# CVE-2026-78409: 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.

- Web page: https://patchblox.com/security/cve/CVE-2026-78409
- Priority: High
- CVSS: 7.0 (high)
- EPSS: 0.0019
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 2, 2026

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | util-linux Debian 14 | 2.42.3-1 | [DEBIAN-CVE-2026-78409](https://patchblox.com/security/data/fix/linux/debian-cve-2026-78409.md) | Sep 17, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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