# CVE-2026-64538: ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

> A null pointer dereference in the Linux kernel IPv6 stack allows remote attackers to cause a denial of service via crafted Router Advertisement packets.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Jul 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-94050, BELL-CVE-2026-64538, DEBIAN-CVE-2026-64538, GHSA-c37c-frr8-rx54, UBUNTU-CVE-2026-64538

## What is CVE-2026-64538?

CVE-2026-64538 is a vulnerability in Linux, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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

## Which versions are affected by CVE-2026-64538?


## How do I fix CVE-2026-64538?

Apply the vendor's patch.

## Is CVE-2026-64538 being exploited?

There is no known exploitation of CVE-2026-64538 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64538 and fix it if it is affected.

What it is: CVE-2026-64538 is a vulnerability in Linux, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64538 in the wild and no public exploit code.

Affected product: Linux. 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-2026-64538/ (Markdown: https://insights.sbomify.com/CVE-2026-64538/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64538
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64538
- OSV: https://osv.dev/vulnerability/CVE-2026-64538

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64538/
Summaries are generated and may contain mistakes; the official description above is the record.