Description: Preflight is an initial request sent by the browser to determine if the actual request is safe to send. This mechanism is part of the CORS (Cross-Origin Resource Sharing) protocol, which allows browsers to make requests to domains different from the one being viewed. The Preflight request is sent using the HTTP OPTIONS method and is primarily used to check if the server allows the operation to be performed, especially in the case of non-simple requests, such as those using HTTP methods other than GET or POST, or that include custom headers. Upon receiving a Preflight request, the server responds with information about CORS policies, indicating whether the actual request can be processed. This process is crucial for web security, as it helps prevent attacks like Cross-Site Request Forgery (CSRF) and ensures that resources are only accessible by authorized domains.
History: The concept of Preflight requests was introduced with the CORS specification in 2010, as part of an effort to improve web security. Before CORS, cross-origin requests were restricted by the same-origin policy, which limited interaction between resources from different domains. With the advent of CORS, more flexible and secure mechanisms were established to allow these interactions, and Preflight requests became an essential tool for validating server access policies.
Uses: Preflight requests are primarily used in web applications that require interaction with APIs from different origins. This is common in web applications that use various frameworks, where multiple requests are made to external servers. They are also useful in situations where sensitive data is sent or advanced HTTP methods are used, as they allow developers to ensure that the server is configured to accept such requests.
Examples: A practical example of a Preflight request is when a web application attempts to send data to a server using the PUT method and a custom header. Before sending the actual request, the browser sends an OPTIONS request to the server to check if the PUT method and the custom header are allowed. If the server responds positively, the actual request is sent. Another example is when an application tries to access a third-party API that requires authentication and specific methods, where the Preflight request ensures that access is permitted.
