diff --git a/src/applications/files/controller/PhabricatorFileLightboxController.php b/src/applications/files/controller/PhabricatorFileLightboxController.php
--- a/src/applications/files/controller/PhabricatorFileLightboxController.php
+++ b/src/applications/files/controller/PhabricatorFileLightboxController.php
@@ -3,6 +3,10 @@
 final class PhabricatorFileLightboxController
   extends PhabricatorFileController {
 
+  public function shouldAllowPublic() {
+    return true;
+  }
+
   public function handleRequest(AphrontRequest $request) {
     $viewer = $request->getViewer();
     $phid = $request->getURIData('phid');