Testing for logout functionality (OTG-SESS-006)

Summary
Session termination is an important part of the session lifecycle. Reducing to a minimum the lifetime of the session tokens decreases the likelihood of a successful session hijacking attack. This can be seen as a control against preventing other attacks like Cross Site Scripting and Cross Site Request Forgery. Such attacks have been known to rely on a user having an authenticated session present. Not having a secure session termination only increases the attack surface for any of these attacks.

A secure session termination requires at least the following components:


 * Availability of user interface controls that allow the user to manually log out.
 * Session termination after a given amount of time without activity (session timeout).
 * Proper invalidation of server-side session state.

There are multiple issues which can prevent the effective termination of a session. For the ideal secure web application, a user should be able to terminate at any time through the user interface. Every page should contain a log out button on a place where it is directly visible. Unclear or ambiguous log out functions could cause the user not trusting such functionality.

Another common mistake in session termination is that the client-side session token is set to a new value while the server-side state remains active and can be reused by setting the session cookie back to the previous value. Sometimes only a confirmation message is shown to the user without performing any further action. This should be avoided.

Some web application frameworks rely solely on the session cookie to identify the logged-on user. The user's ID is embedded in the (encrypted) cookie value. The application server does not do any tracking on the server-side of the session. When logging out, the session cookie is removed from the browser. However, since the application does not do any tracking, it does not know whether a session is logged out or not. So by reusing a session cookie it is possible to gain access to the authenticated session. A well-known example of this is the Forms Authentication functionality in ASP.NET.

Users of web browsers often don't mind that an application is still open and just close the browser or a tab. A web application should be aware of this behavior and terminate the session automatically on the server-side after a defined amount of time.

The usage of a single sign-on (SSO) system instead of an application-specific authentication scheme often causes the coexistence of multiple sessions which have to be terminated separately. For instance, the termination of the application-specific session does not terminate the session in the SSO system. Navigating back to the SSO portal offers the user the possibility to log back in to the application where the log out was performed just before. On the other side a log out function in a SSO system does not necessarily cause session termination in connected applications.

How to Test
Testing for log out user interface: Verify the appearance and visibility of the log out functionality in the user interface. For this purpose, view each page from the perspective of a user who has the intention to log out from the web application.

Result Expected: There are some properties which indicate a good log out user interface:
 * A log out button is present on all pages of the web application.
 * The log out button should be identified quickly by a user who wants to log out from the web application.
 * After loading a page the log out button should be visible without scrolling.
 * Ideally the log out button is placed in an area of the page that is fixed in the view port of the browser and not affected by scrolling of the content.

Testing for server-side session termination: First, store the values of cookies that are used to identify a session. Invoke the log out function and observe the behavior of the application, especially regarding session cookies. Try to navigate to a page that is only visible in an authenticated session, e.g. by usage of the back button of the browser. If a cached version of the page is displayed, use the reload button to refresh the page from the server. If the log out function causes session cookies to be set to a new value, restore the old value of the session cookies and reload a page from the authenticated area of the application. If these test don't show any vulnerabilities on a particular page, try at least some further pages of the application that are considered as security-critical, to ensure that session termination is recognized properly by these areas of the application.

Result Expected: No data that should be visible only by authenticated users should be visible on the examined pages while performing the tests. Ideally the application redirects to a public area or a log in form while accessing authenticated areas after termination of the session. It should be not necessary for the security of the application, but setting session cookies to new values after log out is generally considered as good practice.

Testing for session timeout: Try to determine a session timeout by performing requests to a page in the authenticated area of the web application with increasing delays. If the log out behavior appears, the used delay matches approximately the session timeout value.

Result Expected: The same results as for server-side session termination testing described before are excepted by a log out caused by an inactivity timeout.

The proper value for the session timeout depends on the purpose of the application and should be a balance of security and usability. In a banking applications it makes no sense to keep an inactive session more than 15 minutes. On the other side a short timeout in a wiki or forum could annoy users which are typing lengthy articles with unnecessary log in requests. There timeouts of an hour and more can be acceptable.

Testing for session termination in single sign-on environments (single sign-off): Perform a log out in the tested application. Verify if there is a central portal or application directory which allows the user to log back in to the application without authentication. Test if the application requests the user to authenticate, if the URL of an entry point to the application is requested. While logged in in the tested application, perform a log out in the SSO system. Then try to access an authenticated area of the tested application.

Result Expected: It is expected that the invocation of a log out function in a web application connected to a SSO system or in the SSO system itself causes global termination of all sessions. An authentication of the user should be required to gain access to the application after log out in the SSO system and connected application.

Tools

 * "Burp Suite - Repeater" - http://portswigger.net/burp/repeater.html