# CVE-2026-39977: flatpak-builder: flatpak-builder is a tool to build flatpaks from source

CVE-2026-39977 is fixed in 2 products on 1 platform, first by flatpak-builder 1.4.8-1, released April 9, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-39977
- Priority: Routine
- CVSS: 6.3 (medium)
- EPSS: 0.0030
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 9, 2026

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | flatpak-builder Debian 14 | 1.4.8-1 | [DEBIAN-CVE-2026-39977](https://patchblox.com/security/data/fix/linux/debian-cve-2026-39977.md) | Apr 9, 2026 |
| Linux | flatpak-builder Ubuntu 26.04.1 LTS (resolute) | 1.4.8-1 | [flatpak-builder 1.4.8-1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-flatpak-builder-1.4.8-1.md) | Apr 9, 2026 |
| Linux | flatpak-builder-tests Ubuntu 26.04.1 LTS (resolute) | 1.4.8-1 | [flatpak-builder-tests 1.4.8-1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-flatpak-builder-tests-1.4.8-1.md) | Apr 9, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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