HTML5: CORS Unsafe Methods Allowed

Allowing unsafe HTTP methods such as PUT or DELETE could allow an attacker to inject malicious code, backdoor, deface site, or delete and lock resources to cause denial of service attacks.

The Access-Control-Allow-Methods header, as reflected in the preflight response for the requested resource, indicates that it allows unsafe HTTP methods. An attacker can use HTTP methods such as PUT or DELETE to make unexpected modifications to shared resource and pose a security threat to the overall site security. A user agent rejects any request for this resource with an HTTP method other than the ones that are listed in the Access-Control-Allow-Methods response header.
Cross-Origin Resource Sharing, commonly referred to as CORS, is a technology that 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