CORS Misconfigurations: How Poor Policies Open Doors to Exploits

Modern web applications rely on APIs and interactions between different domains. To allow such interactions, browsers enforce the Same-Origin Policy (SOP), which prevents one domain from accessing resources from another unless explicitly permitted.

This is where Cross-Origin Resource Sharing (CORS) comes in. CORS is a mechanism that enables web servers to specify which origins are allowed to access their resources. When properly configured, it ensures secure cross-domain communication.

But here’s the catch: misconfigured CORS policies can open dangerous vulnerabilities, allowing attackers to steal sensitive information, bypass access controls, or perform unauthorized actions.

In this guide, we’ll explore CORS misconfigurations, how attackers exploit them, and whether they’re still worth hunting for in today’s landscape.

When a browser makes a cross-origin request (e.g., fetching data from api.example.com from app.example.com), it sends a preflight request using the HTTP OPTIONS method. This request asks the server:

If the server responds with a CORS header allowing the origin, the browser processes the request. Otherwise, the request is blocked.

1. Wildcard (*) in Access-Control-Allow-Origin

A wildcard allows any domain to access resources. While useful for public APIs, it becomes a security risk if sensitive data is exposed.

Example:

Impact:

2. Echoing Origin in Access-Control-Allow-Origin

Some servers dynamically reflect the Origin header in the response without validation.

Example:

Impact:

Attackers can craft requests from malicious origins, and the server will allow them access, bypassing the Same-Origin Policy.

3. Allowing Credentials with Wildcards or Any Origin

When Access-Control-Allow-Credentials: true is enabled, browsers allow cookies, tokens, or other credentials to be included in cross-origin requests. Combining this with a wildcard or insecure Access-Control-Allow-Origin creates a severe vulnerability.

Example:

Impact:

Attackers can exploit this to perform Cross-Site Request Forgery (CSRF) or steal sensitive user data.

If you’ve been bug hunting or pentesting for a while, chances are you’ve encountered CORS misconfigurations a few times. You’ve likely seen blog posts or proof-of-concepts showcasing how easily exploitable these vulnerabilities can be.

But let me tell you, I’ve been in your shoes: after the first few attempts to exploit CORS vulnerabilities with no success, it’s easy to lose motivation and start ignoring them altogether.

The truth is, many CORS misconfigurations these days are mitigated by other security layers, like strong authentication tokens or server-side validations. But that doesn’t mean this vulnerability isn’t worth your time.

Here’s why:

1️⃣ High Impact Potential: If you find one of the misconfigurations mentioned earlier , especially those involving credentials , it can lead to massive data exposure or full account compromise.

2️⃣ Easily Overlooked by Developers: Many devs misunderstand how to securely configure CORS policies, making it a ripe area to test.

3️⃣ Unexpected Chains: Even if you can’t exploit a CORS misconfiguration alone, it can be chained with other vulnerabilities (e.g., IDOR or information disclosure) for a more impactful report.

My Advice: Don’t underestimate CORS. If you spot a wildcard or reflective policy, test it thoroughly, because the one time it works, the reward will be worth the effort.

2. Nuclei: Use Nuclei templates to detect insecure CORS policies.

3. Burp Suite:

1. Avoid Wildcards

2. Validate Origins Dynamically

3. Restrict Credentials Usage

4. Minimize Allowed Methods and Headers

5. Disable CORS on Sensitive Endpoints

CORS misconfigurations might not always seem exploitable, but when they are, the impact can be massive. As a bug hunter, don’t overlook these vulnerabilities; they can lead to sensitive data exposure or critical attack chains.

For developers, the lesson is simple: review your CORS policies carefully and adopt strict controls to avoid unnecessary risks.

💬 Have you ever exploited or fixed a CORS misconfiguration? Let’s discuss below!