In computer science, session hijacking, sometimes also known as cookie hijacking is the exploitation of a valid computer session—sometimes also called a session key—to gain unauthorized access to information or services in a computer system. In particular, it is used to refer to the theft of a magic cookie used to authenticate a user to a remote server. It has particular relevance to web developers, as the HTTP cookies used to maintain a session on many web sites can be easily stolen by an attacker using an intermediary computer or with access to the saved cookies on the victim's computer (see HTTP cookie theft).
A popular method is using source-routed IP packets. This allows an attacker at point B on the network to participate in a conversation between A and C by encouraging the IP packets to pass through B's machine. If source-routing is turned off, the attacker can use "blind" hijacking, whereby it guesses the responses of the two machines. Thus, the attacker can send a command, but can never see the response. However, a common command would be to set a password allowing access from somewhere else on the net.
An attacker can also be "inline" between A and C using a sniffing program to watch the conversation. This is known as a "man-in-the-middle attack".