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HIGH

CVE-2026-78409

Util-linux: util-linux: x-mount.subdir detached-tree resolution can escape via intermediate symlinks

CVSS v3

7

HIGH

EPSS Score

0.2 %

exploit probability, as of 2026-09-28

CISA KEV

No

known exploited

Exploitation

poc

SSVC status

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.

Technical details

Published
2026-09-02
Last Modified
2026-09-03

Frequently asked questions

What is 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.

Is CVE-2026-78409 actively exploited?

A proof-of-concept exploit exists for CVE-2026-78409, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-78409?

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

Is CVE-2026-78409 affecting your environment?

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