The best way to find out if an application has any unvalidated redirects or forwards is to:
- Review the code for all uses of redirect or forward (called a transfer in .NET). For each use, identify if the target URL is included in any parameter values. If so, if the target URL isn’t validated against a whitelist, you are vulnerable.
- Also, spider the site to see if it generates any redirects (HTTP response codes 300-307, typically 302). Look at the parameters supplied prior to the redirect to see if they appear to be a target URL or a piece of such a URL. If so, change the URL target and observe whether the site redirects to the new target.
- If code is unavailable, check all parameters to see if they look like part of a redirect or forward URL destination and test those that do.
How to prevent
Safe use of redirects and forwards can be done in a number of ways:
- Simply avoid using redirects and forwards.
- If used, don’t involve user parameters in calculating the destination. This can usually be done.
- If destination parameters can’t be avoided, ensure that the supplied value is valid, and authorized for the user.
- It is recommended that any such destination parameters be a mapping value, rather than the actual URL or portion of the URL, and that server side code translate this mapping to the target URL.
- Applications can use ESAPI to override the sendRedirect() method to make sure all redirect destinations are safe.
- Avoiding such flaws is extremely important as they are a favorite target of phishers trying to gain the user’s trust.
Example Attack Scenarios
Scenario #1: The application has a page called “redirect.jsp” which takes a single parameter named “url”. The attacker crafts a malicious URL that redirects users to a malicious site that performs phishing and installs malware.
Scenario #2: The application uses forwards to route requests between different parts of the site. To facilitate this, some pages use a parameter to indicate where the user should be sent if a transaction is successful. In this case, the attacker crafts a URL that will pass the application’s access control check and then forwards the attacker to administrative functionality for which the attacker isn’t authorized.