Is CVE-2026-78409 being exploited?

No public exploit is currently tracked.

How severe is CVE-2026-78409?

CVE-2026-78409 has a CVSS v3 severity of HIGH (7).

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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.