libssh -- weak Diffie-Hellman secret generation

Andreas Schneider reports:

libssh versions 0.1 and above have a bits/bytes confusion bug and
generate an abnormally short ephemeral secret for the
diffie-hellman-group1 and diffie-hellman-group14 key exchange
methods. The resulting secret is 128 bits long, instead of the
recommended sizes of 1024 and 2048 bits respectively. There are
practical algorithms (Baby steps/Giant steps, Pollard’s rho) that can
solve this problem in O(2^63) operations.
Both client and server are are vulnerable, pre-authentication.
This vulnerability could be exploited by an eavesdropper with enough
resources to decrypt or intercept SSH sessions. The bug was found
during an internal code review by Aris Adamantiadis of the libssh
team.