Insecure Direct Object References (IDOR): Exploiting and Preventing

mccleod1290

Introduction: When Simplicity Betrays Security

IDORs represent dangerous security defects which exist between convenience features and vulnerabilities in modern web application development. A descriptive analysis of a real-life IDOR vulnerability will demonstrate how an innocent chat transcript allowed unauthorized access to sensitive user passwords.

We will analyze both essential technical aspects of the weak point and investigate the underlying reasons behind simple security flaws that continue to exist and provide strategies for creating effective defense mechanisms..

Black-box Testing

Lab URL - https://app.hackthebox.com/challenges/Kryptos%2520Support

At first glances, we note that we are not given source code for this lab. So we take an peak and we got an login page and a portal to enter some details. So quickly our instincts kick, what if we can manipulate the portal and steal cookies of some user? That’s precisely what we are doing at the first part of this challenge. You can read more about stealing cookies with xss online, or refer to previous series in this web pentest blog here . Alternatively you can also take an peek at their official writeup.

Once you got intial access, look around in the settings and you see an option to reset the password.

Now if you look the request behind the reset, it is not validated with any token, and we find uid parameter. What if we change the value?

You can fuzz the uid values, using burp intruder, we see that with uid value as 1 we can change the password for admin user.

And with that we should get our flag.

Grey-box Testing

Unlike the previous blogs, we had white box testing, since online I could not find any (yes I am aware of ctfs on ctf time that post web challenges with source code, but I don’t have access to any of it’s source code or too lazy to find and set up from online, if you are interested you can very well do that as well), but I have found another way, a portswigger lab that kinda explains web application via javascript. Even though this is an apprentice lab (easy one) there is still a lot to learn from and the first two times I solved this lab, I overlooked these tiny details. As a pentester it's our duty to report such trivial findings

The Discovery: Patterns in the Digital Fabric

Security testing often begins with observation - looking for patterns that might reveal underlying weaknesses in an application’s architecture. During a recent engagement, the application’s proxy history revealed an intriguing endpoint:

/download-transcript/{ID}.txt

What made this endpoint immediately suspicious was its adherence to the classic IDOR pattern - a resource accessed via a simple numeric parameter that appeared to be an internal identifier. This pattern often suggests direct mapping between the URL parameter and backend database records, which represents a potential security boundary that can be tested.

Vulnerability Analysis: Peeling Back the Layers

The Technical Mechanics

At its core, this vulnerability stemmed from a fundamental architectural flaw: the application permitted users to access resources through direct references to internal implementation objects - in this case, transcript files - without implementing proper authorization checks.

Let’s break down the vulnerable request pattern:

GET /download-transcript/2.txt HTTP/2
Host: 0a25009004c1db6081ed89ff001e0026.web-security-academy.net
Cookie: session=kp1BS7J2wYMZPvEWYVEGbIYnB4ktSHz5
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/135.0.0.0 Safari/537.36
Accept: */*

The critical vulnerability lies in how the application processes the identifier “2” in the URL path. This numeric ID serves as a direct reference to a specific transcript file stored on the server. The application retrieves the requested file based solely on this identifier, without verifying whether the currently authenticated user should have access to it.

The Exploitation Journey

My investigation followed a methodical approach:

Observation: Identified the /download-transcript/2.txt endpoint in the HTTP history, which showed a pattern suggestive of direct object referencing.(The http request which got highlighted in blue.)

Hypothesis Formation: Theorized that changing the numeric ID might allow access to other users’ transcripts if proper authorization checks were absent.
Controlled Testing: Created a repeater tab in Burp Suite to test accessing /download-transcript/1.txt, maintaining the same session cookie but altering only the resource identifier.
Vulnerability Confirmation: Successfully retrieved the transcript with ID “1”, which belonged to a different user and contained sensitive information, including a password.

The successful retrieval of this unauthorized transcript confirmed the vulnerability. The application failed to validate whether the requesting user had legitimate rights to access the resource identified by “1”, instead blindly retrieving and returning whatever data was associated with that identifier.

Technical Deep Dive: The Anatomy of the Vulnerability

To understand this vulnerability at a deeper level, let’s examine the client-side implementation that interacts with this endpoint. This provides crucial insight into how the vulnerability manifests from end to end.

