Executive brief
Nix is a package manager for Linux and Unix systems that manages software dependencies and builds. Prior to version 2.35.0, when using the recursive-nix experimental feature, a malicious software package could exploit a race condition in how Nix handles symbolic links during file operations. This could allow an attacker to create or truncate files outside the sandbox with the permissions of the Nix daemon user, potentially affecting system integrity on machines using this feature.
Technical details
The vulnerability is a time-of-check/time-of-use (TOCTOU) race condition in the LocalStore restore path involving final symlink handling. When writeFile opens a path with O_TRUNC, it fails to enforce FinalSymlink::DontFollow, allowing a symbolic link substitution attack. The attack requires a malicious derivation running under the recursive-nix experimental feature and winning a filesystem race condition. Successful exploitation allows creation or truncation of an empty file outside the build sandbox with daemon user permissions, though arbitrary content writes are not possible. The issue is fixed in Nix 2.35.0 by exposing FinalSymlink in writeFile arguments and using O_NOFOLLOW guards in the restore path.
Affected products
- NixOS Nix prior to 2.35.0
Timeline
- 2026-08-20: disclosed
- 2026: patched: Fixed in version 2.35.0