CVE-2026-99059: wireguard: noise: reject response consumption after intermediate initiation

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 downwrite(), A completes and then also a handshake initiation that's already been queued up runs in thread C, which itself takes that same downwrite(). The handshake initiation creation succeeds, and sets the state back to waiting-for-response, and calls upwrite(), at which point thread B resumes, because either its finished its calculations or was finally allowed to acquire downwrite(). 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

downread() sA = handshake->state memcpy(cA, handshake->crypto) upread() if (sA != 1) goto fail slowcrypto(cA) downread() sB = handshake->state memcpy(cB, handshake->crypto) upread() if (sB != 1) goto fail slowcrypto(cB) downwrite() if (sA != handshake->state) goto fail memcpy(handshake->crypto, cA) handshake->state = 2 upwrite() downwrite() if (handshake->state != 2) goto fail derivesession(handshake->crypto) upwrite() downwrite() slowcrypto(handshake->crypto) handshake->state = 1 upwrite() downwrite() if (sB != handshake->state) goto fail memcpy(handshake->crypto, cB) handshake->state = 2 upwrite() downwrite() if (handshake->state != 2) goto fail derivesession(handshake->crypto) upwrite()

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.