Insecure Direct Object Reference (IDOR) - A Deep Dive

Insecure Direct Object Reference (IDOR) is a critical security flaw often found in web applications, leading to unauthorized data access. This comprehensive guide delves into the nature of IDOR, its methods of exploitation, real-world examples, and effective remediation strategies.

Broken access controls are among the topmost application security threats that could heavily impact the organization and its user’s data confidentiality and integrity. One of the categories of broken access controls, widely known as Insecure Direct Object References (IDOR), has emerged as a significant concern for organizations and security professionals. This blog aims to demystify IDOR attacks, their occurrence, common attack techniques, remediation strategies, and their potential impact on systems and data integrity.

The Insecure Direct Object Reference (IDOR) is mainly found in Web Applications and APIs (including Mobile applications, Thick Client applications, and any other back-end API communication) that arise when an application uses user-supplied input to access objects directly. This could be anything from database records to files on a server. In IDOR attacks, attackers exploit these references to access data they are not authorized to, leading to unauthorized data exposure or modification.

The implications of IDOR attacks can be severe, ranging from data breaches exposing sensitive user information to unauthorized modifications of system data. It's crucial to note that IDOR vulnerabilities often result from inadequate security practices during application development and design phases.

How an IDOR Attack Happens

IDOR attacks typically follow a predictable pattern, starting with the attacker discovering a direct reference to an internal implementation object. The attacker then manipulates this reference to access unauthorized data. The process usually involves two primary steps: detection and exploitation.

1. Detection: Attackers first identify points in the application where user input, such as URL parameters or form fields, is used to reference internal objects. This is often achieved through techniques like fuzzing or observing the application's behavior in response to modified inputs.
2. Exploitation: Once a potential IDOR vulnerability is identified, the attacker alters the object reference to probe different values. For instance, changing a numeric user ID in a URL could reveal information about other users if the application lacks proper authorization checks.

The simplicity of IDOR attacks lies in their reliance on modifying visible inputs, making them accessible to a wide range of attackers, from amateur hackers to sophisticated threat actors.

Common Attack Techniques and Examples

IDORs are logical vulnerabilities, and there are no limited methods to find and exploit these. Based on the application’s logic and implementation, one can find a unique way to exploit and get a successful IDOR vulnerability. However, there are some common ways to start looking for the vulnerability, as explained below:

1. ID Parameter Manipulation: This is a common form of IDOR attack. It involves altering parameters referencing internal objects, such as database keys or file names. For example, if a URL on a website is “example.com/user?ID=123”, an attacker might change “ID=123” to “ID=124” to attempt to access or modify another user's information.
2. Direct Request: This occurs when an attacker directly accesses a restricted resource or endpoint. For example, if “example.com/admin” is only for administrators, an attacker might try accessing this URL directly with an unauthenticated or limited privileged user session. If the application is vulnerable, the attack will be able to access (view or edit) the endpoint.
3. Mass Assignment: Mass assignment is a vulnerability that occurs when an application automatically assigns user input to object properties without adequate filtering. For instance, if a user object has attributes like “username”, “password”, “email”, and “isAdmin”, an attacker could potentially modify “isAdmin=true” through a request, gaining unauthorized privileges. Similarly, adding parameters like “credit=10000” during a user update operation could unintentionally adjust a user's credit balance if the application doesn't properly filter input fields.
4. Token Prediction: If object reference tokens (like session IDs) are predictable, attackers can guess them to gain unauthorized access.
5. File Path Traversal: Attackers manipulate variables that reference files or directories (e.g., changing “filePath=/user/data” to “filePath=/etc/passwd”).
6. API Endpoint Enumeration: Trying different API endpoints to discover unsecured ones.

The above-mentioned use cases are a few examples of potential IDOR identification and exploitation techniques. Based on the application’s logic and access matrix, an attacker could find more ways to exploit.   

Vulnerable Code Snippets and Remediation

Example 1: URL Parameter Reference

Vulnerable Code:

userID = request.getParameter("id")
userDetails = database.getUserDetails(userID)

Issue: This code directly uses user input to fetch user details without verifying if the user can access the specified ID.

Non-Vulnerable Code:

userID = request.getParameter("id")
if (currentUser.hasAccessTo(userID)):
    userDetails = database.getUserDetails(userID)

Fix: Implement access control checks to verify if the requesting user has the right to access the data.

Example 2: File Access

Vulnerable Code:

var file = request.query.file;
res.sendFile(`/uploads/${file}`);

Issue: The code directly uses the file parameter from the user input, allowing an attacker to access any file.

Remediated Code:

var file = request.query.file;
if (currentUser.isAuthorizedFile(file)):
    res.sendFile(`/uploads/${file}`);

Fix: Add authorization checks to validate whether the user can access the requested file.

Example 3: API Endpoint

Vulnerable Code:

@GetMapping("/user/{userId}/details")
public UserDetails getUserDetails(@PathVariable String userId) {
    return userService.getUserDetails(userId);
}

Issue: The API endpoint directly exposes user details based on the input userId without any access control.

Remediated Code:

@GetMapping("/user/{userId}/details")
public UserDetails getUserDetails(@PathVariable String userId) {
    if (currentUser.isAuthorized(userId)) {
        return userService.getUserDetails(userId);
    }
    throw new UnauthorizedAccessException();
}

Fix: Implement authorization checks to verify if the logged-in user can access the requested user details.

Impact of IDOR

• Data Breaches: Unauthorized access to sensitive data, leading to privacy violations and regulatory implications.
• Data Manipulation: Alteration of critical data, affecting the integrity of the application.
• Loss of Trust: Damage to the organization's reputation, resulting in loss of user trust and potential financial repercussions.

Case Study

A recent example is the CVE-2023-4836 vulnerability in the User Private Files plugin for WordPress. This vulnerability allowed attackers, even with minimal privileges, to craft specialized requests to access and download files from other users' folders. The risks included accessing sensitive files, downloading files without consent, and exposing confidential information.

Remediation of IDOR

1. Implement Proper Access Control: Ensure robust access control checks for each user request.
2. Avoid Direct Object References: Replace direct object references with indirect methods, like reference maps or hashing.
3. Use GUIDs or Random Identifiers: Employ globally unique or random identifiers to make enumeration difficult.
4. Validate User Input: Strictly validate user-supplied parameters for proper length and format.

Conclusion

IDOR attacks present significant security risks, but understanding how they work, recognizing common examples, and implementing effective prevention strategies can significantly reduce these vulnerabilities. Regular updates, robust access controls, avoiding direct object references, using GUIDs, validating user inputs, and education are key to protecting against these security threats.