’d think and the people who do it are thankless heroes.

  • Unix signal delivery semantics are a mistake that we continue to pay the price for decades later.

  • It’s good to eat breakfast when you can.

41 Likes
20

I fear that the 24.05 branch is still at 9.7p1

https://search.nixos.org/packages?channel=24.05&from=0&size=50&sort=relevance&type=packages&query=openssh

(unstable is indeed 9.8p1)

21

The patches were backported, so the version number didn’t change. See slightly further up: Security advisory: OpenSSH CVE-2024-6387 “regreSSHion” – update your servers ASAP - #13 by emily

Hence the:

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22

RIght, sorry, I only read the last status message.

24

Here’s some nix-fu for checking whether you have the backported patch:

nix derivation show $(which ssh) | jq -r '.[].inputSrcs[]' | grep CVE-2024
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25

I believe you need $(readlink -f $(which ssh)), otherwise it’ll be looking at the derivation that produced your /run/current-system/sw/bin, rather than the OpenSSH one. (So don’t panic if this seems to indicate you’re still vulnerable.)

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26

Can we determine if a nixos machine is patched with a tool like vulnix?

27

Yes, vulnix -R $(readlink -f $(which ssh)) will show you if there is an active advisory on it or not. Under the hood, it does something similar to what the nix derivation show/jq/grep proposed a few posts ago does.

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28

It’s worth noting that all these snippets rely on the contents of $PATH, which may or may not match up with whatever is running in the systemd service, especially if user channels or other heavier customization is involved. It’ll work most of the time, of course, but it might not, which is a bit scary. I can’t think of a simple, foolproof and generic check either, though.

Maybe we could make this easier upstream? What do other distros do? “Sadly” this CVE takes too long to exploit to write a simple distro-independent vulnerability checker for it.

3 Likes