What is IDOR Vulnerability? Insecure Direct Object Reference Attack Explained with Real Examples

Learn what an IDOR vulnerability is (Insecure Direct Object Reference), how attackers exploit it, real-world examples, and how to prevent it using access control and secure coding practices.

  Ethical Hacking Techniques   May 23, 2025   18  Add to Reading List

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In the modern web application landscape, security vulnerabilities pose a major risk to user data and system integrity. One such critical flaw is the IDOR vulnerability, short for Insecure Direct Object Reference. This article dives deep into what IDOR is, how it works, real-world examples, and how you can protect your applications against it.

What Is an IDOR Vulnerability?

IDOR (Insecure Direct Object Reference) is a type of access control vulnerability that occurs when an application exposes a reference to an internal object such as a file, directory, or database key, and fails to properly verify whether the user is authorized to access it.

In simple terms, it means a malicious user can manipulate references (like user IDs or file names) in a URL or request to access data they shouldn't.

How Does IDOR Work?

When an application uses predictable values—like user IDs, filenames, or account numbers—as part of a URL or parameter, and doesn't enforce authorization checks, attackers can modify these values to access other users' data.

For example: 

GET /user/profile?userID=123

If an attacker changes this to: 

GET /user/profile?userID=124

...and the server doesn't validate if the user is authorized to view userID 124, the attacker gains access to another user’s profile.

Real-World Example from the Scenario (Image)

The image provided describes the following scenario:

An attacker attempts to exploit an IDOR vulnerability by accessing the following endpoint: 

GET /restricted/accounts/?name=User1 HTTP/1.1
Host: westbank.com

Here’s what’s happening:

  • The attacker replaces their own username in the URL with the username of a valid account: User1.

  • The server does not perform proper access control verification.

  • If the vulnerability exists, the attacker can access User1’s account data.

This is a textbook example of IDOR, where direct object references (like name=User1) are not protected with proper access checks.

Other Types of IDOR Exploits

Here are a few more variations seen in real-world IDOR attacks:

1. URL Path Manipulation

GET /documents/invoice_989.pdf
GET /documents/invoice_990.pdf

Changing the path parameter allows access to someone else's invoice.

2. POST Request with Altered Parameters

POST /transfer
{
  "from_account": "user123",
  "to_account": "user999",
  "amount": "500"
}

By modifying the from_account field, an attacker may perform unauthorized transactions.

3. File Download IDOR

GET /download?fileId=6789

Changing fileId can allow access to files the user shouldn't see.

Common Causes of IDOR

  • Relying on client-side data for authorization decisions.

  • Absence of backend authorization checks.

  • Predictable object identifiers (e.g., sequential user IDs).

  • No user session binding or contextual access checks.

How to Prevent IDOR Vulnerabilities

✅ Enforce Proper Access Controls

Always verify on the server side whether the user has permission to access the requested object.

✅ Use Indirect References

Use hashed or randomized tokens instead of direct IDs (e.g., UUIDs).

✅ Implement Role-Based Access Control (RBAC)

Ensure different roles can only access what they are authorized to.

✅ Avoid Relying Solely on Client-Side Checks

Client-side validation can be bypassed. Never trust the client for access control.

✅ Conduct Security Audits and Penetration Tests

Regularly test your application using tools like:

  • OWASP ZAP

  • Burp Suite

  • Postman

  • Custom scripts

Why Is IDOR Dangerous?

IDOR can expose:

  • User profiles

  • Bank statements

  • Internal documents

  • Sensitive configurations

  • Medical records

In many breaches, IDOR was the root cause. It breaks data confidentiality and can lead to severe consequences including data leaks and compliance violations.

IDOR in the OWASP Top 10

IDOR falls under Broken Access Control, which was ranked as the #1 risk in the OWASP Top 10 - 2021 and continues to be a top concern in 2025.

How Ethical Hackers Use IDOR in Pentesting

Penetration testers often look for:

  • Parameter manipulation opportunities

  • Missing or weak access control mechanisms

  • Sensitive data exposure through enumeration

These are used to demonstrate real risks and help organizations fix them before attackers do.

Conclusion

Insecure Direct Object Reference (IDOR) is a dangerous yet often overlooked vulnerability. By understanding its mechanics and implementing secure coding and access control practices, developers can protect their applications from unauthorized data access and major breaches.

FAQs

What is an IDOR vulnerability in web security?

An IDOR vulnerability, or Insecure Direct Object Reference, is a type of security flaw where an attacker can access internal objects like files or user data by modifying a reference (like a URL parameter) without proper authorization checks.

How does an IDOR attack work?

IDOR attacks work by manipulating object identifiers (such as user IDs or filenames) in a request to access or alter resources that the attacker is not authorized to handle, exploiting missing or weak access controls.

What are real-world examples of IDOR vulnerabilities?

Real-world IDOR examples include accessing another user's profile or bank account by changing user IDs in URLs, downloading unauthorized files, or transferring money from someone else's account.

How can developers prevent IDOR vulnerabilities?

To prevent IDOR, developers should enforce server-side access controls, use indirect references like UUIDs, validate user authorization for every request, and avoid trusting client-side inputs.

Why is IDOR listed in the OWASP Top 10?

IDOR is categorized under Broken Access Control, which is the #1 risk in OWASP Top 10 due to its widespread impact and frequency in real-world applications.

What tools can detect IDOR vulnerabilities?

Tools like OWASP ZAP, Burp Suite, Postman, and manual testing can help identify IDOR vulnerabilities during security assessments and penetration testing.

Can IDOR vulnerabilities lead to data breaches?

Yes, IDOR vulnerabilities can expose sensitive data such as financial records, personal information, or medical data, leading to severe data breaches and compliance issues.

How does ethical hacking relate to IDOR?

Ethical hackers test for IDOR during penetration testing by attempting to access unauthorized resources through parameter manipulation and broken access paths.

Is IDOR still a common vulnerability in 2025?

Yes, despite awareness, IDOR remains common due to developer oversight and improper implementation of access controls, making it a persistent security threat in 2025.

What programming practices help avoid IDOR?

Secure coding practices such as role-based access control (RBAC), tokenization of identifiers, and avoiding sequential object references can help eliminate IDOR risks.

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