CORS: The Gatekeeper of Web Interactions

Imagine you run a community library, and you want to ensure that only trusted members can borrow books. You have a list of people you trust, and you allow them to take books, but you don’t let just anyone walk in and grab a book. This is similar to how CORS (Cross-Origin Resource Sharing) works on the web.

What is CORS?

CORS is a security feature in web browsers that allows a website to request resources from another domain, but only if the other domain allows it. This is necessary because of the Same-Origin Policy (SOP), a security measure that restricts how a document or script loaded from one origin can interact with resources from another origin.

Same-Origin Policy

Think of SOP as a security guard who only lets people from the same neighborhood (or origin) interact. For example, if your website is hosted at example.com, it can freely interact with resources from example.com but cannot access resources from anotherdomain.com unless explicitly allowed.

How CORS Works

CORS uses HTTP headers to tell the browser whether or not to allow requests from different origins. For instance, if example.com wants to fetch data from api.example.com, the server at api.example.com can send back a header saying it’s okay (Access-Control-Allow-Origin: example.com).

Vulnerabilities in CORS Configurations

Just like leaving the library door open for anyone can lead to theft, misconfigurations in CORS can lead to security issues. Let’s explore some common vulnerabilities with real-world analogies and examples.

1. Reflecting the Origin Header

Imagine a scenario where the library decides to trust anyone who claims they’re a friend. If a malicious person says they’re from the neighborhood, they get access to the books. Similarly, some web servers reflect the Origin header they receive in the Access-Control-Allow-Origin response, effectively allowing any domain access to sensitive data.

Example: A request to vulnerable-website.com from malicious-website.com might look like this:

GET /sensitive-data HTTP/1.1
Host: vulnerable-website.com
Origin: https://malicious-website.com
Cookie: sessionid=...

If the server responds with:

HTTP/1.1 200 OK
Access-Control-Allow-Origin: https://malicious-website.com
Access-Control-Allow-Credentials: true

Now, the malicious website can access sensitive data by tricking the server into trusting it.

2. Whitelisted Origin Mistakes

Let’s say the library has a whitelist of trusted members but accidentally includes some sketchy people. In CORS terms, this happens when the whitelist is too broad or misconfigured.

Example: A vulnerability in an e-commerce site may allow any subdomain ending in trusted-site.com to be whitelisted. An attacker can register attacker.trusted-site.com and gained access to the main site’s resources.

3. Exploiting XSS with CORS

Think of a trusted library member who’s actually a thief. If your site trusts another site that has security holes, attackers can exploit those holes.

Example: A subdomain of a site might have an XSS vulnerability. Attackers can use this to inject scripts that make CORS requests, stealing sensitive data from the main site.

How to Prevent CORS-Based Attacks

Here are some practical steps to secure your web applications against CORS-related vulnerabilities:

1. Strict Origin Checking:

   • Always specify exact origins in Access-Control-Allow-Origin headers. Avoid using wildcards (*) or dynamically reflecting origins without validation.

2. Limit Trusted Origins:

   • Only allow trusted sites. Maintain a strict whitelist and regularly review it.

3. Avoid null Origin:

   • Do not use Access-Control-Allow-Origin: null unless absolutely necessary. null can be exploited by cross-origin redirects and sandboxed requests.

4. Secure Internal Networks:

   • Avoid wildcards in internal networks. Ensure internal resources are protected and cannot be accessed by browsers visiting untrusted sites.

5. Server-Side Security:

   • Remember, CORS is a browser security feature, not a substitute for server-side security. Implement strong authentication, session management, and data protection measures on your servers.

Real-World Example: Shopify (2019)

In 2019, Shopify had a misconfigured CORS policy that allowed any subdomain to access its main API. An attacker registered attacker.myshopify.com and exploited this misconfiguration to steal user data. Shopify quickly fixed the issue by tightening their CORS policy.

By understanding and properly configuring CORS, you can prevent your website from falling prey to similar vulnerabilities. Just like running a secure library, it’s about knowing who to trust and keeping a close eye on who gets access to your valuable resources.

Mischief Managed