Linksys WRT54G 3.01.03, 3.03.6, 4.00.7, and possibly other versions before 4.20.7, does not verify user authentication until after an HTTP POST request has been processed, which allows remote attackers to (1) modify configuration using restore.cgi or (2) upload new firmware using upgrade.cgi.
http://www.idefense.com/application/poi/display?id=307&type=vulnerabilities
http://www.idefense.com/application/poi/display?id=306&type=vulnerabilities