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 websites 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). After successfully stealing appropriate session cookies an adversary might use the Pass the Cookie technique to perform session hijacking. Cookie hijacking is commonly used against client authentication on the internet. Modern web browsers use cookie protection mechanisms to protect the web from being attacked.
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 elsewhere 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".
History of HTTP
HTTP protocol versions 0.8 and 0.9 lacked cookies and other features necessary for session hijacking. Version 0.9beta of Mosaic Netscape, released on October 13, 1994, supported cookies.
Early versions of HTTP 1.0 did have some security weaknesses relating to session hijacking, but they were difficult to exploit due to the vagaries of most early HTTP 1.0 servers and browsers. As HTTP 1.0 has been designated as a fallback for HTTP 1.1 since the early 2000s—and as HTTP 1.0 servers are all essentially HTTP 1.1 servers the session hijacking problem has evolved into a nearly permanent security risk.
The introduction of supercookies and other features with the modernized HTTP 1.1 has allowed for the hijacking problem to become an ongoing security problem. Webserver and browser state machine standardization has contributed to this ongoing security problem.
Methods
There are four main methods used to perpetrate a session hijack. These are:
Session fixation, where the attacker sets a user's session ID to one known to them, for example by sending the user an email with a link that contains a particular session ID. The attacker now only has to wait until the user logs in. Session side jacking, where the attacker uses packet sniffing to read network traffic between two parties to steal the session cookie. Many websites use SSL encryption for login pages to prevent attackers from seeing the password, but do not use encryption for the rest of the site once authenticated. This allows attackers that can read the network traffic to intercept all the data that is submitted to the server or web pages viewed by the client. Since this data includes the session cookie, it allows them to impersonate the victim, even if the password itself is not compromised. Unsecured Wi-Fi hotspots are particularly vulnerable, as anyone sharing the network will generally be able to read most of the web traffic between other nodes and the access point. Cross-site scripting, where the attacker tricks the user's computer into running code which is treated as trustworthy because it appears to belong to the server, allowing the attacker to obtain a copy of the cookie or perform other operations. Malware and unwanted programs can use browser hijacking to steal a browser's cookie files without a user's knowledge, and then perform actions (like installing Android apps) without the user's knowledge. An attacker with physical access can simply attempt to steal the session key by, for example, obtaining the file or memory contents of the appropriate part of either the user's computer or the server.
After successfully acquiring appropriate session cookies an adversary might leverage the Pass the Cookie technique to perform session hijacking.