Index: wp-includes/class-IXR.php
===================================================================
--- wp-includes/class-IXR.php	(revision 7599)
+++ wp-includes/class-IXR.php	(working copy)
@@ -571,7 +571,7 @@
     var $message;
     function IXR_Error($code, $message) {
         $this->code = $code;
-        $this->message = $message;
+        $this->message = htmlspecialchars($message);
     }
     function getXml() {
         $xml = <<<EOD