Insecure Direct Object References (IDOR) is a common web application vulnerability that allows attackers to bypass authorization and access resources they are not supposed to. This vulnerability arises when an application uses user-supplied input to fetch an object directly from a data store without performing sufficient access control checks. For OSCP preparation, understanding IDOR is crucial as it's a frequently encountered vulnerability in penetration tests.

IDOR occurs when a web application exposes a reference to an internal implementation object, such as a file, directory, or database record, as a URL parameter, form field, or cookie. If the application doesn't validate that the authenticated user is authorized to access the requested object, an attacker can manipulate these references to access unauthorized data.

Detecting IDOR vulnerabilities involves a combination of manual inspection and automated tools. The key is to identify parameters that seem to reference specific objects and then attempt to manipulate them.

When testing for IDOR, consider the following techniques:

1. Parameter Analysis: Look for parameters in URLs, POST data, or cookies that represent identifiers (e.g., id, user_id, file_name, account_number, document_id).
2. Enumeration: Systematically change the values of these parameters to see if you can access different resources. This can be done manually or with tools like Burp Suite's Intruder.
3. Authorization Bypass: Attempt to access resources belonging to other users or with higher privileges. For example, if you can view your own order history, try to view another user's order history by changing the order ID.
4. File Path Traversal: If parameters reference files, try to use directory traversal sequences (e.g., ../, ..\) to access files outside the intended directory.

Once an IDOR vulnerability is identified, exploitation typically involves modifying the reference to point to a different object. This can be achieved through various methods:

Preventing IDOR vulnerabilities is paramount for secure application development. The most effective mitigation strategies focus on robust access control.

Key mitigation techniques include:

• Implement Strong Access Control: For every request that accesses a resource, verify that the authenticated user has the necessary permissions to access that specific resource. This often involves checking ownership or role-based access control (RBAC).
• Use Indirect References: Instead of directly exposing database IDs or file paths, use indirect references that are mapped to the user's session or context. For example, use session variables to track the user's current document ID.
• Avoid Sequential IDs: While not a complete solution, using non-sequential or randomly generated IDs can make brute-force attacks more difficult.
• Validate Input: Sanitize and validate all user inputs to prevent unexpected behavior, though this is secondary to proper access control for IDOR.
• Least Privilege Principle: Ensure that users and processes only have the minimum privileges necessary to perform their intended functions.

During the OSCP exam, you will encounter web applications that may have IDOR vulnerabilities. Recognizing the patterns, systematically testing for them, and understanding how to exploit them will be critical for gaining unauthorized access to sensitive information or functionality. Practice identifying parameters that reference objects and then attempt to enumerate or bypass access controls. Tools like Burp Suite are invaluable for this process.