Session prediction, also called credential/session prediction, is a method of surreptitiously obtaining data (called a session ID) about an authorized visitor to a Web site. The session ID is normally stored within a cookie or URL.
Session IDs enables user tracking on a Web site. The session ID can also provide automatic authentication for future visits to that site or associated sites. If a cracker obtains session ID data, he can conduct an attack, such as session hijacking or session replay.
Session prediction and hijacking typically occur while a legitimate user is interacting with an affected Web site. The legitimate user may or may not be able to detect the intrusion, depending on his or her level of technical knowledge, and on the nature of the attack. If a Web site does not respond in the normal or expected way to user input, or if it stops responding altogether for an unknown reason, session prediction or hijacking may be taking place.