Vulnerability · Published Apr 9, 2026 · Assigned by GitHub_M
CVE-2026-39977flatpak-builder has a path traversal leading to arbitrary file read on host when installing licence files
High 7.1
In plain English
A high-severity path traversal in flatpak-builder from flatpak. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 23% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:14 | flatpak-builder | 1.4.8-1 | DEBIAN-CVE-2026-39977 |
| Ubuntu:25.10 | flatpak-builder | no fix listed | UBUNTU-CVE-2026-39977 |
| Ubuntu:26.04:LTS | flatpak-builder | 1.4.8-1 | UBUNTU-CVE-2026-39977 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| flatpak | flatpak-builder | >= 1.4.5, < 1.4.8 |
Weakness: CWE-22 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N