Ciphers with 64-bit block sizes used in CBC mode were found to be vulnerable to birthday attack when key renegotiation doesn't happen frequently or at all in long running connections. 3DES cipher as used in TLS protocol is vulnerable to this attack, that allows remote attacker to recover partial plaintext information (XOR of two plaintext blocks). http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-2183