# CVE-2025-21681: openvswitch: fix lockup on tx to unregistering netdev with carrier

> An attacker can cause a system lockup and require a reboot by sending traffic to an unregistering network device in the Linux kernel's Open vSwitch module.

- **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:** 31 Jan 2025
- **Updated:** 23 May 2026
- **Also known as:** AZL-57055, BELL-CVE-2025-21681, DEBIAN-CVE-2025-21681, GHSA-2x4f-r6q9-v77h, UBUNTU-CVE-2025-21681

## What is CVE-2025-21681?

CVE-2025-21681 is a medium-severity denial of service vulnerability in Linux kernel, published on 31 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:

openvswitch: fix lockup on tx to unregistering netdev with carrier

Commit in a fixes tag attempted to fix the issue in the following
sequence of calls:

    do_output
    -> ovs_vport_send
       -> dev_queue_xmit
          -> __dev_queue_xmit
             -> netdev_core_pick_tx
                -> skb_tx_hash

When device is unregistering, the 'dev->real_num_tx_queues' goes to
zero and the 'while (unlikely(hash >= qcount))' loop inside the
'skb_tx_hash' becomes infinite, locking up the core forever.

But unfortunately, checking just the carrier status is not enough to
fix the issue, because some devices may still be in unregistering
state while reporting carrier status OK.

One example of such device is a net/dummy.  It sets carrier ON
on start, but it doesn't implement .ndo_stop to set the carrier off.
And it makes sense, because dummy doesn't really have a carrier.
Therefore, while this device is unregistering, it's still easy to hit
the infinite loop in the skb_tx_hash() from the OVS datapath.  There
might be other drivers that do the same, but dummy by itself is
important for the OVS ecosystem, because it is frequently used as a
packet sink for tcpdump while debugging OVS deployments.  And when the
issue is hit, the only way to recover is to reboot.

Fix that by also checking if the device is running.  The running
state is handled by the net core during unregistering, so it covers
unregistering case better, and we don't really need to send packets
to devices that are not running anyway.

While only checking the running state might be enough, the carrier
check is preserved.  The running and the carrier states seem disjoined
throughout the code and different drivers.  And other core functions
like __dev_direct_xmit() check both before attempting to transmit
a packet.  So, it seems safer to check both flags in OVS as well.

Weakness: CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2025-21681?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21681?

Apply the vendor's patch.

## Is CVE-2025-21681 being exploited?

There is no known exploitation of CVE-2025-21681 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-2025-21681 and fix it if it is affected.

What it is: CVE-2025-21681 is a medium-severity denial of service vulnerability in Linux kernel, published on 31 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-2025-21681 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-2025-21681/ (Markdown: https://insights.sbomify.com/CVE-2025-21681/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21681
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21681
- OSV: https://osv.dev/vulnerability/CVE-2025-21681

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21681/
Summaries are generated and may contain mistakes; the official description above is the record.