---
id: CVE-2026-78409
title: >-
  Util-linux: util-linux: x-mount.subdir detached-tree resolution can escape via
  intermediate symlinks
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 in…
severity: high
cvss: 7
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
cwe:
  - CWE-59
vendor: Red Hat
product: util-linux-main
affected:
  - util-linux-main (all versions)
  - util-linux
  - util-linux
  - util-linux
  - rhel8/flatpak-runtime
  - rhel8/flatpak-sdk
  - util-linux
  - openshift/ose-rhel-coreos-8
  - openshift/ose-rhel-coreos-9
ssvc:
  exploitation: poc
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-09-02T15:29:06.011028Z'
exploitAvailable: true
published: '2026-09-02'
updated: '2026-10-21'
sourceUpdated: '2026-10-21T17:51:49.053Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-78409'
references:
  - url: 'https://access.redhat.com/errata/RHSA-2026:63162'
    label: 'RHSA-2026:63162'
  - url: 'https://access.redhat.com/security/cve/CVE-2026-78409'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2522607'
    label: RHBZ#2522607
  - url: >-
      https://github.com/util-linux/util-linux/security/advisories/GHSA-8f2p-47x3-43mv
tags:
  - cve.org
  - exploit-available
epss: 0.00154
epssPercentile: 0.0417
ingestedAt: '2026-10-21T18:09:27.273Z'
---

## Overview

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

- `util-linux-main (all versions)`
- `util-linux`
- `util-linux`
- `util-linux`
- `rhel8/flatpak-runtime`
- `rhel8/flatpak-sdk`
- `util-linux`
- `openshift/ose-rhel-coreos-8`
- `openshift/ose-rhel-coreos-9`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

### 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.