D-Link DSL-502T, DSL-504T, DSL-562T, and DSL-G604T, when /cgi-bin/firmwarecfg is executed, allows remote attackers to bypass authentication (1) if their IP address already exists in /var/tmp/fw_ip or (2) if their request is the first, which causes /var/tmp/fw_ip to be created and contain their IP address.