# CVE-2026-64542: ipv6: ndisc: fix NULL deref in accept_untracked_na()

> An unprivileged user can cause a kernel crash via a race condition in IPv6 neighbor discovery, leading to denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Jul 2026
- **Updated:** 19 Aug 2026
- **Also known as:** AZL-94044, BELL-CVE-2026-64542, DEBIAN-CVE-2026-64542, GHSA-f33q-wfg9-j2hv, UBUNTU-CVE-2026-64542

## What is CVE-2026-64542?

CVE-2026-64542 is a vulnerability in Linux, published on 27 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

ipv6: ndisc: fix NULL deref in accept_untracked_na()

accept_untracked_na() re-fetches the inet6_dev with __in6_dev_get(dev)
and dereferences idev->cnf.accept_untracked_na without a NULL check,
even though its only caller ndisc_recv_na() already fetched and
NULL-checked idev for the same device.

Both reads of dev->ip6_ptr run in the same RCU read-side critical
section, but a concurrent addrconf_ifdown() can clear dev->ip6_ptr
between them: lowering the MTU below IPV6_MIN_MTU calls addrconf_ifdown()
without the synchronize_net() that orders the unregister path, so the
re-fetch returns NULL and oopses:

 BUG: KASAN: null-ptr-deref in ndisc_recv_na (net/ipv6/ndisc.c:974)
 Read of size 4 at addr 0000000000000364
 Call Trace:
  <IRQ>
  ndisc_recv_na (net/ipv6/ndisc.c:974)
  icmpv6_rcv (net/ipv6/icmp.c:1193)
  ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479)
  ip6_input_finish (net/ipv6/ip6_input.c:534)
  ip6_input (net/ipv6/ip6_input.c:545)
  ip6_mc_input (net/ipv6/ip6_input.c:635)
  ipv6_rcv (net/ipv6/ip6_input.c:351)
  </IRQ>

It is reachable by an unprivileged user via a network namespace.

Pass the caller's already validated idev instead of re-fetching it; the
idev stays alive for the whole RCU critical section, so it is safe even
after dev->ip6_ptr has been cleared.

## Which versions are affected by CVE-2026-64542?


## How do I fix CVE-2026-64542?

Apply the vendor's patch.

## Is CVE-2026-64542 being exploited?

There is no known exploitation of CVE-2026-64542 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64542 and fix it if it is affected.

What it is: CVE-2026-64542 is a vulnerability in Linux, published on 27 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64542 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-2026-64542/ (Markdown: https://insights.sbomify.com/CVE-2026-64542/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64542
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64542
- OSV: https://osv.dev/vulnerability/CVE-2026-64542

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64542/
Summaries are generated and may contain mistakes; the official description above is the record.