diff --git a/www/showthumb.php3 b/www/showthumb.php3
index 895981f3073c98515a6db202ec234af1a888d6ee..ac5bd90d0020143799d3912ab0fae0742db7c723 100644
--- a/www/showthumb.php3
+++ b/www/showthumb.php3
@@ -1,7 +1,7 @@
 <?php
 #
 # EMULAB-COPYRIGHT
-# Copyright (c) 2000-2003 University of Utah and the Flux Group.
+# Copyright (c) 2000-2004 University of Utah and the Flux Group.
 # All rights reserved.
 #
 include("defs.php3");
@@ -19,6 +19,9 @@ if (!isset($idx) ||
     strcmp($idx, "") == 0) {
     USERERROR("You must provide an ID.", 1);
 }
+if (!preg_match("/^\d+$/", $idx)) {
+    PAGEARGERROR("Invalid ID argument.");
+}
 
 #
 # Get the thumb from the DB.