# CVE-2026-89639: cifs: use cifs_invalidate_cache() in cifs_do_truncate() for O_TRUNC

> A race condition in the Linux kernel CIFS module allows serving stale cached data during file truncation, potentially leading to information disclosure.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N`)
- **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-2026-89639, DEBIAN-CVE-2026-89639, GHSA-m6p4-6vjr-qq2h, UBUNTU-CVE-2026-89639

## What is CVE-2026-89639?

CVE-2026-89639 is a high-severity 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: use cifs_invalidate_cache() in cifs_do_truncate() for O_TRUNC

cifs_do_truncate() is invoked from cifs_open() without i_rwsem, so it
cannot use cifs_resize_file_locked() to perform a proper fscache cookie
resize.  Instead, add cifs_invalidate_cache() after cifs_setsize().

cifs_invalidate_cache() calls fscache_invalidate(), which works without
holding i_rwsem: it unconditionally increments inval_counter and sets
FSCACHE_COOKIE_NO_DATA_TO_READ, ensuring that stale cached data is not
served once the cookie is later activated by fscache_use_cookie().
Truncation to zero leaves no valid cached data, making invalidation the
correct semantic here.

## Which versions are affected by CVE-2026-89639?


## How do I fix CVE-2026-89639?

Apply the vendor's patch.

## Is CVE-2026-89639 being exploited?

There is no known exploitation of CVE-2026-89639 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89639 and fix it if it is affected.

What it is: CVE-2026-89639 is a high-severity 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-2026-89639 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-2026-89639/ (Markdown: https://insights.sbomify.com/CVE-2026-89639/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89639
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89639
- OSV: https://osv.dev/vulnerability/CVE-2026-89639

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89639/
Summaries are generated and may contain mistakes; the official description above is the record.