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IDOR - Insecure Direct Object Reference

Some vulnerabilities are loud.
IDOR is quiet. It doesn’t crash servers or inject code.
It simply lets you ask for something you shouldn’t have and get it.

What this page covers

  • What IDOR is: the concept, the mechanics, and the impact
  • How to exploit it: patterns, examples, and testing strategies
  • How to defend against it: access control, validation, and design principles
  • How to detect it: signs, tools, and manual techniques
  • How I’ve practiced it: labs, notes, and real-world scenarios

This isn’t just about broken access.
It’s about understanding how trust is assigned, and how it can be abused.

What IDOR Actually Is

Insecure Direct Object Reference (IDOR) occurs when an application exposes internal object identifiers like user IDs, document IDs, or filenames and fails to properly check whether the requester is authorized to access them.

The Core Idea:

A user can directly reference an object they shouldn’t have access to, simply by changing an identifier in the request.

Common Examples

  • Changing user_id=123 to user_id=124 and accessing another user’s profile
  • Downloading someone else’s file by modifying a URL like /files/2023-0001.pdf
  • Viewing private messages by guessing message IDs in an API call
  • Accessing admin-only resources by tweaking a parameter or header

IDOR is often found in:

  • REST APIs
  • File download endpoints
  • Account management panels
  • Mobile apps with weak server-side checks

What Happens When It’s Exploited

  • Unauthorized access to sensitive data
  • Exposure of personal information (PII, financial records, medical data)
  • Account takeover (if combined with other flaws)
  • Business logic abuse (modifying orders, deleting resources, impersonating users)

IDOR doesn’t need fancy payloads.
It just needs a developer who forgot to ask, “Should this user be allowed to see that?”

How to Mitigate IDOR

1. Enforce Access Control Server-Side

Never rely on client-side logic to determine access.
Always check permissions on the server before returning data.

2. Use Indirect References

Instead of exposing raw IDs, use opaque tokens or UUIDs that are hard to guess.

3. Validate Ownership

For every sensitive object, confirm that the requester owns it or has permission to access it.

4. Log and Monitor Access Patterns

Unusual access sequences or ID enumeration can signal abuse.

5. Apply Role-Based Access Controls (RBAC)

Define clear roles and permissions, and enforce them consistently across endpoints.

Mitigation is about asking the right question:
Not “does this object exist?” but “should this user be allowed to access it?”

How I Practice IDOR

I look for:

  • Predictable identifiers in URLs, headers, or bodies
  • API endpoints that return too much data
  • Mobile apps that expose internal IDs
  • Lack of authorization checks in server responses

I test using:

  • Burp Suite (Repeater and Intruder)
  • Manual fuzzing and ID manipulation
  • Labs like Hack The Box, TryHackMe, and PortSwigger Academy

Every time I find an IDOR, I learn something about how trust is assumed and how fragile that assumption can be.

Final Thought

IDOR is simple.
That’s what makes it dangerous.
It hides in plain sight, waiting for someone curious enough to change a number.

This page is my study of that curiosity.
Not just how to exploit it, but how to recognize it, prevent it, and explain why it matters.