# CVE-2026-64007: netfilter: synproxy: refresh tcphdr after skb_ensure_writable

> A use-after-free in the Linux kernel netfilter synproxy module allows attackers to cause memory corruption or denial of service via crafted TCP packets.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-64007, DEBIAN-CVE-2026-64007, GHSA-rf6c-225m-p2p4, ROOT-OS-UBUNTU-2204-CVE-2026-64007, UBUNTU-CVE-2026-64007

## What is CVE-2026-64007?

CVE-2026-64007 is a critical use after free vulnerability in Linux kernel, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-64007?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64007?

Apply the vendor's patch.

## Is CVE-2026-64007 being exploited?

There is no known exploitation of CVE-2026-64007 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64007 and fix it if it is affected.

What it is: CVE-2026-64007 is a critical use after free vulnerability in Linux kernel, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64007 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-64007/ (Markdown: https://insights.sbomify.com/CVE-2026-64007/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64007
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64007
- OSV: https://osv.dev/vulnerability/CVE-2026-64007

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64007/
Summaries are generated and may contain mistakes; the official description above is the record.