Insecure Direct Object Reference (IDOR) is a common web application vulnerability classified under OWASP Top 10(A5: Broken Access Control). It occurs when an application exposes a reference to an internal implementation object—such as a file, database key, or user ID—without enforcing proper authorization checks. This allows attackers to manipulate the reference (e.g., by changing a URL parameter) to access or manipulate data they shouldn’t.
IDOR has been a persistent issue, contributing to breaches like those in Uber (2016) and Facebook (2019), where millions of records were exposed due to unchecked references.
Primary Causes of IDOR Vulnerabilities
IDOR stems from flawed design and implementation in how applications handle object access. Here are the key causes, broken down for clarity:
1. Direct Use of User-Supplied Input for Object References
Applications often pass user input (e.g., a URL parameter like /user?id=123) directly to backend operations, such as database queries, without validating or mapping it to an authorized object. For instance, if a query like SELECT * FROM users WHERE id = [user_input] lacks checks, an attacker can change id=123 to id=456 to view another user’s profile. This is the root cause in over 80% of IDOR cases, as it bypasses indirect referencing (e.g., using session-based tokens).
2. Inadequate Access Control Mechanisms
Developers fail to implement server-side authorization checks for every object access. Even if client-side restrictions exist (e.g., UI hiding options), they’re easily bypassed. Common pitfalls include assuming client-side enforcement is sufficient or not verifying if the requesting user owns or has permission for the referenced object. This is exacerbated in microservices or APIs where access logic is decentralized.
3. Poor Session and Authentication Handling
When sessions don’t tie requests to specific users or contexts, references become globally accessible. For example, using a static ID instead of a user-specific UUID allows horizontal (same privilege level) or vertical (higher privilege) privilege escalation. Weak session management, like predictable tokens, amplifies this.
4. Insufficient Input Validation and Sanitization
User inputs aren’t filtered for expected formats (e.g., allowing negative IDs or non-numeric values), leading to unintended queries. In file upload/download scenarios, exposing paths like /files/report_123.pdf without ownership checks enables directory traversal or unauthorized downloads.
5. Legacy Code and Rapid Development Practices
Inherited codebases or rushed implementations often reuse patterns without modern security layers, like role-based access control (RBAC). Cloud environments with shared resources (e.g., S3 buckets) can introduce IDOR if permissions aren’t granular.
Real-World Example
Imagine a banking app with an endpoint /account/balance?accountId=1001. If it fetches data solely based on accountId without verifying the caller’s ownership, an attacker could swap it to accountId=1002 to steal another customer’s balance— a classic horizontal IDOR.
How to Mitigate IDOR
• Use Indirect References: Map direct IDs to randomized, user-bound tokens (e.g., via a lookup table).
• Enforce Access Controls: Always validate permissions server-side using frameworks like Spring Security or OWASP guidelines.
• Input Validation: Whitelist parameters and use prepared statements to prevent injection-like manipulations.
• Testing: Include IDOR in penetration testing with tools like Burp Suite; scan for exposed IDs in APIs.
By addressing these causes early in the SDLC, you can reduce IDOR risks significantly. For deeper dives, refer to OWASP’s official cheat sheet.