The OpenSSH client and server are vulnerable to a pre-authentication denial-of-service attack: an asymmetric resource consumption of both memory and CPU.

The OpenSSH client is vulnerable to an active machine-in-the-middle attack if the VerifyHostKeyDNS option is enabled (it is disabled by default): when a vulnerable client connects to a server, an active machine-in-the-middle can impersonate the server by completely bypassing the client's checks of the server's identity.