NEW Add extension point to Director::is_site_url

src/Control/Director.php

@@ -854,6 +854,9 @@ class Director implements TemplateGlobalProvider
      * Useful to check before redirecting based on a URL from user submissions through $_GET or $_POST,
      * and avoid phishing attacks by redirecting to an attackers server.
      *
+     * Provides an extension point to allow extra checks on the URL to allow some external URLs,
+     * e.g. links on secondary domains that point to the same CMS, or subsite domains.
+     *
      * @param string $url
      *
      * @return bool
@@ -871,6 +874,13 @@ class Director implements TemplateGlobalProvider
             return true;
         }
 
+        // Allow extensions to weigh in
+        $isSiteUrl = false;
+        static::singleton()->extend('updateIsSiteUrl', $isSiteUrl, $url);
+        if ($isSiteUrl) {
+            return true;
+        }
+
         // Relative urls always are site urls
         return self::is_relative_url($url);
     }