The Access-Control-Allow-Origin header (a key piece of the CORS protocol) plays a crucial role in determining which websites are allowed to access resources from a server. This header allows a server to specify which origins are allowed to access its resources, thereby providing a security layer to protect against unauthorized access.
However, there is a particular case where developers sometimes use the wildcard * value for the Access-Control-Allow-Origin header, which means that any origin is allowed to access the server's resources. While this might seem like a convenient shortcut, it is a particularly bad idea to use this approach. It is even more unfortunate that many documentation websites and articles geared toward developers broadly recommend this.
The primary reason why using the wildcard * value for Access-Control-Allow-Origin is not recommended is that it opens up the server to potential security risks. By allowing any website to access its resources, the server becomes vulnerable to cross-site scripting (XSS) attacks, where malicious code can be injected into the server's response and executed in the user's browser. This can result in sensitive user data being stolen or manipulated, and the attacker gaining unauthorized access to the server's resources. Moreover, when any origin is allowed to access its resources, anyone can replicate the HTML/CSS/JS that exists on a website, copy it to another domain, www.hackerwebsite.com for instance. Now, to the end user, everything not only looks the same, but it also allows them to access their information on a potentially malicious client that may not only look exactly the way they expect, but also act exactly the same.
For an example, let's say there is a server with the URL "https://example.com" that allows any origin to access its resources by setting the Access-Control-Allow-Origin header to "*". An attacker can create a fake website with the URL "https://malicious.com" and inject a script that sends a request to the server using the user's credentials.
Here are some example cURL commands that demonstrate the attack:
A legitimate request to the server as if made by a client that respects the CORS protocol (A web browser for example):
curl https://example.com/api/data \
-H 'Origin: https://example.com' \
-H 'Authorization: Bearer <access_token>'This request sends an authorization token to the server and retrieves data from the API.
An attack using the malicious website:
curl https://example.com/api/data \
-H 'Origin: https://malicious.com' \
-H 'Authorization: Bearer <access_token>'This request sends the same authorization token to the server but with a different origin, which is the malicious website. Since the server allows any origin to access its resources, the request will be accepted, and the attacker can retrieve sensitive data from the server's API.
In conclusion, using the wildcard (*) value for Access-Control-Allow-Origin is not recommended due to the potential security risks and privacy violations. Instead, developers should take the time to specify the appropriate origins that are allowed to access their server's resources, ensuring that their website is secure, compliant, and provides a good user experience.