From the above image, we got two javascript files, one is chat.js and another is viewTranscript.js, let’s take an peak at viewTranscript.js. This JavaScript function in the application revealed the transcript download mechanism:

function viewTranscript(downloadTranscriptPath) {
    var chatForm = document.getElementById("chatForm");
    var viewTranscriptButton = document.createElement("button");
    viewTranscriptButton.setAttribute("id", "download-transcript");
    viewTranscriptButton.setAttribute("class", "button");
    viewTranscriptButton.innerText = "View transcript";
    viewTranscriptButton.onclick = viewTranscript;
    
    chatForm.appendChild(viewTranscriptButton)
    
    function viewTranscript() {
        var chatArea = document.getElementById("chat-area");
        var messages = chatArea.getElementsByClassName("message")
        var transcript = []
        for (var i = 0; i < messages.length; i++) {
           var message = messages.item(i)
           transcript.push(message.getElementsByTagName("th").item(0).innerText + " " + message.getElementsByTagName("td").item(0).innerText)
        }
        
        var xhr = new XMLHttpRequest();
        xhr.onload = function() {
            window.location = xhr.responseURL;
        }
        xhr.open("POST", downloadTranscriptPath);
        data = new FormData();
        data.append("transcript", transcript.join("<br/>"));
        xhr.send(data);
    }
};

This function reveals several critical details, and we draw the following conclusions:

The application collects chat messages from the DOM and sends them as a transcript to the server via a POST request
The server processes this data and returns a redirect to a URL where the transcript can be downloaded
The redirect leads to the /download-transcript/{ID}.txt endpoint, where the ID is assigned by the server

The vulnerability exists due to the following reasons:

The server generates sequential or predictable IDs for transcripts
When users request a transcript file, the application fails to verify ownership
The server blindly trusts the ID parameter, assuming that the request is legitimate if it contains a valid session cookie

Exploitation Refinement: From Discovery to Impact

From Theory to Proof

Having identified the vulnerability, I systematically verified its severity:

Initial access: Successfully accessed /download-transcript/1.txt despite it belonging to another user

Content analysis: The transcript contained sensitive information including:
- Conversation details
- A user password (the most critical finding)
- Potentially other sensitive data exchanged during chat sessions

Pattern analysis: The sequential nature of the IDs (1, 2, 3…) suggested all transcripts might be accessible, indicating a widespread access control failure.

Suggested Remediation Strategies:

Addressing IDOR vulnerabilities requires a multi-layered approach:

Immediate Tactical Fixes

Implement Resource-Level Authorization Checks

    function downloadTranscript(transcriptId, currentUser) {
        // Retrieve the transcript object
        transcript = getTranscriptById(transcriptId)
        
        // Verify ownership or permission
        if (transcript.owner != currentUser.id && !currentUser.isAdmin) {
            return AccessDeniedError
        }
        
        // Proceed with download if authorized
        return transcript.content
    }

Use Indirect Reference Maps Instead of exposing internal IDs directly, implement a mapping layer:

    // Server-side map (stored in session)
    userResourceMap = {
        "a7f392e": "transcript_2"  // User-specific mapping
    }
    
    // URL becomes /download-transcript/a7f392e
    // Server translates the reference before retrieving the resource

References:

IDOR hunting Checklist

Note that this list is not exhaustive and do take it from grain of salt as it comes from previous ctf experiences and vulnerable labs.

1. Check for any ID value in URL
2. Check for any guessable or numbers that can be altered in URL or in any URL parameters.
3. Check how request on the web application get’s processed in burp suite and see if you can change any numeric values.
4. Don’t forget to check cookies, sometimes they might be encoded with base64, or any base or hex values, sometimes understanding how these cookies are applied on web browser might allow us to create our own cookies for users like admin or administrator.
5. Chain IDOR with other vulnerabilities. (Often some other vulnerability like stealing cookie or authentication bypass using SQL Injection often leads us to an endpoint that is vulnerable to IDOR)

Conclusion

We have explored the IDOR vulnerability to demonstrate that simple implementation should always be coupled with stringent authorization measures for web security. IDOR vulnerabilities present two dangerous characteristics because they combine simple conceptual nature with devastating impact. Such vulnerabilities cause severe data breaches because hackers need no advanced exploit techniques nor specific tech expertise to take advantage of them.

IDOR vulnerabilities can only be properly addressed through the combination of strong authorization controls and indirect reference maps, unpredictable identifiers and deep defense security measures. Security requires proper foundation instead of complex algorithms or sophisticated defense systems to succeed. The comprehension of these security flaws together with suitable protection methods enables us to develop systems that secure user-sensitive information against virtual attackers concealed within electronic frameworks.

At the end of the day, major application security vulnerabilities emerge from basic human mistakes yet cause the most extensive damage.