The remote Debian host is missing a security-related update.
Aris Adamantiadis discovered that libssh, a tiny C SSH library, incorrectly generated a 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. This flaw could allow an eavesdropper with enough resources to decrypt or intercept SSH sessions.
Upgrade the libssh packages. For the oldstable distribution (wheezy), this problem has been fixed in version 0.5.4-1+deb7u3. This update also includes fixes for CVE-2014-8132 and CVE-2015-3146, which were previously scheduled for the next wheezy point release. For the stable distribution (jessie), this problem has been fixed in version 0.6.3-4+deb8u2.