# CVE-2026-78409: Util-linux: util-linux: x-mount.subdir detached-tree resolution can escape via intermediate symlinks

> Local attackers can escape mount restrictions via symlink traversal in util-linux X-mount.subdir, potentially accessing host files.

- **Severity:** High 7.0 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** ALPINE-CVE-2026-78409, BELL-CVE-2026-78409, CVE-2026-104887, DEBIAN-CVE-2026-78409, ROOT-OS-DEBIAN-12-CVE-2026-78409, ROOT-OS-DEBIAN-13-CVE-2026-78409, UBUNTU-CVE-2026-78409

## What is CVE-2026-78409?

CVE-2026-78409 is a high-severity path traversal vulnerability in Red Hat Enterprise Linux 10, published on 2 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

Weakness: CWE-59 (Improper Link Resolution Before File Access)

## Which versions are affected by CVE-2026-78409?


## How do I fix CVE-2026-78409?

Apply the vendor's patch.

## Is CVE-2026-78409 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-78409 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-78409 and fix it if it is affected.

What it is: CVE-2026-78409 is a high-severity path traversal vulnerability in Red Hat Enterprise Linux 10, published on 2 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-78409 exists.

Affected product: Red Hat Enterprise Linux 10. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-78409/ (Markdown: https://insights.sbomify.com/CVE-2026-78409/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-78409
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-78409
- OSV: https://osv.dev/vulnerability/CVE-2026-78409

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-78409/
Summaries are generated and may contain mistakes; the official description above is the record.