GHSA-9rww-v4mm-x4jg (https://github.com/flatpak/flatpak/security/advisories/GHSA-9rww-v4mm-x4jg)

Description: When extracting OCI layer archives, Flatpak rebases archive entry pathnames to the destination directory using g_build_filename, and sets ARCHIVE_EXTRACT_SECURE_NODOTDOT to reject .. components in both pathnames and hardlink targets. However, hardlink targets were not rebased to the destination directory. A crafted archive entry with an absolute hardlink target (e.g. /etc/shadow) causes libarchive to call link() with that path directly, hardlinking the host file into the extraction directory and making its contents readable. An attacker controlling an OCI registry can serve a crafted layer archive that exploits this during flatpak install or flatpak update.

Mitigation: Only install applications from trusted OCI registries. Flatpak remotes using the default OSTree transport are not affected. Versions 1.16.x and older are not believed to be vulnerable (introduced in 1.17.0). Fixed in 1.18.1. Reported by @swick.