In the latest OWASP Top 10:2021 list, Broken Access Control has surged to the number one position—up from fifth place in the previous version. This marks it as the most exploited and widespread vulnerability in modern web applications. According to OWASP’s global testing data, 94% of applications were found to have at least one instance of broken access control, with over 318,000 occurrences mapped across 34 Common Weakness Enumerations (CWEs)—more than any other category.
What Is Broken Access Control?
Access control defines who can access what within a system. When these controls fail, attackers can gain unauthorized privileges, view sensitive data, or even perform administrative actions that compromise the entire application.
Simply put, broken access control is when an application fails to properly restrict actions that users are not allowed to perform.
Common Examples in Real-World Applications
Bypassing Role Restrictions: Changing a user role from “user” to “admin” through URL tampering or API manipulation.
Insecure Direct Object References (IDOR): Accessing someone else’s account or files by altering a parameter value such as a user ID.
CORS Misconfiguration: Allowing resources to be accessed from untrusted origins through weak cross-origin settings.
JWT or Session Token Tampering: Modifying tokens or cookies to escalate privileges.
Force Browsing: Manually visiting restricted paths such as
/adminor/confidential/reports.
Even applications developed with strong authentication can fail if authorization checks are missing or inconsistently applied across modules.
Why It’s So Prevalent
Access control logic is often decentralized, implemented inconsistently by different developers, or handled on the client side where attackers can easily manipulate requests. Organizations underestimate how complex enforcing fine-grained permissions can be—especially in APIs and microservices.
How to Prevent Broken Access Control
Deny by Default: Restrict every action unless explicitly allowed by access control rules.
Centralize Enforcement: Implement a unified access control layer on the server side.
Enforce Resource Ownership: Validate that users only access data they own.
Secure Tokens: Use short-lived JWTs and revoke them following OAuth guidelines.
Test Rigorously: Include functional access control checks in automated unit and integration tests.
Monitor and Log: Detect repeated access control failures and alert administrators for investigation.
Limit CORS Exposure: Allow only trusted origins for cross-origin resource sharing.
Example in Action
Imagine a banking web app where user account information is fetched via a parameter:
texthttps://examplebank.com/account?acct=12345
If a malicious user changes the account number to another user’s ID, and the app doesn’t verify ownership, the attacker gains access to the victim’s data. This is a textbook case of Broken Access Control through parameter tampering.
Final Thoughts
Broken Access Control remains the most dangerous and commonly exploited vulnerability in 2021—and even in 2025, it continues to appear in bug bounties and penetration tests. It’s not just a coding mistake; it’s often a design-level flaw that gives attackers the keys to your system. By adopting a deny-by-default mindset and implementing robust, centralized authorization, developers can safeguard their applications from this critical threat.
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