Ghost before 5.42.1 allows remote attackers to read arbitrary files within the active theme's folder via /assets/built%2F..%2F..%2F/ directory traversal. This occurs in frontend/web/middleware/static-theme.js.
https://github.com/TryGhost/Ghost/compare/v5.42.0...v5.42.1
https://github.com/TryGhost/Ghost/commit/378dd913aa8d0fd0da29b0ffced8884579598b0f