CVE-2026-78409

Name	CVE-2026-78409
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.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
util-linux (PTS)	bookworm	2.38.1-5+deb12u3	vulnerable
	bookworm (security)	2.38.1-5+deb12u1	vulnerable
	trixie (security), trixie	2.41.5-0+deb13u1	vulnerable
	forky	2.42.2-4	vulnerable
	sid	2.42.3-1	fixed

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
util-linux	source	(unstable)	2.42.3-1

[trixie] - util-linux <no-dsa> (Minor issue)
https://github.com/util-linux/util-linux/security/advisories/GHSA-8f2p-47x3-43mv