ENTERSOFT KNOWLEDGE BASE

Unvalidated Redirects and Forwards
VULNERABILITIES

The best way to find out if an application has any unvalidated redirects or forwards is to:

  1. 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.
  2. 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.
  3. 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:

  1. Simply avoid using redirects and forwards.
  2. If used, don’t involve user parameters in calculating the destination. This can usually be done.
  3. If destination parameters can’t be avoided, ensure that the supplied value is valid, and authorized for the user.
  4. 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.
  5. Applications can use ESAPI to override the sendRedirect() method to make sure all redirect destinations are safe.
  6. Avoiding such flaws is extremely important as they are a favorite target of phishers trying to gain the user’s trust.

    7. 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.

        http://www.example.com/redirect.jsp?url=evil.com

    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.

        http://www.example.com/boring.jsp?fwd=admin.jsp