# CVE-2025-39931: crypto: af_alg - Set merge to zero early in af_alg_sendmsg

> A local attacker can cause a kernel crash by triggering an error in af_alg_sendmsg, leading to a denial of service.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** 4 Oct 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-68175, AZL-75324, BELL-CVE-2025-39931, DEBIAN-CVE-2025-39931, GHSA-rfcq-q5wv-mpcg, ROOT-OS-DEBIAN-11-CVE-2025-39931, ROOT-OS-DEBIAN-12-CVE-2025-39931, ROOT-OS-DEBIAN-13-CVE-2025-39931, ROOT-OS-UBUNTU-2204-CVE-2025-39931, ROOT-OS-UBUNTU-2404-CVE-2025-39931, UBUNTU-CVE-2025-39931

## What is CVE-2025-39931?

CVE-2025-39931 is a high-severity vulnerability in Linux kernel, published on 4 Oct 2025. 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:

crypto: af_alg - Set merge to zero early in af_alg_sendmsg

If an error causes af_alg_sendmsg to abort, ctx->merge may contain
a garbage value from the previous loop.  This may then trigger a
crash on the next entry into af_alg_sendmsg when it attempts to do
a merge that can't be done.

Fix this by setting ctx->merge to zero near the start of the loop.

Weakness: CWE-908 (Use of Uninitialized Resource)

## Which versions are affected by CVE-2025-39931?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-39931?

Apply the vendor's patch.

## Is CVE-2025-39931 being exploited?

There is no known exploitation of CVE-2025-39931 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39931 and fix it if it is affected.

What it is: CVE-2025-39931 is a high-severity vulnerability in Linux kernel, published on 4 Oct 2025. 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-2025-39931 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2025-39931/ (Markdown: https://insights.sbomify.com/CVE-2025-39931/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39931
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39931
- OSV: https://osv.dev/vulnerability/CVE-2025-39931

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39931/
Summaries are generated and may contain mistakes; the official description above is the record.