diff --git a/includes/common.inc b/includes/common.inc
index c4b6236a3777..c7025fa25ac0 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -2139,7 +2139,9 @@ function drupal_get_css($css = NULL) {
     }
 
     if ($is_writable && $preprocess_css) {
-      $filename = md5(serialize($types) . $query_string) . '.css';
+      // Prefix filename to prevent blocking by firewalls which reject files
+      // starting with "ad*".
+      $filename = 'css_' . md5(serialize($types) . $query_string) . '.css';
       $preprocess_file = drupal_build_css_cache($types, $filename);
       $output .= '<link type="text/css" rel="stylesheet" media="' . $media . '" href="' . base_path() . $preprocess_file . '" />' . "\n";
     }
@@ -2605,7 +2607,9 @@ function drupal_get_js($scope = 'header', $javascript = NULL) {
 
   // Aggregate any remaining JS files that haven't already been output.
   if ($is_writable && $preprocess_js && count($files) > 0) {
-    $filename = md5(serialize($files) . $query_string) . '.js';
+    // Prefix filename to prevent blocking by firewalls which reject files
+    // starting with "ad*".
+    $filename = 'js_' . md5(serialize($files) . $query_string) . '.js';
     $preprocess_file = drupal_build_js_cache($files, $filename);
     $preprocessed .= '<script type="text/javascript" src="' . base_path() . $preprocess_file . '"></script>' . "\n";
   }