Insecure Direct Object References (IDOR): A Deep Dive into the #1 Vulnerability Found by Penetration Testers in 2023

Insecure Direct Object References (IDOR) vulnerabilities have been a consistent thorn in the side of web application security. In 2023, it was identified as the top vulnerability discovered by penetration testers, highlighting its prevalence and potential for exploitation. This article provides a comprehensive exploration of IDOR, its implications, and how it can be mitigated.

What is IDOR?

Insecure Direct Object References occur when an application provides direct access to objects based on user-supplied input. As a result, attackers can bypass authorisation and directly access resources in the system.

Exploitation of IDOR

Exploiting IDOR vulnerabilities can lead to unauthorized data exposure, data modification, or even deletion. In some cases, attackers can gain full control over user accounts. For instance, the infamous Facebook IDOR vulnerability allowed an attacker to delete any photo album on Facebook.

IDOR vulnerabilities have been found in various applications, from web and mobile applications to enterprise and cloud applications. For instance, the Uber IDOR vulnerability exposed personal information of Uber drivers and passengers. Another example is the Yahoo! Mail IDOR vulnerability that allowed an attacker to delete any email of Yahoo! Mail users.

Exploiting IDOR: Practical Examples

  1. Direct Reference to Database Objects: Consider a website that uses the following URL to access the customer account page, by retrieving information from the back-end database: Here, the customer number is used directly as a record index in queries that are performed on the back-end database. If no other controls are in place, an attacker can simply modify the customer_number value, bypassing access controls to view the records of other customers. This is an example of an IDOR vulnerability leading to horizontal privilege escalation.
  2. Direct Reference to Static Files: IDOR vulnerabilities often arise when sensitive resources are located in static files on the server-side filesystem. For example, a website might save chat message transcripts to disk using an incrementing filename, and allow users to retrieve these by visiting a URL like the following: In this situation, an attacker can simply modify the filename to retrieve a transcript created by another user and potentially obtain user credentials and other sensitive data.

Popular Tutorials for Exploiting IDOR

Several tutorials provide insights into exploiting IDOR vulnerabilities. For instance, the ippsec IDOR tutorial on YouTube provides a practical demonstration of exploiting IDOR. Additionally, various write-ups on platforms like HackerOne provide real-world examples of IDOR exploitation.

Mitigation Strategies

Mitigating IDOR vulnerabilities involves implementing proper access control checks, such as ensuring that the application verifies if the logged-in user is authorised to access the requested resource. Additionally, applications should avoid exposing direct references to internal resources and should use per-user or session indirect object references.

Best Practices

Several best practices can help prevent IDOR vulnerabilities. These include:

  1. Implementing proper access control checks: Ensure that the application verifies if the logged-in user is authorised to access the requested resource.
  2. Avoiding exposure of direct references to internal resources: Use per-user or session indirect object references instead.
  3. Regularly updating and patching systems: Keep your systems up-to-date to protect against known vulnerabilities.
  4. Conducting regular security audits: Regularly check your systems for vulnerabilities and fix them promptly.
  5. Changing the request method or requested file type: If one request method doesn’t work, try others. Also, try changing the file type of the requested file as it may lead to the server processing authorization differently.

Case Studies

Several case studies highlight the impact of IDOR vulnerabilities. For instance, the Uber IDOR case study provides insights into how an IDOR vulnerability can lead to significant data exposure.

Further Reading

For more information on IDOR vulnerabilities, you can refer to the following resources:

In conclusion, IDOR vulnerabilities pose a significant threat to application security. By understanding the nature of these vulnerabilities, learning how to exploit them for penetration testing purposes, and implementing robust security measures, organisations can significantly reduce their risk profile.