# CVE-2026-89839: f2fs: use the mount idmap for the owner check in f2fs_xattr_advise_set()

> Local attackers can bypass owner checks on idmapped mounts to modify extended attributes, potentially leading to privilege escalation or unauthorized access.

- **Severity:** Not scored
- **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:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101223, BELL-CVE-2026-89839, DEBIAN-CVE-2026-89839, GHSA-hf9x-gfgm-hfj2, UBUNTU-CVE-2026-89839

## What is CVE-2026-89839?

CVE-2026-89839 is a missing authorization vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

f2fs: use the mount idmap for the owner check in f2fs_xattr_advise_set()

f2fs_xattr_advise_set() calls inode_owner_or_capable() with &nop_mnt_idmap
before allowing the "system.advise" xattr to be set, instead of the idmap
that the VFS passes to the ->set() handler.

f2fs supports idmapped mounts, so on such a mount this checks the caller's
fsuid against the unmapped on-disk owner rather than the mapped owner: the
actual owner can be wrongly denied with -EPERM and an unrelated caller
wrongly allowed.  Pass the handler's idmap instead.

## Which versions are affected by CVE-2026-89839?


## How do I fix CVE-2026-89839?

Apply the vendor's patch.

## Is CVE-2026-89839 being exploited?

There is no known exploitation of CVE-2026-89839 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89839 and fix it if it is affected.

What it is: CVE-2026-89839 is a missing authorization vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89839 in the wild and no public exploit code.

Affected product: Linux. 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-2026-89839/ (Markdown: https://insights.sbomify.com/CVE-2026-89839/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89839
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89839
- OSV: https://osv.dev/vulnerability/CVE-2026-89839

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89839/
Summaries are generated and may contain mistakes; the official description above is the record.