# CVE-2025-40297: net: bridge: fix use-after-free due to MST port state bypass

> A use-after-free in the Linux kernel bridge module allows attackers to cause denial of service or execute code when MST and VLAN filtering are configured.

- **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:** 8 Dec 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2025-40297, DEBIAN-CVE-2025-40297, GHSA-9jpf-8hq9-w5f5, ROOT-OS-DEBIAN-12-CVE-2025-40297, ROOT-OS-DEBIAN-13-CVE-2025-40297, ROOT-OS-UBUNTU-2404-CVE-2025-40297, UBUNTU-CVE-2025-40297

## What is CVE-2025-40297?

CVE-2025-40297 is a high-severity use after free vulnerability in Linux, published on 8 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:

net: bridge: fix use-after-free due to MST port state bypass

syzbot reported[1] a use-after-free when deleting an expired fdb. It is
due to a race condition between learning still happening and a port being
deleted, after all its fdbs have been flushed. The port's state has been
toggled to disabled so no learning should happen at that time, but if we
have MST enabled, it will bypass the port's state, that together with VLAN
filtering disabled can lead to fdb learning at a time when it shouldn't
happen while the port is being deleted. VLAN filtering must be disabled
because we flush the port VLANs when it's being deleted which will stop
learning. This fix adds a check for the port's vlan group which is
initialized to NULL when the port is getting deleted, that avoids the port
state bypass. When MST is enabled there would be a minimal new overhead
in the fast-path because the port's vlan group pointer is cache-hot.

[1] https://syzkaller.appspot.com/bug?extid=dd280197f0f7ab3917be

## Which versions are affected by CVE-2025-40297?


## How do I fix CVE-2025-40297?

Apply the vendor's patch.

## Is CVE-2025-40297 being exploited?

There is no known exploitation of CVE-2025-40297 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40297 and fix it if it is affected.

What it is: CVE-2025-40297 is a high-severity use after free vulnerability in Linux, published on 8 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-2025-40297 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-2025-40297/ (Markdown: https://insights.sbomify.com/CVE-2025-40297/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40297
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40297
- OSV: https://osv.dev/vulnerability/CVE-2025-40297

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40297/
Summaries are generated and may contain mistakes; the official description above is the record.