Understanding Preflight Requests

What are preflight requests?

Preflight requests are an important security measure that help protect against cross-site request forgery (CSRF) attacks. They are a type of HTTP request that is sent by a client (which could be a browser or any other type of HTTP client), to a server in order to check whether a subsequent HTTP request is safe to send.

Preflight requests are typically used when a client (typically as a web browser) wants to send a request to a server that uses a different HTTP method or header than what is allowed by the server’s security policies. For example, a client may want to send a PUT request to a server, but the server’s security policies only allow GET and POST requests. In this case, the client would send a preflight request to the server to check whether the PUT request is allowed.

Preflight requests are usually sent as an OPTIONS request, which is an HTTP method that allows a client to retrieve information about the communication options available for a given resource. When the server receives a preflight request, it checks its security policies to determine whether the subsequent request is allowed. If the request is allowed, the server sends a response back to the client indicating that the request is safe to send. If the request is not allowed, the server sends an error response.

Preflight requests are also used in cross-origin resource sharing (CORS), which is a mechanism that allows web pages to make requests to a server in a different domain than the one that served the web page. CORS uses preflight requests to ensure that only authorized requests are sent to servers in other domains.

As a developer, it is important to understand how preflight requests work and how they can be used to ensure the safety of your resources. By implementing preflight requests, you can help protect against CSRF attacks and ensure that only authorized requests are sent to your server.

How to implement preflight requests

To implement preflight requests, you will need to configure your server to handle OPTIONS requests and to check the headers of subsequent requests to ensure that they are allowed by your security policies. Here are the general steps for implementing preflight requests:

• Configure your server to handle OPTIONS requests: Preflight requests are usually sent as OPTIONS requests, so you will need to configure your server to handle these requests. This typically involves writing a handler function that checks the headers of the request and determines whether it is allowed by your security policies.

• Check the headers of subsequent requests: When your server receives a preflight request, it should check the headers of the subsequent request to ensure that it is allowed by your security policies. If the request is allowed, your server should send a response indicating that it is safe to send. If the request is not allowed, your server should send an error response.

• Set the appropriate CORS headers: When your server sends a response to a preflight request, it should also set the appropriate CORS headers to indicate whether the subsequent request is allowed. These headers include the “Access-Control-Allow-Origin” header, which specifies the domains that are allowed to access the resource, and the “Access-Control-Allow-Methods” header, which specifies the HTTP methods that are allowed for the resource.

• Once you have implemented preflight requests on your server, it is important to test your implementation to ensure that it is working correctly. You can use a tool such as Postman or cURL to send preflight requests to your server and verify that you are receiving the expected responses.

By following these steps, you can implement preflight requests on your server and help protect against CSRF attacks and ensure that only authorized requests are sent to your resources.