4.27.10 ‘Handles SSH-2 key re-exchange badly’

Some SSH servers cannot cope with repeat key exchange at all, and will ignore attempts by the client to start one. Since PuTTY pauses the session while performing a repeat key exchange, the effect of this would be to cause the session to hang after an hour (unless you have your rekey timeout set differently; see section 4.19.2 for more about rekeys). Other, very old, SSH servers handle repeat key exchange even more badly, and disconnect upon receiving a repeat key exchange request.

If this bug is detected, PuTTY will never initiate a repeat key exchange. If this bug is enabled when talking to a correct server, the session should still function, but may be less secure than you would expect.

This is an SSH-2-specific bug.