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Re:OpenSSL Security Advisory: TLS heartbeat read overrun by jlgaddis on Monday April 07, 2014 @05:54PM (#7549829)
2011-12-31, it looks like, so it's been there for a couple of years already (introduced between 1.0.0f and 1.0.1):

http://git.openssl.org/gitweb/?p=openssl.git;a=commitdiff;h=...

Re:OpenSSL Security Advisory: TLS heartbeat read overrun by ge0rg on Tuesday April 08, 2014 @06:47AM (#7552555)
Wow, a code change introduced around New Year's Eve (depending on timezone), and authored by Robin Seggelmann, who also brought us DTLS-SCTP.