Verify that authenticated API calls are made from our domain name.

Evil forms on other websites could otherwise potentially be configured
to have action="https://gnusocial.example/api/statuses/update.json" or
whatever. XHR is already blocked with CORS stuff.

Really, why do browsers allow cross domain POSTs at all? Sigh. The web.

lib/apiauthaction.php

@@ -85,8 +85,10 @@ class ApiAuthAction extends ApiAction
         // NOTE: $this->scoped and $this->auth_user has to get set in
         // prepare(), not handle(), as subclasses use them in prepares.
 
-        // Allow regular login session
-        if (common_logged_in()) {
+        // Allow regular login session, but we have to double-check the
+        // HTTP_REFERER value to avoid cross domain POSTing since the API
+        // doesn't use the "token" form field.
+        if (common_logged_in() && common_local_referer()) {
             $this->scoped = Profile::current();
             $this->auth_user = $this->scoped->getUser();
             if (!$this->auth_user->hasRight(Right::API)) {

lib/util.php

@@ -264,6 +264,11 @@ function common_logged_in()
     return (!is_null(common_current_user()));
 }
 
+function common_local_referer()
+{
+    return parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST) === common_config('site', 'server');
+}
+
 function common_have_session()
 {
     return (0 != strcmp(session_id(), ''));