Add %HTML.TargetNoreferrer, which adds rel="noreferrer" when target attribute is set

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

library/HTMLPurifier/AttrTransform/TargetNoreferrer.php

@@ -0,0 +1,37 @@
+<?php
+
+// must be called POST validation
+
+/**
+ * Adds rel="noreferrer" to any links which target a different window
+ * than the current one.  This is used to prevent malicious websites
+ * from silently replacing the original window, which could be used
+ * to do phishing.
+ * This transform is controlled by %HTML.TargetNoreferrer.
+ */
+class HTMLPurifier_AttrTransform_TargetNoreferrer extends HTMLPurifier_AttrTransform
+{
+    /**
+     * @param array $attr
+     * @param HTMLPurifier_Config $config
+     * @param HTMLPurifier_Context $context
+     * @return array
+     */
+    public function transform($attr, $config, $context)
+    {
+        if (isset($attr['rel'])) {
+            $rels = explode(' ', $attr['rel']);
+        } else {
+            $rels = array();
+        }
+        if (isset($attr['target']) && !in_array('noreferrer', $rels)) {
+            $rels[] = 'noreferrer';
+        }
+        if (!empty($rels) || isset($attr['rel'])) {
+            $attr['rel'] = implode(' ', $rels);
+        }
+
+        return $attr;
+    }
+}
+