# CVE-2026-64009: xfrm: Check for underflow in xfrm_state_mtu

> A local attacker can trigger an integer underflow in the Linux kernel's xfrm subsystem, leading to an out-of-bounds write and potential code execution or system crash.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-64009, DEBIAN-CVE-2026-64009, GHSA-666q-fgc6-3x4w, ROOT-OS-UBUNTU-2204-CVE-2026-64009, UBUNTU-CVE-2026-64009

## What is CVE-2026-64009?

CVE-2026-64009 is a high-severity vulnerability in Linux kernel, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

xfrm: Check for underflow in xfrm_state_mtu

Leo Lin reported OOB write issue in esp component:

  xfrm_state_mtu() returns u32 but performs its arithmetic in unsigned
  modulo-2^32 space using an attacker-influenced "header_len + authsize +
  net_adj" subtracted from a small "mtu" argument. A nobody user can
  install an IPv4 ESP tunnel SA with a large authentication key
  (XFRMA_ALG_AUTH_TRUNC, e.g. hmac(sha512), 64-byte key, 64-byte trunc),
  configure a small interface MTU (68 bytes), and set XFRMA_TFCPAD to a
  large value. When a single UDP datagram is then sent through the
  tunnel, xfrm_state_mtu() underflows to a near-2^32 value, and
  esp_output() consumes it as a signed int via:

        padto      = min(x->tfcpad, xfrm_state_mtu(x, mtu_cached))
        esp.tfclen = padto - skb->len   (assigned to int)

  esp.tfclen ends up negative (e.g. -207). It is sign-extended to size_t
  when passed to memset() inside esp_output_fill_trailer(), producing a
  ~16 EB write of zeroes at skb_tail_pointer(skb). KASAN logs it as
  "Write of size 18446744073709551537 at addr ffff888...".

Check for underflow and return 1. This causes the sendmsg attempt to
fail with ENETUNREACH.

Weakness: CWE-787 (Out-of-bounds Write), CWE-191 (Integer Underflow (Wrap or Wraparound))

## Which versions are affected by CVE-2026-64009?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64009?

Apply the vendor's patch.

## Is CVE-2026-64009 being exploited?

There is no known exploitation of CVE-2026-64009 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-2026-64009 and fix it if it is affected.

What it is: CVE-2026-64009 is a high-severity vulnerability in Linux kernel, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64009 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-2026-64009/ (Markdown: https://insights.sbomify.com/CVE-2026-64009/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64009
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64009
- OSV: https://osv.dev/vulnerability/CVE-2026-64009

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64009/
Summaries are generated and may contain mistakes; the official description above is the record.