OpenVPN, when using a 64-bit block cipher, makes it easier for remote attackers to obtain cleartext data via a birthday attack against a long-duration encrypted session, as demonstrated by an HTTP-over-OpenVPN session using Blowfish in CBC mode, aka a "Sweet32" attack.
https://security.gentoo.org/glsa/201611-02
https://h20566.www2.hpe.com/portal/site/hpsc/public/kb/docDisplay?docId=emr_na-c05369403
http://www.securitytracker.com/id/1036695
http://www.securityfocus.com/bid/92631
http://www-01.ibm.com/support/docview.wss?uid=swg21995039