util-linux: The X-mount.subdir option uses a detached-tree fast path on Linux 6.15 and later and passes the configured subdirecto...
CVE-2026-78409 is fixed in util-linux on 1 platform by DEBIAN-CVE-2026-78409 (version 2.42.3-1), released September 17, 2026. It is not reported as exploited. Patch Radar priority: High.
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.
Where it's fixed
| Product | Fixed in | Fix | Released |
|---|---|---|---|
| Linux | |||
| util-linux · Debian 14 | 2.42.3-1 | DEBIAN-CVE-2026-78409 | Sep 17, 2026 |
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