Index: modules/user/user.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.module,v
retrieving revision 1.887
diff -u -p -r1.887 user.module
--- modules/user/user.module	10 Jan 2008 20:22:57 -0000	1.887
+++ modules/user/user.module	15 Jan 2008 05:05:14 -0000
@@ -1965,7 +1965,8 @@ function user_comment(&$comment, $op) {
   // Validate signature.
   if ($op == 'view') {
     if (variable_get('user_signatures', 0) && !empty($comment->signature)) {
-      $comment->signature = check_markup($comment->signature, $comment->format);
+      // Be sure to filter signatures using a consistent format.
+      $comment->signature = check_markup($comment->signature, FILTER_FORMAT_DEFAULT);
     }
     else {
       $comment->signature = '';