# CVE-2026-89631: smb: client: reject a tree connect response whose byte count is too small

> A malicious SMB server can trigger an out-of-bounds read in the Linux kernel SMB client, leaking kernel memory to userspace via debug data.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-100496, BELL-CVE-2026-89631, DEBIAN-CVE-2026-89631, GHSA-95qf-fgcq-pcrc, UBUNTU-CVE-2026-89631

## What is CVE-2026-89631?

CVE-2026-89631 is a critical vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

smb: client: reject a tree connect response whose byte count is too small

CIFSTCon() bounds its strnlen() over the byte area with the server's
ByteCount minus two, which for ByteCount 0 or 1 goes negative as an int
and converts to a huge size_t.  The later subtraction wraps the __u16
bytes_left, and that is what bounds cifs_strndup_from_utf16(): a bound of
up to 65535 against a ~16 KB cifs_req_poolp object runs off the end of the
slab object, and the bytes reach userspace through tcon->nativeFileSystem
in /proc/fs/cifs/DebugData.

Reject a byte area too small for what the parser consumes.  Two bytes is
the least it can consume, and no conformant response carries fewer.  The
new trace point is the 129th smb_eio_trace entry, which __mode(byte)
cannot represent, so the attribute goes with it.

## Which versions are affected by CVE-2026-89631?


## How do I fix CVE-2026-89631?

Apply the vendor's patch.

## Is CVE-2026-89631 being exploited?

There is no known exploitation of CVE-2026-89631 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89631 and fix it if it is affected.

What it is: CVE-2026-89631 is a critical vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89631 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-2026-89631/ (Markdown: https://insights.sbomify.com/CVE-2026-89631/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89631
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89631
- OSV: https://osv.dev/vulnerability/CVE-2026-89631

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89631/
Summaries are generated and may contain mistakes; the official description above is the record.