unattended-upgrades before 0.86.1 does not properly authenticate packages when the (1) force-confold or (2) force-confnew dpkg options are enabled in the DPkg::Options::* apt configuration, which allows remote man-in-the-middle attackers to upload and execute arbitrary packages via unspecified vectors.
http://www.ubuntu.com/usn/USN-2657-1
http://www.securitytracker.com/id/1032738