A vulnerability in the TLS protocol allows a man-in-the-middle
attacker to downgrade vulnerable TLS connections using ephemeral
Diffie-Hellman key exchange to 512-bit export-grade cryptography. This
vulnerability is known as Logjam (CVE-2015-4000).

OpenSSL has added protection for TLS clients by rejecting handshakes
with DH parameters shorter than 768 bits. This limit will be increased
to 1024 bits in a future release.

OpenSSL 1.0.2 users should upgrade to 1.0.2b
OpenSSL 1.0.1 users should upgrade to 1.0.1n

Fixes for this issue were developed by Emilia Käsper and Kurt Roeckx
of the OpenSSL development team.