# CVE-2025-21639: sctp: sysctl: rto_min/max: avoid using current->nsproxy

> A null pointer dereference in the Linux kernel SCTP subsystem can cause a system crash when specific network namespace conditions are met.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 19 Jan 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-57225, BELL-CVE-2025-21639, DEBIAN-CVE-2025-21639, GHSA-6w8c-3g24-p9jh, UBUNTU-CVE-2025-21639

## What is CVE-2025-21639?

CVE-2025-21639 is a medium-severity vulnerability in Linux kernel, published on 19 Jan 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:

sctp: sysctl: rto_min/max: avoid using current->nsproxy

As mentioned in a previous commit of this series, using the 'net'
structure via 'current' is not recommended for different reasons:

- Inconsistency: getting info from the reader's/writer's netns vs only
  from the opener's netns.

- current->nsproxy can be NULL in some cases, resulting in an 'Oops'
  (null-ptr-deref), e.g. when the current task is exiting, as spotted by
  syzbot [1] using acct(2).

The 'net' structure can be obtained from the table->data using
container_of().

Note that table->data could also be used directly, as this is the only
member needed from the 'net' structure, but that would increase the size
of this fix, to use '*data' everywhere 'net->sctp.rto_min/max' is used.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2025-21639?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21639?

Apply the vendor's patch.

## Is CVE-2025-21639 being exploited?

There is no known exploitation of CVE-2025-21639 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-2025-21639 and fix it if it is affected.

What it is: CVE-2025-21639 is a medium-severity vulnerability in Linux kernel, published on 19 Jan 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-2025-21639 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-2025-21639/ (Markdown: https://insights.sbomify.com/CVE-2025-21639/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21639
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21639
- OSV: https://osv.dev/vulnerability/CVE-2025-21639

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21639/
Summaries are generated and may contain mistakes; the official description above is the record.