A missing bounds check was found in the way OpenSSL handled TLS heartbeat extension packets. This flaw could be used to reveal up to 64k of memory from a connected client or server.
Only 1.0.1 releases of OpenSSL are affected including 1.0.1f (and 1.0.2 betas)
The following upstream commit introduced TLS/DTLS heatbeat support and also this issue:
http://git.openssl.org/gitweb/?p=openssl.git;a=commitdiff;h=4817504