diff --git a/CHANGELOG.md b/CHANGELOG.md
index a156950e..bf157773 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@
 * Improve: log client IP on SSL error and SSL protocol+cipher if successful
 * Improve: catch htpasswd hash verification errors
 * Improve: add support for more bcrypt algos on autodetection, extend logging for autodetection fallback to PLAIN in case of hash length is not matching
+* Add: warning in case of started standalone and not listen on loopback interface but trusting external authentication
 
 ## 3.4.1
 * Add: option [auth] dovecot_connection_type / dovecot_host / dovecot_port
diff --git a/DOCUMENTATION.md b/DOCUMENTATION.md
index 8b6cbcbf..7bde4cbb 100644
--- a/DOCUMENTATION.md
+++ b/DOCUMENTATION.md
@@ -506,7 +506,9 @@ RequestHeader set X-Remote-User expr=%{REMOTE_USER}
 ```
 
 > **Security:** Untrusted clients should not be able to access the Radicale
-> server directly. Otherwise, they can authenticate as any user.
+> server directly. Otherwise, they can authenticate as any user by simply
+> setting related HTTP header. This can be prevented by restrict listen to
+> loopback interface only or at least a local firewall rule.
 
 #### Secure connection between Radicale and the reverse proxy
 
diff --git a/radicale.wsgi b/radicale.wsgi
index 87c330b9..211b6c78 100644
--- a/radicale.wsgi
+++ b/radicale.wsgi
@@ -3,4 +3,8 @@ Radicale WSGI file (mod_wsgi and uWSGI compliant).
 
 """
 
+import os
 from radicale import application
+
+# set an environment variable
+os.environ.setdefault('SERVER_GATEWAY_INTERFACE', 'Web')
diff --git a/radicale/auth/__init__.py b/radicale/auth/__init__.py
index 62a7b34f..43ce953b 100644
--- a/radicale/auth/__init__.py
+++ b/radicale/auth/__init__.py
@@ -30,9 +30,10 @@ Take a look at the class ``BaseAuth`` if you want to implement your own.
 """
 
 import hashlib
+import os
 import threading
 import time
-from typing import Sequence, Set, Tuple, Union, final
+from typing import List, Sequence, Set, Tuple, Union, final
 
 from radicale import config, types, utils
 from radicale.log import logger
@@ -55,15 +56,36 @@ CACHE_LOGIN_TYPES: Sequence[str] = (
                                     "pam",
                                    )
 
+INSECURE_IF_NO_LOOPBACK_TYPES: Sequence[str] = (
+                                    "remote_user",
+                                    "http_x_remote_user",
+                                   )
+
 AUTH_SOCKET_FAMILY: Sequence[str] = ("AF_UNIX", "AF_INET", "AF_INET6")
 
 
 def load(configuration: "config.Configuration") -> "BaseAuth":
     """Load the authentication module chosen in configuration."""
-    if configuration.get("auth", "type") == "none":
-        logger.warning("No user authentication is selected: '[auth] type=none' (insecure)")
-    if configuration.get("auth", "type") == "denyall":
-        logger.warning("All access is blocked by: '[auth] type=denyall'")
+    _type = configuration.get("auth", "type")
+    if _type == "none":
+        logger.warning("No user authentication is selected: '[auth] type=none' (INSECURE)")
+    elif _type == "denyall":
+        logger.warning("All user authentication is blocked by: '[auth] type=denyall'")
+    elif _type in INSECURE_IF_NO_LOOPBACK_TYPES:
+        sgi = os.environ.get('SERVER_GATEWAY_INTERFACE') or None
+        if not sgi:
+            hosts: List[Tuple[str, int]] = configuration.get("server", "hosts")
+            localhost_only = True
+            address_lo = []
+            address = []
+            for address_port in hosts:
+                if address_port[0] in ["localhost", "localhost6", "127.0.0.1", "::1"]:
+                    address_lo.append(utils.format_address(address_port))
+                else:
+                    address.append(utils.format_address(address_port))
+                    localhost_only = False
+            if localhost_only is False:
+                logger.warning("User authentication '[auth] type=%s' is selected but server is not only listen on loopback address (potentially INSECURE): %s", _type, " ".join(address))
     return utils.load_plugin(INTERNAL_TYPES, "auth", "Auth", BaseAuth,
                              configuration)
 
diff --git a/radicale/auth/http_x_remote_user.py b/radicale/auth/http_x_remote_user.py
index 120342ab..df8324d6 100644
--- a/radicale/auth/http_x_remote_user.py
+++ b/radicale/auth/http_x_remote_user.py
@@ -2,7 +2,7 @@
 # Copyright © 2008 Nicolas Kandel
 # Copyright © 2008 Pascal Halter
 # Copyright © 2008-2017 Guillaume Ayoub
-# Copyright © 2017-2018 Unrud <unrud@outlook.com>
+# Copyright © 2017-2021 Unrud <unrud@outlook.com>
 #
 # This library is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
diff --git a/radicale/auth/remote_user.py b/radicale/auth/remote_user.py
