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). 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".
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.
There are four main methods used to perpetrate a session hijack. These are:
After successfully acquiring appropriate session cookies an adversary might leverage the Pass the Cookie technique to perform session hijacking.
In October 2010, a Mozilla Firefox extension called Firesheep was released that made it easy for session hijackers to attack users of unencrypted public Wi-Fi. Websites like Facebook, Twitter, and any that the user adds to their preferences allow the Firesheep user to easily access private information from cookies and threaten the public Wi-Fi user's personal property. Only months later, Facebook and Twitter responded by offering (and later requiring) HTTP Secure throughout.
An app named "WhatsApp Sniffer" was made available on Google Play in May 2012, able to display messages from other WhatsApp users connected to the same network as the app user. At that time WhatsApp used an XMPP infrastructure with encryption, not plain-text communication.
DroidSheep is a simple Android tool for web session hijacking (sidejacking). It listens for HTTP packets sent via a wireless (802.11) network connection and extracts the session id from these packets in order to reuse them. DroidSheep can capture sessions using the libpcap library and supports: open (unencrypted) networks, WEP encrypted networks, and WPA/WPA2 encrypted networks (PSK only). This software uses libpcap and arpspoof. The apk was made available on Google Play but it has been taken down by Google.
CookieCadger is a graphical Java app that automates sidejacking and replay of HTTP requests, to help identify information leakage from applications that use unencrypted GET requests. It is a cross-platform open-source utility based on the Wireshark suite which can monitor wired Ethernet, insecure Wi-Fi, or load a packet capture file for offline analysis. Cookie Cadger has been used to highlight the weaknesses of youth team sharing sites such as Shutterfly (used by AYSO soccer league) and TeamSnap.
Methods to prevent session hijacking include: