HTML5: CORS Prolonged Caching of Preflight Response

Unsafe caching of the preflight response can result in unauthorized cross domain access to sensitive resources based on an invalid policy.

Cross-Origin Resource Sharing (CORS), allows a domain to define a policy for its resources to be accessed by a web page hosted on a different domain using cross domain XML HTTP Requests (XHR). Historically, the browser restricts cross domain XHR requests to abide by the same origin policy. At its basic form, the same origin policy sets the script execution scope to the resources available on the current domain and prohibits any communication to domains outside this scope. Therefore, execution and incorporation of remote methods and functions hosted on domains outside of the current domain are effectively prohibited. While CORS is supported on all major browsers, it also requires that the domain correctly defines the CORS policy in order to have its resources shared with another domain. These restrictions are managed by access policies typically included in specialized response headers, such as:

- Access-Control-Allow-Origin
- Access-Control-Allow-Headers
- Access-Control-Allow-Methods
- Access-Control-Max-Age

The browser generates a preflight OPTIONS request whenever the cross domain request made by the web page is anything other than a simple HTTP request. A GET or POST HTTP request with no special headers or credentials is considered a simple request. A response for a preflight request exposes the server's CORS policy via specialized headers mentioned previously. After examining the required permissions, the browser makes the actual request that the web page initially performed. This extra preflight request adds overhead and hence the server can configure its preflight response to be cached.
Prolonged caching of a preflight response can pose a security threat as the policy can be updated on the server while a browser will still allow unauthorized access to resources based on the original cached policy. The time a response is allowed to be cached is conveyed using an Access-Control-Max-Age response header and a value of more than 30 minutes is considered to be prolonged.