Markus@ (OpenBSD developer) has just committed source code changes which disable version 1 of the OpenSSH application protocol. Old clients and servers that rely on protocol version 1 will no longer communicate with future versions of OpenSSH.
For over eight years OpenSSH clients and servers have defaulted to protocol version 2 and negotiated down to 1 when necessary. OpenSSH version 5.3 released on October 1, 2009 includes this behavior. However that negotiation is gone in subsequent versions making protocol version 2 mandatory.Prepackaged versions of OpenSSH supplied by vendors or embedded in routers, switches, and consoles may be affected if the vendor has not updated in recent years. Now may be a prudent time to check for updates.
Here are links to the relevant file diffs for those interested in the gory details of this change.
readconf.c
servconf.c
sshd_config
ssh_config.5
sshd_config.5
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