libssh2 - security update

Andreas Schneider reported that libssh2, an SSH2 protocol
implementation used by many applications, did not generate
sufficiently long Diffie-Hellman secrets.

This vulnerability could be exploited by an eavesdropper to decrypt
and to intercept SSH sessions.

For the oldoldstable distribution (squeeze), this has been fixed in
version 1.2.6-1+deb6u2. Although the changelog refers to sha256,
this version only supports DH SHA-1 key exchange and it is that key
exchange method that has been fixed.

For the oldstable (wheezy) and stable (jessie) distributions, this
will be fixed soon.