Nix is a package manager for Linux and other Unix systems. Prior to 2.35.0, a malicious derivation executed with the recursive-nix experimental feature can exploit a time-of-check/time-of-use race involving final symlink handling in the LocalStore restore path. The race can cause writeFile to follow a substituted final symlink when opening a path with OTRUNC instead of enforcing FinalSymlink::DontFollow, allowing the Nix process or nix-daemon to create or truncate an empty file outside the build sandbox with the daemon user's permissions. The primitive does not provide arbitrary-content writes and requires winning the race. This issue is fixed in version 2.35.0.

An issue was discovered in Nix before 2.34.7. Writing to arbitrary files can occur via "nix-prefetch-url --unpack" or "nix store prefetch-file --unpack" directory traversal. The fixed versions are 2.34.7, 2.33.6, 2.32.8, 2.31.5, 2.30.5, 2.29.4, and 2.28.7 (introduced in 2.24.7);

An issue was discovered in Nix before 2.34.7 and Lix before 2.95.2. Unbounded recursion in the NAR (Nix Archive) parser could lead to a stack-to-heap overflow when the parser is run on a coroutine stack. The stack is allocated without a guard page, which means that a stack overflow could overwrite memory on the heap and could allow arbitrary code execution as the Nix daemon (run as root in multi-user installations) if ASLR hardening is bypassed. This can be exploited by all users able to connect to the daemon (e.g., in Nix, this is configurable via the allowed-users setting, defaulting to all users). The fixed versions are 2.34.7, 2.33.6, 2.32.8, 2.31.5, 2.30.5, 2.29.4, and 2.28.7 for Nix (introduced in 2.24.4); and 2.95.2, 2.94.2, and 2.93.4 for Lix (introduced in 2.93.0).

Summary

Nix and Lix daemon implementations are affected by buffer overflows vulnerabilities that allow a local attacker to gain arbitrary code execution as the daemon user (root in multi-user installations).

The vulnerabilities are identified as: - Nix: GHSA-vh5x-56v6-4368, CVE ID pending attribution. - Lix: CVE ID pending attribution.

This is a coordinated disclosure between the Nix and Lix projects.

Guix is NOT affected by this vulnerability.

Am I affected?

To exploit this issue, a local attacker needs access to talk to the Nix daemon. All systems that allow connections to their daemons are affected. Only users that are allowed to connect to the daemon (via allowed-users and trusted-users) can reliably trigger the issue. Substituters can in theory trigger the issue but cannot make enough attempts to mount attacks in practice.

Additionally, this vulnerability requires ASLR weakening techniques to lead to a compromise.

Fixes

The vulnerabilities are fixed in the following versions:

- Nix: - Affected versions: ≥ 2.24.4 - Fixed versions: 2.34.7, 2.33.6, 2.32.8, 2.31.5, 2.30.5, 2.29.4, 2.28.7

Nix security release also includes patches that address an unrelated path traversal vulnerability GHSA-gr92-w2r5-qw5p (CVE ID pending attribution).

- Lix: - Affected versions: ≥ 2.93.0 - Fixed versions: 2.93.4, 2.94.2, 2.95.2

Acknowledgement

- We would like to thank @edef with the help of Sander (@sandydoo) for reporting the issues and working with the development teams to suggest and confirm the fixes. - Thanks to eldritch horrors (@pennae) and Raito Bezarius (@RaitoBezarius) on the Lix side for the mitigation. - Thanks to @xokdvium on the Nix side for the mitigation. - Thanks to @hexa and @tgerbet on the NixOS security team for coordinating this.

References

https://discourse.nixos.org/t/security-advisory-local-privilege-escalation-in-lix-and-nix/77407 Nix issues: - https://github.com/NixOS/nix/security/advisories/GHSA-vh5x-56v6-4368 - https://github.com/NixOS/nix/security/advisories/GHSA-gr92-w2r5-qw5p Lix in-depth review blog post: not yet published

Nix through 2.22.1 mishandles certain usage of hash caches, which makes it easier for attackers to replace current source code with attacker-controlled source code by luring a maintainer into accepting a malicious pull request.