Middleware unduly allow OPTIONS requests

[jub0bs]

Problem

Contrary to popular belief,

• not all OPTIONS requests are CORS-preflight requests, and
• OPTIONS only needs to be listed as an allowed method in the CORS configuration when the server wishes to allow clients to send actual (as opposed to preflight) OPTIONS requests, which is rare.

However, all CORS middleware produced by this library implicitly allow OPTIONS requests, even when OPTIONS isn't listed in their configuration's AllowedMethods field. As a result, those middleware unduly allow browser-based clients to send OPTIONS requests to the server.

Proof of concept

Consider the following server (borrowed from the README):

package main

import (
    "net/http"

    "github.com/rs/cors"
)

func main() {
    mux := http.NewServeMux()
    mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Content-Type", "application/json")
        w.Write([]byte("{\"hello\": \"world\"}"))
    })
    handler := cors.Default().Handler(mux)
    http.ListenAndServe(":8080", handler)
}

Note that cors.Default() allows GET, HEAD, POST, but does not allow OPTIONS.

Start the server locally, then simulate a preflight request preceding an actual OPTIONS request by running the following command:

curl -v -XOPTIONS \
  -H "Access-Control-Request-Method: OPTIONS" \
  -H "Origin: https://example.com" \
  localhost:8080

Note that OPTIONS is (but shouldn't be) listed in the response's Access-Control-Allow-Methods header:

HTTP/1.1 204 No Content
Access-Control-Allow-Methods: OPTIONS
Access-Control-Allow-Origin: *
Vary: Origin, Access-Control-Request-Method, Access-Control-Request-Headers
-snip-

Therefore, preflight would succeed and the browser would (undesirably) proceed to send the client's actual OPTIONS request.

Remediation

 // isMethodAllowed checks if a given method can be used as part of a cross-domain request
 // on the endpoint
 func (c *Cors) isMethodAllowed(method string) bool {
-       if len(c.allowedMethods) == 0 {
-               // If no method allowed, always return false, even for preflight request
-               return false
-       }
-       if method == http.MethodOptions {
-               // Always allow preflight requests
-               return true
-       }
        for _, m := range c.allowedMethods {
                if m == method {
                        return true

[jub0bs]

The proposed change is a breaking one (in terms of behaviour), but I don't expect that many CORS-aware servers wish to allow clients to send actual OPTIONS requests.