Hydra is a Continuous Integration service for Nix based projects. Evaluation of untrusted non-flake nix code could potentially access secrets that are accessible by the hydra user/group. This should not affect the signing keys, that are owned by the hydra-queue-runner and hydra-www users respectively.
https://github.com/nix-community/nix-eval-jobs/releases/tag/v2.28.1
https://github.com/NixOS/nixpkgs/pull/397919
https://github.com/NixOS/hydra/security/advisories/GHSA-j7w7-965w-vjxw
https://github.com/NixOS/hydra/commit/8d750265135b7e203520036a742afdf301b4013f