diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index a99a8e79cfa..cd6ecaf589f 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -821,6 +821,10 @@ function authenticate_user_login($username, $password) {
         $auth = $user->auth;
     }
     
+    if (detect_munged_arguments($auth, 0)) {   // For safety on the next require
+        return false;
+    }
+
     if (!file_exists("$CFG->dirroot/auth/$auth/lib.php")) {
         $auth = "manual";    // Can't find auth module, default to internal
     }