A01:2021 – Broken Access Control
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Moving up from the fifth position, 94% of applications were tested for some form of broken access control with the average incidence rate of 3.81%, and has the most occurrences in the contributed dataset with over 318k. Notable Common Weakness Enumerations (CWEs) included are CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, CWE-201: Exposure of Sensitive Information Through Sent Data, and CWE-352: Cross-Site Request Forgery.
Access control enforces policy such that users cannot act outside of their intended permissions. Failures typically lead to unauthorized information disclosure, modification, or destruction of all data or performing a business function outside the user's limits. Common access control vulnerabilities include:
Violation of the principle of least privilege or deny by default, where access should only be granted for particular capabilities, roles, or users, but is available to anyone.
Bypassing access control checks by modifying the URL (parameter tampering or force browsing), internal application state, or the HTML page, or by using an attack tool modifying API requests.
Permitting viewing or editing someone else's account, by providing its unique identifier (insecure direct object references)
Accessing API with missing access controls for POST, PUT and DELETE.
Elevation of privilege. Acting as a user without being logged in or acting as an admin when logged in as a user.
Metadata manipulation, such as replaying or tampering with a JSON Web Token (JWT) access control token, or a cookie or hidden field manipulated to elevate privileges or abusing JWT invalidation.
CORS misconfiguration allows API access from unauthorized/untrusted origins.
Force browsing to authenticated pages as an unauthenticated user or to privileged pages as a standard user.
How to Prevent
Access control is only effective in trusted server-side code or server-less API, where the attacker cannot modify the access control check or metadata.
Except for public resources, deny by default.
Implement access control mechanisms once and re-use them throughout the application, including minimizing Cross-Origin Resource Sharing (CORS) usage.
Model access controls should enforce record ownership rather than accepting that the user can create, read, update, or delete any record.
Unique application business limit requirements should be enforced by domain models.
Disable web server directory listing and ensure file metadata (e.g., .git) and backup files are not present within web roots.
Log access control failures, alert admins when appropriate (e.g., repeated failures).
Rate limit API and controller access to minimize the harm from automated attack tooling.
Stateful session identifiers should be invalidated on the server after logout. Stateless JWT tokens should rather be short-lived so that the window of opportunity for an attacker is minimized. For longer lived JWTs it's highy recommended to follow the OAuth standards to revoke access.
Developers and QA staff should include functional access control unit and integration tests.
Example Attack Scenarios
Scenario #1: The application uses unverified data in a SQL call that is accessing account information:
pstmt.setString(1, request.getParameter("acct")); ResultSet results = pstmt.executeQuery( );
An attacker simply modifies the browser's 'acct' parameter to send whatever account number they want. If not correctly verified, the attacker can access any user's account.
Scenario #2: An attacker simply forces browses to target URLs. Admin rights are required for access to the admin page.
If an unauthenticated user can access either page, it's a flaw. If a non-admin can access the admin page, this is a flaw.