Hydra is a Continuous Integration service for Nix based projects. Attackers can execute arbitrary code in the browser context of Hydra and execute authenticated HTTP requests. The abused feature allows Nix builds to specify files that Hydra serves to clients. One use of this functionality is serving NixOS `.iso` files. The issue is only with html files served by Hydra. The issue has been patched on https://hydra.nixos.org around 2024-04-21 14:30 UTC. The nixpkgs package were fixed in unstable and 23.11. Users with custom Hydra packages can apply the fix commit to their local installations. The vulnerability is only triggered when opening HTML build artifacts, so not opening them until the vulnerability is fixed works around the issue.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.