Abstract: A method for preventing an illegal user from logging in an online application with an authentic user's user log-in information is provided in the present invention. The system associates user log-in information with personal communication device information specified by the authentic user. In response to receiving the user log-in information inputted by a user, the system retrieves the personal communication device information associated with the user log-in information, and sends a short message to the personal communication device to notify the authentic user of the logging operation. If no confirming message is received from the authentic user, log in is rejected.