CVE-2026-39977: flatpak flatpak-builder Path Traversal

Vulnerability summary

CVE-2026-39977 is a high (CVSS 7.1) Path Traversal vulnerability in flatpak flatpak-builder. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 30.6% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 09-04-2026
   PublishedVulnerability first disclosed.
2. 09-04-2026
   Last modifiedVulnerability information updated.

Description

flatpak-builder is a tool to build flatpaks from source. From 1.4.5 to before 1.4.8, the license-files manifest key takes an array of paths to user defined licence files relative to the source directory of the module. The paths from that array are resolved using g_file_resolve_relative_path() and validated to stay inside the source directory using two checks - g_file_get_relative_path() which does not resolve symlinks and g_file_query_file_type() with G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS which only applies to the final path component. The copy operation runs on host. This can be exploited by using a crafted manifest and/or source to read arbitrary files from the host and capture them into the build output. This vulnerability is fixed in 1.4.8.

• CWE-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.