4.4 Authorisation of direct object references¶
Verify that access to sensitive records is protected, such that only authorized objects or data is accessible to each user (for example, protect against users tampering with a parameter to see or alter another user’s account).
Levels: 1, 2, 3
A direct object reference occurs when a developer exposes a reference to an internal implementation object, such as a file, directory, database record, or key, as a URL or form parameter. An attacker can manipulate direct object references to access other objects without authorization, unless an access control check is in place. For example, in Internet Banking applications, it is common to use the account number as the primary key. Therefore, it is tempting to use the account number directly in the web interface. Even if the developers have used parameterized SQL queries to prevent SQL injection, if there is no extra check that the user is the account holder and authorized to see the account, an attacker tampering with the account number parameter can see or change all accounts. This type of attack occurred to the Australian Taxation Office’s GST Start Up Assistance site in 2000, where a legitimate but hostile user simply changed the ABN (a company tax id) present in the URL. The user farmed around 17,000 company details from the system, and then e-mailed each system. This was a major embarrassment to the Government of the 17,000 companies with details of his attack. This type of vulnerability is very common, but is largely untested in current applications.