There is a bits/bytes confusion bug resulting in generation of a significantly 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.
Using such drastically reduced amount of random bits for Diffie Hellman weakened the handshake security significantly. This vulnerability could be exploited by an eavesdropper with enough resources to decrypt or intercept SSH sessions.