CVE-2026-96808

Flatpak before 1.18.1 contains a symlink-traversal flaw (CWE-61) in the revokefs writer used by flatpak-system-helper to receive repository data from unprivileged callers: it rejected literal '..' path components but did not block traversal through symlinks. A malicious local user with an active session could open two revokefs sessions via the system helper, place a symlink in one pointing into the other's directory, and retain a file descriptor to modify files after they had been validated and imported. This enables tampering with ostree commit objects in the system repository after they passed signature verification, yielding root-controlled file writes to attacker-chosen paths and full local root privilege escalation (CVSS 3.1: 7.4, high; attack complexity is high and an active local session is required). Anyone running a vulnerable Flatpak build that uses the revokefs-backed system helper — common in mainstream Linux desktop distributions — is affected. No public proof of concept is known and there is no indication of exploitation in the wild.

What to do: Upgrade Flatpak to 1.18.1 or later (or apply your distribution's patched security package) and verify with 'flatpak --version'. Restrict interactive local accounts on multi-user desktops and kiosk-style shared systems, since exploitation requires an active local session. Watch for anomalous writes to /var/lib/flatpak ostree objects or unexpected setuid binaries owned by root following any local session activity on unpatched hosts.