# CVE-2026-89633: smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2()

> A missing bounds check in the Linux kernel SMB client allows remote attackers to trigger out-of-bounds memory reads and writes, potentially leading to code execution or system crashes.

- **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:** 21 Oct 2026
- **Also known as:** AZL-100592, BELL-CVE-2026-89633, DEBIAN-CVE-2026-89633, GHSA-6jcv-m8xc-577h, UBUNTU-CVE-2026-89633

## What is CVE-2026-89633?

CVE-2026-89633 is a critical memory corruption 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 OOB read/write from unvalidated DataOffset in coalesce_t2()

coalesce_t2() computes data pointers directly from server-supplied
DataOffset fields with no validation against buffer bounds:

  data_area_of_tgt = (char *)&pSMBt->hdr.Protocol +
                     get_unaligned_le16(&pSMBt->t2_rsp.DataOffset);
  data_area_of_src = (char *)&pSMBs->hdr.Protocol +
                     get_unaligned_le16(&pSMBs->t2_rsp.DataOffset);
  data_area_of_tgt += total_in_tgt;
  ...
  memcpy(data_area_of_tgt, data_area_of_src, total_in_src);

A small DataOffset can push a pointer below the actual byte area,
overwriting header fields; a large one can push it past the buffer
end, causing out-of-bounds heap reads (source) or writes (target).
The BCC overflow guard does not prevent this: BCC reflects how much
data is present, while DataOffset controls where in the buffer it
starts.

The "validate target area" comment present since the function was
first written in 2005 was a placeholder that was never implemented.

Add lower- and upper-bound checks for both data pointers before the
memcpy, and before any target header fields are modified.

## Which versions are affected by CVE-2026-89633?


## How do I fix CVE-2026-89633?

Apply the vendor's patch.

## Is CVE-2026-89633 being exploited?

There is no known exploitation of CVE-2026-89633 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89633 and fix it if it is affected.

What it is: CVE-2026-89633 is a critical memory corruption 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-2026-89633 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-2026-89633/ (Markdown: https://insights.sbomify.com/CVE-2026-89633/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89633
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89633
- OSV: https://osv.dev/vulnerability/CVE-2026-89633

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89633/
Summaries are generated and may contain mistakes; the official description above is the record.