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.

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 gfileresolverelativepath() and validated to stay inside the source directory using two checks - gfilegetrelativepath() which does not resolve symlinks and gfilequeryfiletype() with GFILEQUERYINFONOFOLLOWSYMLINKS 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.