CVE-2026-78409

Summary

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.

Affected Software

Weaknesses

• CWE-59: Improper Link Resolution Before File Access ('Link Following')

Workarounds

Upgrade to util-linux v2.41.6, v2.42.3, or later. Until a fix is applied, do not allow unprivileged /etc/fstab entries that specify X-mount.subdir.

ADP Enrichment

CISA ADP Vulnrichment

• SSVC:
• Exploitation: poc
  • Automatable: no
  • Technical Impact: total

Additional References

• https://github.com/util-linux/util-linux/security/advisories/GHSA-8f2p-47x3-43mv

References

• https://access.redhat.com/security/cve/CVE-2026-78409
• https://bugzilla.redhat.com/show_bug.cgi?id=2522607
• https://github.com/util-linux/util-linux/security/advisories/GHSA-8f2p-47x3-43mv

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