---
id: "CVE-2026-99059"
kind: "vulnerability"
aliases: ["CVE-2026-99059"]
title: "Linux kernel WireGuard: duplicate handshake response can start a second session"
severity: "unknown"
cvss: null
epss: 0.00209
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["linux-kernel", "runtime", "self-hosted", "wireguard"]
not_affected_if: ["no-wireguard"]
published_at: "2026-10-23T13:17:29+00:00"
first_seen_at: "2026-10-23T21:57:33+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99059", "first_seen_at": "2026-10-23T21:57:33+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-99059", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel WireGuard: duplicate handshake response can start a second session

A race in the Linux kernel's WireGuard noise handshake lets a duplicated handshake response be consumed after a newer initiation has already run, so two threads can derive the same session. The sources do not say what an attacker gains beyond a reused session state.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- `uname -r` and compare against the fixed kernel builds listed by your distribution
- `lsmod | grep wireguard` or `modinfo wireguard` to confirm WireGuard is loaded or built in
- unknown whether a specific kernel version is affected

## Fix

- Update the Linux kernel to a build that includes the upstream fix for this WireGuard noise change; the exact fixed version is unknown from the sources
- Check your distribution's kernel advisory for the patched package

## Timeline

- 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99059
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-99059

## From the sources

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

wireguard: noise: reject response consumption after intermediate initiation

Two threads begin processing the identical response message, received
twice. The first thread, A, runs. While it's running, the second one,
B, gets partway through, and during that slow calculation, or even while
blocking on down_write(), A completes and then also a handshake
initiation that's already been queued up runs in thread C, which itself
takes that same down_write(). The handshake initiation creation
succeeds, and sets the state back to waiting-for-response, and calls
up_write(), at which point thread B resumes, because either its finished
its calculations or was finally allowed to acquire down_write(). Thread
B then copies the state back to the peer, and begins a new session,
using that state, which is the same session as the one made in thread A.

Thread A			Thread B			Thread C

down_read()
 sA = handshake->state
 memcpy(cA, handshake->crypto)
up_read()
if (sA != 1)
 goto fail
slow_crypto(cA)
				down_read()
				 sB = handshake->state
				 memcpy(cB, handshake->crypto)
				up_read()
				if (sB != 1)
				 goto fail
				slow_crypto(cB)
down_write()
 if (sA != handshake->state)
  goto fail
 memcpy(handshake->crypto, cA)
 handshake->state = 2
up_write()
down_write()
 if (handshake->state != 2)
  goto fail
 derive_session(handshake->crypto)
up_write()
								down_write()
								 slow_crypto(handshake->crypto)
								 handshake->state = 1
								up_write()
				down_write()
				 if (sB != handshake->state)
				   goto fail
				 memcpy(handshake->crypto, cB)
				 handshake->state = 2
				up_write()
				down_write()
				 if (handshake->state != 2)
				  goto fail
				 derive_session(handshake->crypto)
				up_write()

This seems basically impossible to hit in a meaningful way in practice,
but ensure that it absolutely cannot happen by comparing the ephemeral
private key that's on the stack with the latest one that the peer's
handshake state has.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_