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Closing, see #1823 |
vlastee
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Sep 25, 2026
SendMessage checked the key exchange wait handle before acquiring the socket write lock. When a server-initiated re-exchange started in that window, the client's SSH_MSG_KEXINIT could be sent first, after which the already in-flight data message violated RFC 4253 section 7.1. Strict servers (e.g. ProFTPD mod_sftp) then fail the exchange or drop the connection. SendMessage now re-checks the wait handle while holding the write lock and goes back to waiting when a re-exchange has started in the meantime. The packet is also built entirely under the write lock, so a completing re-exchange can no longer swap the client cipher, MAC or compression state in the middle of building a packet. The race does not reproduce against the OpenSSH test server (which is why the attempt in sshnet#1774 stayed green): OpenSSH queues non key exchange output while a re-exchange is in progress and tolerates the client data that slips in. ProFTPD mod_sftp does not, so this adds an integration test which reproduces the failure with concurrent SFTP uploads against a ProFTPD server configured to re-key every 1 MB. Without the fix the test failed 7 out of 8 runs; with it, it passes consistently. Fixes sshnet#1764.
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