diff --git a/app/Observers/WebMentionObserver.php b/app/Observers/WebMentionObserver.php
new file mode 100644
index 00000000..b15c3b3e
--- /dev/null
+++ b/app/Observers/WebMentionObserver.php
@@ -0,0 +1,65 @@
+<?php
+
+namespace App\Observers;
+
+use HTMLPurifier;
+use App\WebMention;
+use HTMLPurifier_Config;
+
+class WebMentionObserver
+{
+    /**
+     * Listen for the created event.
+     *
+     * @param  WebMention  $webmention
+     * @return void
+     */
+    public function created(WebMention $webmention)
+    {
+        $this->addFilteredHTML($webmention);
+    }
+
+    /**
+     * Listen for the updated event.
+     *
+     * @param  WebMention  $webmention
+     * @return void
+     */
+    public function updated(WebMention $webmention)
+    {
+        $this->addFilteredHTML($webmention);
+    }
+
+    /**
+     * Filter the HTML in a reply webmention.
+     *
+     * @param  WebMention  The WebMention model
+     * @return void
+     */
+    private function addFilteredHTML(WebMention $webmention)
+    {
+        $mf2 = json_decode($webmention->mf2);
+        if (isset($mf2['items'][0]['properties']['content'][0]['html'])) {
+            $mf2['items'][0]['properties']['content'][0]['html_purified'] = $this->useHTMLPurifier(
+                $mf2['items'][0]['properties']['content'][0]['html']
+            );
+        }
+        $webmention->mf2 = json_encode($mf2);
+        $webmetion->save();
+    }
+
+    /**
+     * Set up and use HTMLPurifer on some HTML.
+     *
+     * @param  string  The HTML to be processed
+     * @return string  The processed HTML
+     */
+    private function useHTMLPurifier($html)
+    {
+        $config = HTMLPurifier_Config::createDefault();
+        $config->set('Cache.SerializerPath', storage_path() . '/HTMLPurifier');
+        $purifier = new HTMLPurifier($config);
+
+        return $purifier->purify($html);
+    }
+}