CVE-2026-39977: Flatpak Flatpak-Builder

Medium severity, CVSS 6.3. EPSS: 0.4% chance of exploitation in the next 30 days.

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.

Affected products

• Flatpak Flatpak-Builder: from 1.4.5, before 1.4.8 (fixed in 1.4.8)

Published 2026-04-09. Last modified 2026-06-17.