ssh: add UnauthClientConn.RequestKeyExchange for client-requested pre-auth rekey - #89
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…-auth rekey Expose the ability to trigger a key re-exchange from an unauthenticated client connection. The transport already queues packets written during a rekey and flushes them on completion, so the connection remains usable immediately after the request. This enables pre-authentication state-transition research, e.g. probing how a server handles connection-protocol messages after a rekey that was requested before userauth completed (needed to detect RouterOS CVE-2026-67279, where a pre-auth rekey makes the server accept session channels without authentication).
hdm
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RouterOS loses track of the incomplete userauth state when the client requests a key re-exchange before authenticating, and then accepts connection-protocol messages anyway (fixed in 7.24.2 / 7.23.4 / 6.49.21). An unauthenticated client can open a session channel without any credentials. The new vuln-mikrotik-preauth-rekey check requests a rekey immediately after the initial key exchange, skips the userauth service entirely, and tries to open a session channel. It needs no credentials and no victim key material (unlike the CVE-2026-67276 check, which needs the victim's public key). Two guards keep it precise: - negative control: the same pre-auth channel open without the rekey must be refused, otherwise the server accepts pre-auth sessions generally (a different defect) - RouterOS attribution: the server banner (ROSSSH) or the proof command output must identify RouterOS before naming the CVE Requires the client-requested rekey API from runZeroInc/excrypto#89; go.mod temporarily replaces excrypto with the fork branch and should be bumped to a tagged excrypto release once that lands. Live-tested against a vulnerable CHR 7.24.1 VM (detected) and against a compliant Go sshd (control refuses the channel, not reported).
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Summary
Adds
(*UnauthClientConn).RequestKeyExchange(), letting a research client trigger a key re-exchange at an arbitrary point — in particular before userauth completes.The call is synchronous: it blocks until the key exchange completes (or the connection fails) and returns the exchange's error. This guarantees that on return, subsequent packets are sent over the rekeyed transport — callers cannot race the rekey with their next write. (
kexLoopgains arequestKexSyncchannel whose requesters are recorded and notified when the next exchange completes or the loop exits; the existing asynchronous threshold-based path is unchanged. No interface changes, no behavior changes for existing callers.)Motivation
Pre-authentication state-transition research. Concretely: MikroTik RouterOS CVE-2026-67279 — a client-requested rekey before authentication makes the server accept connection-protocol messages (session channels, exec) without any login. Detecting this requires initiating a rekey at a point where neither
x/crypto/sshnor this fork currently exposes one. A detection check for SSHamble that uses this API is ready and will be PRed against runZeroInc/sshamble, referencing this change.Refs:
Test
TestUnauthClientConnRequestKeyExchange(new,client_research_test.go): performs a client-requested rekey immediately after the initial handshake and then completes password authentication over the rekeyed transport. Fullx/crypto/sshsuite passes.(Note:
go test -raceon this package fails onmainas well — a pre-existing race in the FIPS-140 global indicator, unrelated to this change.)