Top 10 2010-A8-Failure to Restrict URL Access

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Threat Agents	Attack Vectors	Security Weakness		Technical Impacts	Business Impacts
______	Exploitability EASY	Prevalence UNCOMMON	Detectability AVERAGE	Impact MODERATE	______
Anyone with network access can send your application a request. Could anonymous users access a private page or regular users a privileged page?	Attacker, who is an authorized system user, simply changes the URL to a privileged page. Is access granted? Anonymous users could access private pages that aren’t protected.	Applications are not always protecting page requests properly. Sometimes, URL protection is managed via configuration, and the system is misconfigured. Sometimes, developers must include the proper code checks, and they forget. Detecting such flaws is easy. The hardest part is identifying which pages (URLs) exist to attack.		Such flaws allow attackers to access unauthorized functionality. Administrative functions are key targets for this type of attack.	Consider the business value of the exposed functions and the data they process. Also consider the impact to your reputation if this vulnerability became public.

Am I Vulnerable to Failure to Restrict URL Access?

The best way to find out if an application has failed to properly restrict URL access is to verify every page. Consider for each page, is the page supposed to be public or private. If a private page:

1. Is authentication required to access that page?
2. Is it supposed to be accessible to ANY authenticated user? If not, is an authorization check made to ensure the user has permission to access that page?

External security mechanisms frequently provide authentication and authorization checks for page access. Verify they are properly configured for every page. If code level protection is used, verify that code level protection is in place for every required page. Penetration testing can also verify whether proper protection is in place.

How Do I Prevent Failure to Restrict URL Access?

Preventing unauthorized URL access requires selecting an approach for requiring proper authentication and proper authorization for each page. Frequently, such protection is provided by one or more components external to the application code. Regardless of the mechanism(s), all of the following are recommended:

1. The authentication and authorization policies be role based, to minimize the effort required to maintain these policies.
2. The policies should be highly configurable, in order to minimize any hard coded aspects of the policy.
3. The enforcement mechanism(s) should deny all access by default, requiring explicit grants to specific users and roles for access to every page.
4. If the page is involved in a workflow, check to make sure the conditions are in the proper state to allow access.

Example Attack Scenarios

The attacker simply force browses to target URLs. Consider the following URLs which are both supposed to require authentication. Admin rights are also required for access to the “admin_getappInfo” page.

• http://example.com/app/getappInfo
• http://example.com/app/admin_getappInfo

If the attacker is not authenticated, and access to either page is granted, then unauthorized access was allowed. If an authenticated, non-admin, user is allowed to access the “admin_getappInfo” page, this is a flaw, and may lead the attacker to more improperly protected admin pages.

Such flaws are frequently introduced when links and buttons are simply not displayed to unauthorized users, but the application fails to protect the pages they target.

References

OWASP

• OWASP Top 10-2007 on Failure to Restrict URL Access
• ESAPI Access Control API
• OWASP Development Guide: Chapter on Authorization
• OWASP Testing Guide: Testing for Path Traversal
• OWASP Article on Forced Browsing

For additional access control requirements, see the ASVS requirements area for Access Control (V4)

External

• CWE Entry 285 on Improper Access Control (Authorization)

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