CVE-2026-78409

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.

Affected versions: util-linux v2.42 through v2.42.2. The Linux >= 6.15 detached-tree X-mount.subdir fast path was introduced in ae19f7546ccb (2025-04-15) and first released in v2.42. Earlier releases, including v2.40 and v2.41, are not affected. Restricted-user SUID mount(8) reproduction also requires Linux >= 6.15. Fixed in v2.41.6 and v2.42.3.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Upgrade to util-linux v2.41.6, v2.42.3, or later. Until a fix is applied, do not allow unprivileged /etc/fstab entries that specify X-mount.subdir.