# CVE-2026-63920: ipv6: validate extension header length before copying to cmsg

> An unprivileged user can trigger a slab-out-of-bounds read in the Linux kernel IPv6 stack via crafted network packets, potentially leaking memory or causing a crash.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/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:** 19 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-63920, DEBIAN-CVE-2026-63920, GHSA-8539-cx26-hf58, ROOT-OS-UBUNTU-2204-CVE-2026-63920, UBUNTU-CVE-2026-63920

## What is CVE-2026-63920?

CVE-2026-63920 is a high-severity memory corruption vulnerability in Linux kernel, published on 19 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: validate extension header length before copying to cmsg

ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}
cmsgs (and their IPV6_2292* legacy counterparts) by trusting the
on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.
The length was validated only at parse time (ipv6_parse_hopopts(),
etc.).  An nftables payload-write expression can rewrite hdrlen after
parsing and before the skb reaches recvmsg; the write itself is
in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040
bytes from an 8-byte header.  nftables is reachable from an
unprivileged user namespace, so this is an unprivileged
slab-out-of-bounds read:

  BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540
   put_cmsg+0x3ac/0x540
   udpv6_recvmsg+0xca0/0x1250
   sock_recvmsg+0xdf/0x190
   ____sys_recvmsg+0x1b1/0x620

Add ipv6_get_exthdr_len() which validates that at least two bytes
are accessible before reading the hdrlen field, then checks the
computed length against skb_tail_pointer(skb), returning 0 on
failure.  Extension headers are kept in the linear skb area by
pskb_may_pull() during input, so skb_tail_pointer() is the correct
bound.

Use ipv6_get_exthdr_len() at all non-AH call sites: the five
standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)
and the three standard cases in the extension-header walk loop
(DSTOPTS, ROUTING, default).  AH retains an inline bounds check
because its length formula differs ((ptr[1]+2)<<2).

The walk loop also gets a pre-read bounds check at the top to
validate ptr before any case accesses ptr[0] or ptr[1].

When the walk loop detects a corrupted header, return from the
function instead of continuing to process later socket options.

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-63920?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-63920?

Apply the vendor's patch.

## Is CVE-2026-63920 being exploited?

There is no known exploitation of CVE-2026-63920 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63920 and fix it if it is affected.

What it is: CVE-2026-63920 is a high-severity memory corruption vulnerability in Linux kernel, published on 19 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-2026-63920 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-63920/ (Markdown: https://insights.sbomify.com/CVE-2026-63920/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63920
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63920
- OSV: https://osv.dev/vulnerability/CVE-2026-63920

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63920/
Summaries are generated and may contain mistakes; the official description above is the record.