util-linux versions through 2.41.5 and 2.42.2 fail to check mount helper exit status before running post-mount hooks, allowing unprivileged users to execute privileged operations on pre-existing filesystems. Attackers can exploit X-mount.idmap or X-mount.owner hooks to clone filesystems with inherited suid bits or modify target inode permissions after a helper fails, achieving privilege escalation.

CVE-2026-76642 affects util-linux versions through 2.41.5 and 2.42.2, where the libmount component fails to verify the exit status of mount helpers before executing post-mount hooks. This flaw allows unprivileged users to trigger privileged operations on pre-existing filesystems. Attackers can exploit X-mount.idmap or X-mount.owner hooks to clone filesystems with inherited suid bits or manipulate target inode permissions following a helper failure. The vulnerability enables local privilege escalation on affected Linux systems. Patches have been committed to the util-linux repository and a GitHub Security Advisory (GHSA-m25x-3hj9-m26f) has been published. The affected code paths are identified in libmount/src/context_mount.c at lines 476 and 892. Three separate commits address the vulnerability. VulnCheck has also published an independent advisory detailing the exploitation mechanism.