index 98e255a0..d813da5a 100644
--- a/radicale/auth/remote_user.py
+++ b/radicale/auth/remote_user.py
@@ -2,7 +2,7 @@
 # Copyright © 2008 Nicolas Kandel
 # Copyright © 2008 Pascal Halter
 # Copyright © 2008-2017 Guillaume Ayoub
-# Copyright © 2017-2018 Unrud <unrud@outlook.com>
+# Copyright © 2017-2021 Unrud <unrud@outlook.com>
 #
 # This library is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
diff --git a/radicale/server.py b/radicale/server.py
index ed6d73a3..c07c4809 100644
--- a/radicale/server.py
+++ b/radicale/server.py
@@ -58,19 +58,7 @@ elif sys.platform == "win32":
 
 
 # IPv4 (host, port) and IPv6 (host, port, flowinfo, scopeid)
-ADDRESS_TYPE = Union[Tuple[Union[str, bytes, bytearray], int],
-                     Tuple[str, int, int, int]]
-
-
-def format_address(address: ADDRESS_TYPE) -> str:
-    host, port, *_ = address
-    if not isinstance(host, str):
-        raise NotImplementedError("Unsupported address format: %r" %
-                                  (address,))
-    if host.find(":") == -1:
-        return "%s:%d" % (host, port)
-    else:
-        return "[%s]:%d" % (host, port)
+ADDRESS_TYPE = utils.ADDRESS_TYPE
 
 
 class ParallelHTTPServer(socketserver.ThreadingMixIn,
@@ -321,20 +309,20 @@ def serve(configuration: config.Configuration,
             try:
                 getaddrinfo = socket.getaddrinfo(address_port[0], address_port[1], 0, socket.SOCK_STREAM, socket.IPPROTO_TCP)
             except OSError as e:
-                logger.warning("cannot retrieve IPv4 or IPv6 address of '%s': %s" % (format_address(address_port), e))
+                logger.warning("cannot retrieve IPv4 or IPv6 address of '%s': %s" % (utils.format_address(address_port), e))
                 continue
-            logger.debug("getaddrinfo of '%s': %s" % (format_address(address_port), getaddrinfo))
+            logger.debug("getaddrinfo of '%s': %s" % (utils.format_address(address_port), getaddrinfo))
             for (address_family, socket_kind, socket_proto, socket_flags, socket_address) in getaddrinfo:
-                logger.debug("try to create server socket on '%s'" % (format_address(socket_address)))
+                logger.debug("try to create server socket on '%s'" % (utils.format_address(socket_address)))
                 try:
                     server = server_class(configuration, address_family, (socket_address[0], socket_address[1]), RequestHandler)
                 except OSError as e:
-                    logger.warning("cannot create server socket on '%s': %s" % (format_address(socket_address), e))
+                    logger.warning("cannot create server socket on '%s': %s" % (utils.format_address(socket_address), e))
                     continue
                 servers[server.socket] = server
                 server.set_app(application)
                 logger.info("Listening on %r%s",
-                            format_address(server.server_address),
+                            utils.format_address(server.server_address),
                             " with SSL" if use_ssl else "")
         if not servers:
             raise RuntimeError("No servers started")
diff --git a/radicale/utils.py b/radicale/utils.py
index 87836a65..4683fe02 100644
--- a/radicale/utils.py
+++ b/radicale/utils.py
@@ -20,7 +20,7 @@
 import ssl
 import sys
 from importlib import import_module, metadata
-from typing import Callable, Sequence, Type, TypeVar, Union
+from typing import Callable, Sequence, Tuple, Type, TypeVar, Union
 
 from radicale import config
 from radicale.log import logger
@@ -36,6 +36,11 @@ RADICALE_MODULES: Sequence[str] = ("radicale", "vobject", "passlib", "defusedxml
                                    "pam")
 
 
+# IPv4 (host, port) and IPv6 (host, port, flowinfo, scopeid)
+ADDRESS_TYPE = Union[Tuple[Union[str, bytes, bytearray], int],
+                     Tuple[str, int, int, int]]
+
+
 def load_plugin(internal_types: Sequence[str], module_name: str,
                 class_name: str, base_class: Type[_T_co],
                 configuration: "config.Configuration") -> _T_co:
@@ -74,6 +79,17 @@ def packages_version():
     return " ".join(versions)
 
 
+def format_address(address: ADDRESS_TYPE) -> str:
+    host, port, *_ = address
+    if not isinstance(host, str):
+        raise NotImplementedError("Unsupported address format: %r" %
+                                  (address,))
+    if host.find(":") == -1:
+        return "%s:%d" % (host, port)
+    else:
+        return "[%s]:%d" % (host, port)
+
+
 def ssl_context_options_by_protocol(protocol: str, ssl_context_options):
     logger.debug("SSL protocol string: '%s' and current SSL context options: '0x%x'", protocol, ssl_context_options)
     # disable any protocol by default