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;
+    }
+}
+

library/HTMLPurifier/ConfigSchema/schema/HTML.TargetNoreferrer.txt

@@ -0,0 +1,9 @@
+HTML.TargetNoreferrer
+TYPE: bool
+VERSION: 4.8.0
+DEFAULT: TRUE
+--DESCRIPTION--
+If enabled, noreferrer rel attributes are added to links which have
+a target attribute associated with them.  This prevents malicious
+destinations from overwriting the original window.
+--# vim: et sw=4 sts=4

library/HTMLPurifier/HTMLModule/TargetNoreferrer.php

@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * Module adds the target-based noreferrer attribute transformation to a tags.  It
+ * is enabled by HTML.TargetNoreferrer
+ */
+class HTMLPurifier_HTMLModule_TargetNoreferrer extends HTMLPurifier_HTMLModule
+{
+    /**
+     * @type string
+     */
+    public $name = 'TargetNoreferrer';
+
+    /**
+     * @param HTMLPurifier_Config $config
+     */
+    public function setup($config) {
+        $a = $this->addBlankElement('a');
+        $a->attr_transform_post[] = new HTMLPurifier_AttrTransform_TargetNoreferrer();
+    }
+}

library/HTMLPurifier/HTMLModuleManager.php

@@ -271,6 +271,11 @@ class HTMLPurifier_HTMLModuleManager
         if ($config->get('HTML.TargetBlank')) {
             $modules[] = 'TargetBlank';
         }
+        // NB: HTML.TargetNoreferrer must be AFTER HTML.TargetBlank
+        // so that its post-attr-transform gets run afterwards.
+        if ($config->get('HTML.TargetNoreferrer')) {
+            $modules[] = 'TargetNoreferrer';
+        }
 
         // merge in custom modules
         $modules = array_merge($modules, $this->userModules);