[Bug 2302] with DH-GEX, ssh (and sshd) should not fall back to unconfigured DH groups or at least document this behaviour and use a stronger group

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=2302

--- Comment #13 from Darren Tucker <dtucker at zip.com.au> ---
(In reply to Christoph Anton Mitterer from comment #10)
[...]
> Even though an attacker cannot (AFAIU??) for a connection to
> downgrade to the weaker groups,

The server's DH-GEX exchange hash includes the DH group sizes it
received from the client.  If these are modified in transit the
exchange hash will not match.

> it still doesn't give the server
> admin a good way to "block out" weak clients.

Do any such clients actually exist?  RFC4419 says DH-GEX
implementations SHOULD have a max group size of 8k.

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