CVE-2025-54800 is a high-severity cross-site scripting (XSS) vulnerability affecting the Hydra continuous integration (CI) service used in NixOS projects. Hydra automates builds and testing for Nix-based software projects. The vulnerability arises from improper neutralization of input during web page generation, specifically allowing malicious JavaScript code to be injected into the Hydra database via a crafted package. This malicious code is then automatically executed in the browser of any user who visits the affected build pages, including pages displaying hydra-release-name information. The root cause is that untrusted package metadata or build output is not properly sanitized before being embedded in the web interface, enabling arbitrary script execution. Exploitation requires no privileges or authentication but does require user interaction (visiting the build page). The vulnerability was patched in commit dea1e16, which presumably introduced proper input sanitization or output encoding to prevent script injection. Until patched, the workaround is to avoid building untrusted packages or accessing the builds page. The CVSS v4.0 score is 7.1 (high), reflecting network attack vector, low attack complexity, no privileges or authentication required, user interaction needed, and high impact on integrity and limited impact on confidentiality. No known exploits are currently reported in the wild. This vulnerability falls under CWE-79, a common and dangerous web security flaw that can lead to session hijacking, defacement, or redirection to malicious sites.

For European organizations using NixOS and Hydra for continuous integration, this vulnerability poses a significant risk. Exploitation could allow attackers to execute arbitrary JavaScript in the browsers of developers, CI operators, or other stakeholders viewing build pages. This could lead to theft of session tokens, unauthorized actions within the CI system, or delivery of further malware payloads. The integrity of the CI environment is critical for software supply chain security; a compromised CI system can lead to injection of malicious code into production software. European organizations relying on open-source Nix-based projects or contributing to them may be targeted, especially if they build third-party or community packages without strict vetting. The vulnerability could also be leveraged in targeted attacks against organizations with sensitive or critical software development pipelines. While no active exploits are known, the ease of exploitation and the public availability of the patch increase the urgency to remediate. The impact on availability is minimal, but the integrity and confidentiality of developer sessions and build data are at risk.

1. Immediately upgrade Hydra to a version including the patch from commit dea1e16 or later to ensure proper input sanitization and output encoding. 2. Until patching is possible, avoid building untrusted or third-party packages that could contain malicious scripts. 3. Restrict access to the Hydra web interface to trusted users only, ideally via VPN or network segmentation, to reduce exposure. 4. Implement Content Security Policy (CSP) headers on the Hydra web interface to limit the execution of unauthorized scripts. 5. Monitor build logs and database entries for suspicious or unexpected script content. 6. Educate developers and CI users about the risk of visiting untrusted build pages and encourage use of updated browsers with XSS protections. 7. Review and harden CI pipeline security policies to include package vetting and code review before builds. 8. Consider deploying web application firewalls (WAFs) that can detect and block XSS payloads targeting Hydra interfaces.