Index: boost.module
===================================================================
--- boost.module
+++ boost.module
@@ -1564,6 +1564,13 @@
  *   File extension for this mime type
  */
 function boost_cache_set($path, $data, $extension = BOOST_FILE_EXTENSION) {
+  
+  global $base_path;
+  $files = $base_path . variable_get('file_directory_path', 'files');
+  if (strpos($_SERVER['REQUEST_URI'], $files) === 0) {
+    return FALSE;
+  }
+  
   if (empty($data)) {
     return FALSE;
   }