# CVE-2026-80840: ipv6: seg6: clear IPv4 control block on IPIP decapsulation

> A memory corruption vulnerability in the Linux kernel's IPv6 Segment Routing allows remote attackers to trigger a slab-out-of-bounds write, potentially leading to code execution or system crash.

- **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:** 4 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-99426, BELL-CVE-2026-80840, DEBIAN-CVE-2026-80840, GHSA-56f8-x2fw-63m3, UBUNTU-CVE-2026-80840

## What is CVE-2026-80840?

CVE-2026-80840 is a memory corruption vulnerability in Linux, published on 4 Sep 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: seg6: clear IPv4 control block on IPIP decapsulation

End.DX4 and End.DT4 decapsulate an IPv4 packet through
decap_and_validate() and send it directly to IPv4 routing. The inner
packet therefore bypasses ip_rcv_core(), which normally clears IPCB
before IPv4 interprets skb->cb.

The skb instead retains IP6CB data from the outer packet. IP6CB and
IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps
IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and
ts.

The sender can make the stale optlen byte nonzero with a valid outer
extension-header chain. The reproducers put an eight-byte Destination
Options header immediately after the 40-byte IPv6 header and before the
Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled
Destination Options offset in both lastopt and nhoff, setting them to
40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees
optlen = 40 and rr = 40.

Both tcp_v4_save_options() and __ip_options_echo() skip option copying
when optlen is zero. Here optlen is 40, so the TCP SYN path allocates
room for 40 bytes of option data and calls __ip_options_echo(). The
stale rr value makes that function read inner packet byte 41 as the
Record Route option length. The reproducers set that sender-controlled
byte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte
option-data area.

Separate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5
kernel both produced:

  BUG: KASAN: slab-out-of-bounds in __ip_options_echo()
  Write of size 255

The relevant End.DX4 call path is:

  __ip_options_echo
  tcp_v4_route_req
  tcp_conn_request
  tcp_v4_conn_request
  tcp_rcv_state_process
  tcp_v4_do_rcv
  tcp_v4_rcv
  ip_protocol_deliver_rcu
  ip_local_deliver_finish
  ip_local_deliver
  input_action_end_dx4_finish
  input_action_end_dx4

The relevant End.DT4 call path is:

  __ip_options_echo
  tcp_v4_route_req
  tcp_conn_request
  tcp_v4_conn_request
  tcp_rcv_state_process
  tcp_v4_do_rcv
  tcp_v4_rcv
  ip_protocol_deliver_rcu
  ip_local_deliver_finish
  ip_local_deliver
  input_action_end_dt4

tcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so
it does not appear as a separate frame.

When decap_and_validate() handles IPPROTO_IPIP, save the ingress
interface from IP6CB, clear IPCB, and restore the saved value. Doing
this in the common decapsulation path covers End.DX4, End.DT4, and
End.DT46's IPv4 arm.

Use IP6CB(skb)->iif rather than skb->skb_iif. These actions run after
l3mdev processing, which can replace skb_iif with the L3 master;
IP6CB iif still records the receiving interface set at IPv6 ingress.

## Which versions are affected by CVE-2026-80840?


## How do I fix CVE-2026-80840?

Apply the vendor's patch.

## Is CVE-2026-80840 being exploited?

There is no known exploitation of CVE-2026-80840 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80840 and fix it if it is affected.

What it is: CVE-2026-80840 is a memory corruption vulnerability in Linux, published on 4 Sep 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-2026-80840 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-2026-80840/ (Markdown: https://insights.sbomify.com/CVE-2026-80840/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80840
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80840
- OSV: https://osv.dev/vulnerability/CVE-2026-80840

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80840/
Summaries are generated and may contain mistakes; the official description above is the record.