A flaw in the Nix package manager’s sandboxed build process allows a builder to create a symlink that redirects output registration to an arbitrary writable file. The Nix daemon, which ordinarily runs as root in multi‑user installations, follows that link when moving the build result, thereby overwriting the target file. This results in arbitrary file modification and can elevate a non‑privileged user to full root control over the host system. The weakness maps to CWE‑61, the classic directory traversal and symlink injection issue that leads to untrusted file write.

The vulnerability affects Linux installations of the Nix package manager, specifically sandboxed builds performed by the Nix daemon. Versions earlier than 2.34.5 (and the corresponding patches 2.33.4, 2.32.7, 2.31.4, 2.30.4, 2.29.3, and 2.28.6) are susceptible. macOS sandboxed builds are not impacted.

The CVSS score is 9, indicating critical severity, and the exploitability score is not available. The vulnerability is not yet catalogued in CISA’s KEV list. An attacker only needs the ability to submit a derivation to the Nix daemon, which by default permits all users on a multi‑user system. This makes exploitation highly feasible and the potential impact total system compromise. Immediate remediation is strongly recommended.