# CVE-2026-89641: cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()

> A use-after-free in the Linux kernel CIFS module allows attackers to cause denial of service or execute code via crafted file size operations.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/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:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89641, DEBIAN-CVE-2026-89641, GHSA-f4f3-66v7-cmrj, UBUNTU-CVE-2026-89641

## What is CVE-2026-89641?

CVE-2026-89641 is a high-severity use after free vulnerability in Linux, published on 11 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:

cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()

When the else branch of cifs_file_set_size() finds a writable file handle
via find_writable_file(), it borrows tcon and server from the handle's
tlink, attempts the handle-based set_file_size() RPC, and then releases
the handle with cifsFileInfo_put().

If set_file_size() fails, execution falls through to the path-based
fallback, which reuses the borrowed tcon and server under the
"if (tcon == NULL)" guard.  Since tcon is not NULL at that point, the
guard is skipped.  If cifsFileInfo_put() dropped the last reference on a
tlink that was already removed from the tlink tree (TCON_LINK_IN_TREE
cleared, as happens during reconnection or session teardown),
cifs_put_tlink() will have freed tcon; the subsequent set_path_size()
call is then a use-after-free.

Setting tcon = NULL after cifsFileInfo_put() causes the existing guard
to take the cifs_sb_tlink() path, which acquires a fresh reference for
the path-based operation or fails cleanly if the session is gone.

## Which versions are affected by CVE-2026-89641?


## How do I fix CVE-2026-89641?

Apply the vendor's patch.

## Is CVE-2026-89641 being exploited?

There is no known exploitation of CVE-2026-89641 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89641 and fix it if it is affected.

What it is: CVE-2026-89641 is a high-severity use after free vulnerability in Linux, published on 11 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-2026-89641 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-2026-89641/ (Markdown: https://insights.sbomify.com/CVE-2026-89641/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89641
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89641
- OSV: https://osv.dev/vulnerability/CVE-2026-89641

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89641/
Summaries are generated and may contain mistakes; the official description above is the record.