It came to light back in May that an exploit was possible whereby an attacker could force a server to downgrade its encryption to “export-grade”. I remember something about it at the time, but I don’t remember thinking it was a big deal.
Boy was I wrong.
The problem is that many servers use the same Diffie-Hellman groups for key exchange. An attacker with a large amount of computing power could pre-compute most of the bits needed to break the most common DH groups, and then use a man-in-the-middle to force a connection into a 512-bit export-grade mode… and then break it.
See Weak Diffie-Hellman and the Logjam Attack for details.
(And of course, removing export-grade DH from my servers has caused me to break many things…)