CVE-2026-78409 Util-linux: util-linux: x-mount.subdir detached-tree resolution can escape via intermediate symlinks
Local attackers can escape mount restrictions via symlink traversal in util-linux X-mount.subdir, potentially accessing host files.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-78409?
CVE-2026-78409 is a high-severity path traversal vulnerability in Red Hat Enterprise Linux 10, published on 2 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
The X-mount.subdir option uses a detached-tree fast path on Linux 6.15 and later and passes the configured subdirectory to open_tree() with AT_SYMLINK_NOFOLLOW. That flag does not stop intermediate symlink traversal or keep resolution inside the newly mounted filesystem. A local unprivileged user with an fstab-authorized X-mount.subdir entry can attach a host path at the intended mountpoint.
Weakness: CWE-59
Which versions are affected by CVE-2026-78409?
How do I fix CVE-2026-78409?
Apply the vendor's patch.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-78409 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-78409 exists.
Sources
Also known as ALPINE-CVE-2026-78409, BELL-CVE-2026-78409, CVE-2026-104887, DEBIAN-CVE-2026-78409, ROOT-OS-DEBIAN-12-CVE-2026-78409, ROOT-OS-DEBIAN-13-CVE-2026-78409 and 1 more.