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application does not generate a new session identifier whenever a user authenticates, the potential exists for a session fixation attack, in which the attacker forces a known session identifier onto a user In a generic session xation exploit, an attacker creates a new session in a Web application without logging in and records the associated session identi er The attacker then causes the victim to authenticate against the server using that session identi er, which results in the attacker gaining access to the user s account through the active session Imagine the following scenario: 1 The attacker walks up to a public terminal and navigates to the login page for a poorly built Web application The application issues a session cookie as part of rendering the login page 2 The attacker records the session cookie and walks away from the terminal 3 A few minutes later, a victim approaches the terminal and logs in 4 Because the application continues to use the same session cookie it originally created for the attacker, the attacker now knows the victim s session identi er and can take control of the session from another computer This attack scenario requires several things for the attacker to have a chance at success: access to an unmonitored public terminal, the capability to keep the compromised session active, and a victim interested in logging into the vulnerable application on the public terminal In most circumstances, the rst two challenges are surmountable, given a suf cient investment of time Finding a victim who is both using a public terminal and interested in logging into the vulnerable application is possible as well, as long as the site is reasonably popular and the attacker is not picky about who the victim will be For example, a Web e-mail kiosk would be a prime target An attacker can do away with the need for a shared public terminal if the application server makes it possible to force a session identi er on a user by means of a link on a Web page or in an e-mail message For instance, Apache Tomcat allows an attacker to specify a session identi er as a URL parameter like this: https://wwwexamplecom/indexjsp jsessionid=abc123 If the value of the jsessionid parameter refers to an existing session, Tomcat will begin using it as the session identi er To limit session fixation, a Web application must issue a new session identifier at the same time it authenticates a user Many application servers make this more difficult by providing separate facilities for managing
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authorization and session management For example, the Java Servlet specification requires a container to provide the URL j_security_check, but it does not require that the container issue a new session identifier when authentication succeeds This leads to a vulnerability in the standard recommended method for setting up a login page, which involves creating a form that looks like this:
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<form method="POST" action="j_security_check" > Username: <input type="text" name="j_username"> Password:<input type="password" name="j_password"> <input type="submit" name="action" value="Log In"> </form>
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If the application has already created a session before the user authenticates, some implementations of j_security_check (including the one in Tomcat) will continue to use the already established session identi er If that identi er were supplied by an attacker, the attacker would have access to the authenticated session It is worth noting that, by default, Web browsers associate cookies with the top-level domain for a given URL If multiple applications reside under the same top-level domain, such as bankexamplecom and adsexamplecom, a vulnerability in one application can allow an attacker to fix the session identifier that will be used in all interactions with any application on the domain examplecom If your application needs to maintain state across an authentication boundary, the code in Example 913 outlines the session management portion of the authentication process Note that it creates the new session before authenticating the user to avoid a race condition in which an authenticated user is brie y associated with the old session identi er
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Example 913 This login method invalidates any existing session and creates a new session before attempting to authenticate the user
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public void doLogin(HttpServletRequest request) { HttpSession oldSession = requestgetSession(false); if (oldSession != null) { // create new session if there was an old session oldSessioninvalidate(); HttpSession newSession = requestgetSession(true); // transfer attributes from old to new Enumeration enum = oldSessiongetAttributeNames(); while (enumhasMoreElements()) {
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