flatpak-builder can access files outside the build directory.

Vulnerability Description

Flatpak is a Linux application sandboxing and distribution framework. A path traversal vulnerability affects versions of Flatpak prior to 1.12.3 and 1.10.6. flatpak-builder applies `finish-args` last in the build. At this point the build directory will have the full access that is specified in the manifest, so running `flatpak build` against it will gain those permissions. Normally this will not be done, so this is not problem. However, if `--mirror-screenshots-url` is specified, then flatpak-builder will launch `flatpak build --nofilesystem=host appstream-utils mirror-screenshots` after finalization, which can lead to issues even with the `--nofilesystem=host` protection. In normal use, the only issue is that these empty directories can be created wherever the user has write permissions. However, a malicious application could replace the `appstream-util` binary and potentially do something more hostile. This has been resolved in Flatpak 1.12.3 and 1.10.6 by changing the behaviour of `--nofilesystem=home` and `--nofilesystem=host`.

References

• https://github.com/flatpak/flatpak/security/advisories/GHSA-8ch7-5j3h-g4fx
• https://github.com/flatpak/flatpak/commit/445bddeee657fdc8d2a0a1f0de12975400d4fc1a
• https://github.com/flatpak/flatpak/commit/4d11f77aa7fd3e64cfa80af89d92567ab9e8e6fa
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/APFTBYGJJVJPFVHRXUW5PII5XOAFI4KH/
• https://www.debian.org/security/2022/dsa-5049
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/IXKBERLJRYV7KXKGXOLI6IOXVBQNN4DP/
• https://security.gentoo.org/glsa/202312-12