The installer in Yahoo! Messenger 4.0, 5.0 and 5.5 does not verify package signatures which could allow remote attackers to install trojan programs via DNS spoofing.
http://www.securityfocus.com/bid/5579
http://www.iss.net/security_center/static/9984.php
http://cert.uni-stuttgart.de/archive/bugtraq/2002/08/msg00384.html