Path Traversal in Flatpak Flatpak-Builder
CVE-2026-39977
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.
Vulnerability class: Path Traversal (Directory Traversal)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.3 (Medium). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N.
EPSS exploit prediction
EPSS: 0.004 (29.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 29.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001
Affected products
- Flatpak Flatpak-Builder — versions >= 1.4.5, < 1.4.8
Weakness classification (CWE)
References
- github.com/flatpak/flatpak-builder/security/advisories/GHSA-6gm9-3g7m-3965 (Exploit, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-39977?
- CVE-2026-39977 is a medium-severity vulnerability in Flatpak Flatpak-Builder, classified under Path Traversal. CVSS score: 6.3/10. Published 2026-04-09.
- How severe is CVE-2026-39977?
- Medium severity. CVSS v3 base score is 6.3 out of 10.