4.27.13 ‘Only supports pre-RFC4419 SSH-2 DH GEX’

The SSH key exchange method that uses Diffie-Hellman group exchange was redesigned after its original release, to use a slightly more sophisticated setup message. Almost all SSH implementations switched over to the new version. (PuTTY was one of the last.) A few old servers still only support the old one.

If this bug is detected, and the client and server negotiate Diffie-Hellman group exchange, then PuTTY will send the old message now known as SSH2_MSG_KEX_DH_GEX_REQUEST_OLD in place of the new SSH2_MSG_KEX_DH_GEX_REQUEST.

This is an SSH-2-specific bug.