# CVE-2026-89637: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

> A use-after-free vulnerability in the Linux kernel SMB client allows attackers to cause memory corruption or crashes via malformed secondary T2 responses.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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:** 19 Oct 2026
- **Also known as:** AZL-100694, BELL-CVE-2026-89637, DEBIAN-CVE-2026-89637, GHSA-q27p-5553-xhrr, UBUNTU-CVE-2026-89637

## What is CVE-2026-89637?

CVE-2026-89637 is a critical use after free 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: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

When a valid primary TRANSACT2 response has been received (mid->resp_buf
set, mid->multiRsp true) and a subsequent secondary response causes
cifs_check_trans2() to return false -- either because the SMB header is
invalid (malformed != 0) or because check2ndT2() rejects the PDU --
handle_mid() overwrites mid->resp_buf with the new buffer (leaking the
primary buffer) and, because mid->multiRsp is set, skips the
server->smallbuf/bigbuf NULL-out.  When the user thread frees
mid->resp_buf, server->smallbuf or server->bigbuf is left dangling; the
demux thread reuses it for the next packet, resulting in a use-after-free.

Combine both early-exit conditions and, when mid->multiRsp is already
set, abort the pending transaction inline: set multiEnd, call
dequeue_mid() with malformed=true, and return true so handle_mid() exits
without touching mid->resp_buf or the server buffer pointers.

## Which versions are affected by CVE-2026-89637?


## How do I fix CVE-2026-89637?

Apply the vendor's patch.

## Is CVE-2026-89637 being exploited?

There is no known exploitation of CVE-2026-89637 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89637 and fix it if it is affected.

What it is: CVE-2026-89637 is a critical use after free 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-2026-89637 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-2026-89637/ (Markdown: https://insights.sbomify.com/CVE-2026-89637/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89637
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89637
- OSV: https://osv.dev/vulnerability/CVE-2026-89637

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89637/
Summaries are generated and may contain mistakes; the official description above is the record.