# CVE-2026-89563: ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

> A use-after-free vulnerability in the Linux kernel's IPv6 tunnel implementation allows attackers to cause a denial of service or potentially execute code.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100262, BELL-CVE-2026-89563, DEBIAN-CVE-2026-89563, GHSA-7pg6-vmqr-62jx, UBUNTU-CVE-2026-89563

## What is CVE-2026-89563?

CVE-2026-89563 is a high-severity use after free vulnerability in Linux, published on 11 Sep 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:

ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit()

ip6_tnl_xmit() may need to expand headroom before it can push the
outer IPv6 and optional encap headers. It currently does that with
skb_realloc_headroom(), copies skb->sk ownership, consumes the original
skb, and then continues processing with the replacement skb kept only in
its local variable.

That is safe only if the helper cannot fail afterwards. But this helper
still has post-reallocation error exits. collect_md tunnels reject
non-NONE encap after the replacement, and ip6_tnl_encap() can also fail
later. In those cases the helper returns an error to its callers while
the caller still only has the original skb pointer.

Both ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on
error, so they can end up freeing an skb that ip6_tnl_xmit() already
consumed.

Use skb_cow_head() instead. It provides the required headroom and
writability without privately replacing the caller-owned skb, so later
error returns cannot leave callers with a stale pointer.

The Ethernet users, ip6gretap and ip6erspan, clear IFF_TX_SKB_SHARING
and already call skb_cow_head() before entering ip6_tnl_xmit(). They do
not rely on the removed skb_shared() reallocation. This also makes the
IPv6 tunnel path consistent with ip_tunnel_xmit().

## Which versions are affected by CVE-2026-89563?


## How do I fix CVE-2026-89563?

Apply the vendor's patch.

## Is CVE-2026-89563 being exploited?

There is no known exploitation of CVE-2026-89563 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89563 and fix it if it is affected.

What it is: CVE-2026-89563 is a high-severity use after free vulnerability in Linux, published on 11 Sep 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-2026-89563 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-2026-89563/ (Markdown: https://insights.sbomify.com/CVE-2026-89563/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89563
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89563
- OSV: https://osv.dev/vulnerability/CVE-2026-89563

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89563/
Summaries are generated and may contain mistakes; the official description above is the record.