diff --git a/radicale/__init__.py b/radicale/__init__.py
index a036b4ef..c467ed36 100644
--- a/radicale/__init__.py
+++ b/radicale/__init__.py
@@ -275,7 +275,11 @@ class Application(object):
             user, password = self.decode(base64.b64decode(
                 authorization.encode("ascii")), environ).split(":", 1)
         else:
-            user = password = None
+            password = None
+            try:
+                user = environ["REMOTE_USER"]
+            except KeyError:
+                user = None
 
         is_authenticated = auth.is_authenticated(user, password)
         is_valid_user = is_authenticated or not user