Netscape Communicator before version 4.73 and Navigator 4.07 do not properly validate SSL certificates, which allows remote attackers to steal information by redirecting traffic from a legitimate web server to their own malicious server, aka the "Acros-Suencksen SSL" vulnerability.
http://www.securityfocus.com/bid/1188
http://www.redhat.com/support/errata/RHSA-2000-028.html