# CVE-2026-31700: net/packet: fix TOCTOU race on mmap'd vnet_hdr in tpacket_snd()

> A time-of-check time-of-use race condition in the Linux kernel's packet socket implementation allows local attackers to bypass security checks and potentially escalate privileges.

- **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:** 1 May 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-85448, BELL-CVE-2026-31700, DEBIAN-CVE-2026-31700, GHSA-hxpw-4fhf-x5hx, ROOT-OS-DEBIAN-11-CVE-2026-31700, ROOT-OS-DEBIAN-12-CVE-2026-31700, ROOT-OS-UBUNTU-2204-CVE-2026-31700, ROOT-OS-UBUNTU-2404-CVE-2026-31700, UBUNTU-CVE-2026-31700

## What is CVE-2026-31700?

CVE-2026-31700 is a high-severity vulnerability in Linux kernel, published on 1 May 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:

net/packet: fix TOCTOU race on mmap'd vnet_hdr in tpacket_snd()

In tpacket_snd(), when PACKET_VNET_HDR is enabled, vnet_hdr points
directly into the mmap'd TX ring buffer shared with userspace. The
kernel validates the header via __packet_snd_vnet_parse() but then
re-reads all fields later in virtio_net_hdr_to_skb(). A concurrent
userspace thread can modify the vnet_hdr fields between validation
and use, bypassing all safety checks.

The non-TPACKET path (packet_snd()) already correctly copies vnet_hdr
to a stack-local variable. All other vnet_hdr consumers in the kernel
(tun.c, tap.c, virtio_net.c) also use stack copies. The TPACKET TX
path is the only caller of virtio_net_hdr_to_skb() that reads directly
from user-controlled shared memory.

Fix this by copying vnet_hdr from the mmap'd ring buffer to a
stack-local variable before validation and use, consistent with the
approach used in packet_snd() and all other callers.

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization), CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2026-31700?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31700?

Apply the vendor's patch.

## Is CVE-2026-31700 being exploited?

There is no known exploitation of CVE-2026-31700 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-2026-31700 and fix it if it is affected.

What it is: CVE-2026-31700 is a high-severity vulnerability in Linux kernel, published on 1 May 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-2026-31700 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-2026-31700/ (Markdown: https://insights.sbomify.com/CVE-2026-31700/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31700
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31700
- OSV: https://osv.dev/vulnerability/CVE-2026-31700

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31700/
Summaries are generated and may contain mistakes; the official description above is the record.