diff --git a/modules/filter/filter.module b/modules/filter/filter.module
index e603b254..30f442bd 100644
--- a/modules/filter/filter.module
+++ b/modules/filter/filter.module
@@ -1116,7 +1116,13 @@ function _filter_xss_attributes($attr) {
         // Attribute name, href for instance
         if (preg_match('/^([-a-zA-Z]+)/', $attr, $match)) {
           $attrname = strtolower($match[1]);
-          $skip = ($attrname == 'style' || substr($attrname, 0, 2) == 'on');
+          $skip = (
+            $attrname == 'style' ||
+            substr($attrname, 0, 2) == 'on' ||
+            substr($attrname, 0, 1) == '-' ||
+            // Ignore long attributes to avoid unnecessary processing overhead.
+            strlen($attrname) > 96
+          );
           $working = $mode = 1;
           $attr = preg_replace('/^[-a-zA-Z]+/', '', $attr);
         }