Nix sandbox escape: file write via symlink at FOD `.tmp` copy destination

Description

Nix is a package manager for Linux and other Unix systems. A bug in the fix for CVE-2024-27297 allowed for arbitrary overwrites of files writable by the Nix process orchestrating the builds (typically the Nix daemon running as root in multi-user installations) by following symlinks during fixed-output derivation output registration. This affects sandboxed Linux builds - sandboxed macOS builds are unaffected. The location of the temporary output used for the output copy was located inside the build chroot. A symlink, pointing to an arbitrary location in the filesystem, could be created by the derivation builder at that path. During output registration, the Nix process (running in the host mount namespace) would follow that symlink and overwrite the destination with the derivation's output contents. In multi-user installations, this allows all users able to submit builds to the Nix daemon (allowed-users - defaulting to all users) to gain root privileges by modifying sensitive files. This vulnerability is fixed in 2.34.5, 2.33.4, 2.32.7, 2.31.4, 2.30.4, 2.29.3, and 2.28.6.

Scores

EPSS (exploitation in 30 days)

0.20% · top 91% of all CVEs (6 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: no · Technical impact: total

The list uses the assigner's score when there is one, then CISA's, then NVD's: Critical 9.0.

Weakness

• CWE-61: UNIX Symbolic Link (Symlink) Following (opens in a new tab)All about CWE-61 and its CVEs

Affected products

• NixOS · nix

  • affected >= 2.21, < 2.28.6
  • affected >= 2.29.0, < 2.29.3
  • affected >= 2.30.0, < 2.30.4
  • affected >= 2.31.0, < 2.31.4
  • affected >= 2.32.0, < 2.32.7
  • affected >= 2.33.0, < 2.33.4
  • affected >= 2.34.0, < 2.34.5
  • affected >= 2.20.5, <= 2.20.9
  • affected >= 2.19.4, <= 2.19.7
  • affected >= 2.18.2, <= 2.18.9

References (6)

• https://github.com/NixOS/nix/security/advisories/GHSA-g3g9-5vj6-r3gj (opens in a new tab)x_refsource_CONFIRM
• https://github.com/NixOS/nix/pull/10178 (opens in a new tab)x_refsource_MISC
• https://github.com/NixOS/nix/commit/244f3eee0bbc7f11e9b383a15ed7368e2c4becc9 (opens in a new tab)x_refsource_MISC
• https://github.com/NixOS/nix/commit/4bc5a3510fa3735798f9ed3a2a30a3ea7b32343a (opens in a new tab)x_refsource_MISC
• https://github.com/NixOS/nix/commit/7794354a982449927ee7401cdeb573ddd16c4688 (opens in a new tab)x_refsource_MISC
• https://github.com/NixOS/nix/commit/a3163b9eabb952b4aa96e376dea95ebcca97b31a (opens in a new tab)x_refsource_MISC

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-39860](https://trackcve.app/badge/CVE-2026-39860.svg)](https://trackcve.app/cve/CVE-2026-39860)

Record last updated 9 Apr 2026, 13:42 UTC.