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In computer science, session 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 a hacker at point A on the network to participate in a conversation between B and C by encouraging the IP packets to pass through its machine.
If source-routing is turned off, the hacker can use "blind" hijacking, whereby it guesses the responses of the two machines. Thus, the hacker 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.
A hacker can also be "inline" between B and C using a sniffing program to watch the conversation. This is known as a "man-in-the-middle attack".
Session hijacking was not possible with early versions 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.
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 him, 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 sidejacking, where the attacker uses packet sniffing to read network traffic between two parties to steal the session cookie. Many web sites 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 him 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.
- Alternatively, 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.
- 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.
Methods to prevent session hijacking include:
- Encryption of the data traffic passed between the parties; in particular the session key, though ideally all traffic for the entire session by using SSL/TLS. This technique is widely relied-upon by web-based banks and other e-commerce services, because it completely prevents sniffing-style attacks. However, it could still be possible to perform some other kind of session hijack.
- Use of a long random number or string as the session key. This reduces the risk that an attacker could simply guess a valid session key through trial and error or brute force attacks.
- Regenerating the session id after a successful login. This prevents session fixation because the attacker does not know the session id of the user after s/he has logged in.
- Some services make secondary checks against the identity of the user. For example, a web server could check with each request made that the IP address of the user matched the one last used during that session. This does not prevent attacks by somebody who shares the same IP address, however, and could be frustrating for users whose IP address is liable to change during a browsing session.
- Alternatively, some services will change the value of the cookie with each and every request. This dramatically reduces the window in which an attacker can operate and makes it easy to identify whether an attack has taken place, but can cause other technical problems (for example, two legitimate, closely timed requests from the same client can lead to a token check error on the server).
- Users may also wish to log out of websites whenever they are finished using them. However this will not protect against attacks such as Firesheep.
In October 2010, a Mozilla Firefox extension called Firesheep has exploited and made it easy for users of unencrypted public Wi-Fi to be attacked by session hijackers. 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 users personal property. Only months later, Facebook and Twitter responded by offering (and later requiring) HTTP Secure throughout.
WhatsApp sniffer 
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. WhatsApp uses an XMPP infrastructure with unencrypted, 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 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. The source is available here
See also 
- "Warning of webmail wi-fi hijack". BBC News. August 3, 2007.
- "Schneier on Security: Firesheep". 27 October, 2010. Retrieved 29 May, 2011.
- See "NetBadge: How To Log Out".
- See also "Be Card Smart Online - Always log out".
- "Firefox extension steals Facebook, Twitter, etc. sessions". The H. 25 October 2010.
- "Facebook now SSL-encrypted throughout". The H. 27 January 2011.
- "Twitter adds ‘Always use HTTPS’ option". The H. 16 March 2011.
- "Sniffer tool displays other people's WhatsApp messages". The H. 13 May 2012.
- "DroidSheep Blog".