Privoxy before 3.0.21 does not properly handle Proxy-Authenticate and Proxy-Authorization headers in the client-server data stream, which makes it easier for remote HTTP servers to spoof the intended proxy service via a 407 (aka Proxy Authentication Required) HTTP status code.
http://lists.opensuse.org/opensuse-updates/2013-03/msg00118.html
http://ijbswa.cvs.sourceforge.net/viewvc/ijbswa/current/ChangeLog?revision=1.188&view=markup