# CVE-2026-74521: ksmbd: use memcmp() to compare ClientGUIDs

> An attacker can bypass SMB3 multichannel session binding checks by exploiting incorrect ClientGUID comparison logic in the Linux kernel ksmbd module.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 3 Oct 2026
- **Also known as:** BELL-CVE-2026-74521, DEBIAN-CVE-2026-74521, GHSA-336h-563m-gwm9, UBUNTU-CVE-2026-74521

## What is CVE-2026-74521?

CVE-2026-74521 is a critical authentication bypass vulnerability in Linux, published on 15 Aug 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:

ksmbd: use memcmp() to compare ClientGUIDs

ClientGUID is a fixed-size binary value and can contain embedded NUL
bytes. strncmp() stops comparing at the first NUL byte, so different
ClientGUID values can incorrectly be treated as equal.

Use memcmp() in SMB3 multichannel session binding and
FSCTL_VALIDATE_NEGOTIATE_INFO to compare all SMB2_CLIENT_GUID_SIZE
bytes.

## Which versions are affected by CVE-2026-74521?


## How do I fix CVE-2026-74521?

Apply the vendor's patch.

## Is CVE-2026-74521 being exploited?

There is no known exploitation of CVE-2026-74521 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74521 and fix it if it is affected.

What it is: CVE-2026-74521 is a critical authentication bypass vulnerability in Linux, published on 15 Aug 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-2026-74521 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-2026-74521/ (Markdown: https://insights.sbomify.com/CVE-2026-74521/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74521
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74521
- OSV: https://osv.dev/vulnerability/CVE-2026-74521

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74521/
Summaries are generated and may contain mistakes; the official description above is the record.