A bug in the revokefs writer used by the flatpak-system-helper allows a malicious local user to craft symlinks that traverse from one revokefs session into another. Because the writer only rejects literal '..' components and does not guard against symlink traversal, the attacker can obtain file descriptors to files in a different session after those files have been imported and validated. This capability lets the attacker tamper with ostree commit objects that have already passed signature verification, effectively granting root‑controlled writes to arbitrary paths on the system. The weakness is a classic directory traversal flaw (CWE‑61).

The vulnerability exists in Flatpak releases before version 1.18.1. Flatpak is a package management system distributed via the Flatpak:Flatpak vendor. For distributions that keep long‑term support, the backport containing the same fixes is available in the flatpak-1.16.x branch, so those earlier releases are also at risk until the backported updates are installed.

The CVSS score of 7.4 indicates a high severity. Attack requires a local user to be in an active session that can create two revokefs sessions via the unprivileged helper; it does not rely on remote network exposure. EPSS is not available, and the vulnerability is not yet listed in CISA's KEV catalog. Although the exploit path is somewhat complex and requires specific privileges, the high CVSS makes it a priority for patching, and the lack of a widely known exploit suggests the risk is moderate until a public exploit appears.