diff --git a/doc/man-sections/plugin-options.rst b/doc/man-sections/plugin-options.rst
index 51c574fe..9266429e 100644
--- a/doc/man-sections/plugin-options.rst
+++ b/doc/man-sections/plugin-options.rst
@@ -55,3 +55,12 @@ plug-ins must be prebuilt and adhere to the OpenVPN Plug-In API.
   (such as tls-verify, auth-user-pass-verify, or client-connect), then
   every module and script must return success (:code:`0`) in order for the
   connection to be authenticated.
+
+  **WARNING**:
+        Plug-ins may do deferred execution, meaning the plug-in will
+        return the control back to the main OpenVPN process and provide
+        the plug-in result later on via a different thread or process.
+        OpenVPN does **NOT** support multiple authentication plug-ins
+        **where more than one plugin** tries to do deferred authentication.
+        If this behaviour is detected, OpenVPN will shut down upon first
+        authentication.
diff --git a/src/openvpn/plugin.c b/src/openvpn/plugin.c
index e3a89293..51136fea 100644
--- a/src/openvpn/plugin.c
+++ b/src/openvpn/plugin.c
@@ -802,7 +802,7 @@ plugin_call_ssl(const struct plugin_list *pl,
         const char **envp;
         const int n = plugin_n(pl);
         bool error = false;
-        bool deferred = false;
+        bool deferred_auth_done = false;
 
         setenv_del(es, "script_type");
         envp = make_env_array(es, false, &gc);
@@ -824,7 +824,34 @@ plugin_call_ssl(const struct plugin_list *pl,
                     break;
 
                 case OPENVPN_PLUGIN_FUNC_DEFERRED:
-                    deferred = true;
+                    if ((type == OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY)
+                        && deferred_auth_done)
+                    {
+                        /*
+                         * Do not allow deferred auth if a deferred auth has
+                         * already been started.  This should allow a single
+                         * deferred auth call to happen, with one or more
+                         * auth calls with an instant authentication result.
+                         *
+                         * The plug-in API is not designed for multiple
+                         * deferred authentications to happen, as the
+                         * auth_control_file file will be shared across all
+                         * the plug-ins.
+                         *
+                         * Since this is considered a critical configuration
+                         * error, we bail out and exit the OpenVPN process.
+                         */
+                        error = true;
+                        msg(M_FATAL,
+                            "Exiting due to multiple authentication plug-ins "
+                            "performing deferred authentication.  Only one "
+                            "authentication plug-in doing deferred auth is "
+                            "allowed.  Ignoring the result and stopping now, "
+                            "the current authentication result is not to be "
+                            "trusted.");
+                        break;
+                    }
+                    deferred_auth_done = true;
                     break;
 
                 default:
@@ -844,7 +871,7 @@ plugin_call_ssl(const struct plugin_list *pl,
         {
             return OPENVPN_PLUGIN_FUNC_ERROR;
         }
-        else if (deferred)
+        else if (deferred_auth_done)
         {
             return OPENVPN_PLUGIN_FUNC_DEFERRED;
         }