Index: modules/forum/forum.pages.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/forum/forum.pages.inc,v
retrieving revision 1.3
diff -u -p -r1.3 forum.pages.inc
--- modules/forum/forum.pages.inc	9 Oct 2009 00:59:59 -0000	1.3
+++ modules/forum/forum.pages.inc	4 Dec 2009 15:20:32 -0000
@@ -10,6 +10,13 @@
  * Menu callback; prints a forum listing.
  */
 function forum_page($tid = 0) {
+
+  // Ensure $tid is an integer.
+  $tid = filter_var($tid, FILTER_VALIDATE_INT, array("options" => array('min_range' => 0)));
+  if ($tid === FALSE) {
+    return MENU_NOT_FOUND;
+  }
+
   $topics = '';
   $forum_per_page = variable_get('forum_per_page', 25);
   $sortby = variable_get('forum_order', 1);