# CVE-2025-40256: xfrm: also call xfrm_state_delete_tunnel at destroy time for states that were never added

> A memory leak in the Linux kernel's xfrm subsystem allows local attackers to cause denial of service by triggering specific IPcomp tunnel state failures.

- **Severity:** Not scored
- **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 Dec 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40256, DEBIAN-CVE-2025-40256, GHSA-wxqp-f8fg-rxmv, UBUNTU-CVE-2025-40256

## What is CVE-2025-40256?

CVE-2025-40256 is a memory corruption vulnerability in Linux, published on 4 Dec 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:

xfrm: also call xfrm_state_delete_tunnel at destroy time for states that were never added

In commit b441cf3f8c4b ("xfrm: delete x->tunnel as we delete x"), I
missed the case where state creation fails between full
initialization (->init_state has been called) and being inserted on
the lists.

In this situation, ->init_state has been called, so for IPcomp
tunnels, the fallback tunnel has been created and added onto the
lists, but the user state never gets added, because we fail before
that. The user state doesn't go through __xfrm_state_delete, so we
don't call xfrm_state_delete_tunnel for those states, and we end up
leaking the FB tunnel.

There are several codepaths affected by this: the add/update paths, in
both net/key and xfrm, and the migrate code (xfrm_migrate,
xfrm_state_migrate). A "proper" rollback of the init_state work would
probably be doable in the add/update code, but for migrate it gets
more complicated as multiple states may be involved.

At some point, the new (not-inserted) state will be destroyed, so call
xfrm_state_delete_tunnel during xfrm_state_gc_destroy. Most states
will have their fallback tunnel cleaned up during __xfrm_state_delete,
which solves the issue that b441cf3f8c4b (and other patches before it)
aimed at. All states (including FB tunnels) will be removed from the
lists once xfrm_state_fini has called flush_work(&xfrm_state_gc_work).

## Which versions are affected by CVE-2025-40256?


## How do I fix CVE-2025-40256?

Apply the vendor's patch.

## Is CVE-2025-40256 being exploited?

There is no known exploitation of CVE-2025-40256 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40256 and fix it if it is affected.

What it is: CVE-2025-40256 is a memory corruption vulnerability in Linux, published on 4 Dec 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-2025-40256 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-2025-40256/ (Markdown: https://insights.sbomify.com/CVE-2025-40256/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40256
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40256
- OSV: https://osv.dev/vulnerability/CVE-2025-40256

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40256/
Summaries are generated and may contain mistakes; the official description above is the record.