What Is CORS?

What Is CORS?

 By Charles Joseph | Cybersecurity Advocate
 Last update: November 25, 2023

Imagine you’re at home (this represents your website), and you want to order a pizza (this represents the data you want from another website). Normally, for security reasons, the pizza place only delivers to the address from where the order was placed – it doesn’t want to give your pizza to just anyone.

CORS (Cross-Origin Resource Sharing) is like you giving the pizza place special instructions or permission to deliver your pizza to your neighbor’s house (another website). This way, the pizza place knows it’s okay to give your pizza (data) to that address (website).

In more technical terms, CORS is a set of rules that web developers use when they want to share data between different websites – a process that is normally not allowed because of security rules. By using CORS, developers can safely share data across different sites.

Does CORS Instruct Your Browser to Not Read Data?

When a browser sends a request to a server, the server responds with both the requested data and some additional information in the form of headers. One of these headers is the ‘Access-Control-Allow-Origin’ header, which tells the browser which origins (websites) are allowed to read the response.

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If your website’s domain isn’t included in this header, the browser will block your site from reading the response for security reasons, even though the server has technically already sent the data. This is a security measure to prevent unauthorized sites from accessing sensitive data.

Therefore, a website can only access data from another site if the other site’s server allows it through its CORS policy.

Related Questions

1. Why is CORS important in web development?

CORS is crucial in web development because it allows servers to specify who can access their resources and how they can be accessed. Without CORS, a web application could be at risk because it wouldn’t be able to share resources with other domains. This is particularly important in today’s interconnected world of web apps, where resources are often shared across different domains.

2. What happens if a server doesn’t implement CORS?

If a server doesn’t implement CORS, then browsers will enforce the same-origin policy. This policy prevents a web page from making requests to a different domain than the one the web page came from. If the server doesn’t use CORS to tell the browser it’s okay to access resources, the browser will block the request.

3. How do you enable CORS?

Enabling CORS involves adding specific headers to HTTP responses from your server. The most common header is ‘Access-Control-Allow-Origin,’ which specifies the domains allowed to access resources. In Node.js, for example, you can use middleware like ‘cors’ to easily add these headers.

4. Can CORS prevent CSRF attacks?

While CORS is designed to allow more flexibility with requests from different origins, it doesn’t inherently prevent Cross-Site Request Forgery (CSRF) attacks. CORS and CSRF are two different concepts addressing different security concerns. To prevent CSRF attacks, you would need to implement additional security measures like synchronizer token patterns or same-site cookies.

5. Is it safe to enable CORS for all domains?

Enabling CORS for all domains can pose a security risk. By doing so, you’re allowing any website to make requests to your server and access your data, which can expose sensitive information to malicious websites. It’s best to only enable CORS for specific, trusted domains.

What Is CORS? (Video)

"Amateurs hack systems, professionals hack people."
-- Bruce Schneier, a renown computer security professional