Security fix: Nix derivation sandbox escape

Announcements
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tl;dr: CVE-2024-38531 (see Sandbox escape · Advisory · NixOS/nix · GitHub if the CVE index doesn’t show it yet) was published earlier today. Please update your Nix installation.

Discovered by @lheckemann some time ago, and recently reported with a reproducer by @alois31, a security issue in Nix allowed allowed a build process to access and change the permissions of the build directory. After creating a setuid binary in a globally accessible location, a malicious local user could assume the permissions of a Nix daemon worker and hijack all future builds.

Concretely, this would happen when all the following conditions are met:

  • The attacker has access to a local user
  • The local user has access to the Nix daemon (but does not have to be a trusted user)
  • seccomp is disabled (if the sandbox is disabled or filter-syscalls if set to false on Linux) or ineffective (fixed in #10501)

It has been patched in the following Nix versions:

How to update

  • The fix will first land in nixos-unstable-small, and then nixpkgs-unstable.
  • Backports to nixos-23.11 And nixos-24.05 are on the way, and are likely to land first on nixos-23.11-small and nixos-24.05-small.
  • If you don’t, or if you can’t afford to wait, you can directly fetch the source from the patches above.
13 Likes
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No, it is not there yet. It wasn’t even in cache.nixos.org at the moment of your post.

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Yes, true, excuse the copypasta. I’m just a dog in a lab coat, never done this before.

14 Likes
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Because I wasn’t able to find the PR that updated nixpkgs with a quick search for 2.23.1, I’m linking it here after doing a deeper search.

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I have been using nosuid,nodev mounts for /nix and /tmp, doesn’t that also prevent this?

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No, it doesn’t. But running an up-to-date Nix does.