diff --git a/conan/cli/commands/remote.py b/conan/cli/commands/remote.py
index 35cffa9b8b3..b1737fe06c1 100644
--- a/conan/cli/commands/remote.py
+++ b/conan/cli/commands/remote.py
@@ -201,10 +201,13 @@ def remote_login(conan_api, parser, subparser, *args):
     for r in remotes:
         previous_info = conan_api.remotes.user_info(r)
 
-        user, password = creds.auth(r, args.username, args.password)
-        if args.username is not None and args.username != user:
-            raise ConanException(f"User '{args.username}' doesn't match user '{user}' in "
-                                 f"credentials.json or environment variables")
+        if args.username is not None and args.password is not None:
+            user, password = args.username, args.password
+        else:
+            user, password = creds.auth(r, args.username)
+            if args.username is not None and args.username != user:
+                raise ConanException(f"User '{args.username}' doesn't match user '{user}' in "
+                                     f"credentials.json or environment variables")
 
         conan_api.remotes.user_login(r, user, password)
         info = conan_api.remotes.user_info(r)
diff --git a/conans/client/rest/conan_requester.py b/conans/client/rest/conan_requester.py
index f59fac373d0..73974a79c4e 100644
--- a/conans/client/rest/conan_requester.py
+++ b/conans/client/rest/conan_requester.py
@@ -33,11 +33,11 @@ class _SourceURLCredentials:
     """
     def __init__(self, cache_folder):
         self._urls = {}
-        self.auth_source_plugin = None
+        self._auth_source_plugin = None
         if not cache_folder:
             return
         auth_source_plugin_path = HomePaths(cache_folder).auth_source_plugin_path
-        self.auth_source_plugin = _load_auth_source_plugin(auth_source_plugin_path)
+        self._auth_source_plugin = _load_auth_source_plugin(auth_source_plugin_path)
         creds_path = os.path.join(cache_folder, "source_credentials.json")
         if not os.path.exists(creds_path):
             return
@@ -68,9 +68,9 @@ def _get_auth(credentials):
 
     def add_auth(self, url, kwargs):
         # First, try to use "auth_source_plugin"
-        if self.auth_source_plugin:
+        if self._auth_source_plugin:
             try:
-                c = self.auth_source_plugin(url)
+                c = self._auth_source_plugin(url)
             except Exception as e:
                 msg = f"Error while processing 'auth_source_remote.py' plugin"
                 msg = scoped_traceback(msg, e, scope="/extensions/plugins")
@@ -206,8 +206,8 @@ def _call_method(self, method, url, **kwargs):
                 os.environ.clear()
                 os.environ.update(old_env)
 
+
 def _load_auth_source_plugin(auth_source_plugin_path):
     if os.path.exists(auth_source_plugin_path):
         mod, _ = load_python_file(auth_source_plugin_path)
-        if hasattr(mod, "auth_source_plugin"):
-            return mod.auth_source_plugin
+        return getattr(mod, "auth_source_plugin", None)
diff --git a/conans/client/rest/remote_credentials.py b/conans/client/rest/remote_credentials.py
index d9ad7ddafdd..701af57766e 100644
--- a/conans/client/rest/remote_credentials.py
+++ b/conans/client/rest/remote_credentials.py
@@ -12,11 +12,13 @@
 from conans.errors import ConanException, scoped_traceback
 from conans.util.files import load
 
+
 class RemoteCredentials:
     def __init__(self, cache_folder, global_conf):
         self._global_conf = global_conf
         self._urls = {}
-        self.auth_remote_plugin = _load_auth_remote_plugin(HomePaths(cache_folder).auth_remote_plugin_path)
+        auth_plugin_path = HomePaths(cache_folder).auth_remote_plugin_path
+        self._auth_remote_plugin = _load_auth_remote_plugin(auth_plugin_path)
         creds_path = os.path.join(cache_folder, "credentials.json")
         if not os.path.exists(creds_path):
             return
@@ -31,15 +33,11 @@ def __init__(self, cache_folder, global_conf):
         except Exception as e:
             raise ConanException(f"Error loading 'credentials.json' {creds_path}: {repr(e)}")
 
-    def auth(self, remote, user=None, password=None):
-        if user is not None and password is not None:
-            return user, password
-
+    def auth(self, remote, user=None):
         # First get the auth_remote_plugin
-
-        if self.auth_remote_plugin is not None:
+        if self._auth_remote_plugin is not None:
             try:
-                plugin_user, plugin_password = self.auth_remote_plugin(remote, user=user)
+                plugin_user, plugin_password = self._auth_remote_plugin(remote, user=user)
             except Exception as e:
                 msg = f"Error while processing 'auth_remote.py' plugin"
                 msg = scoped_traceback(msg, e, scope="/extensions/plugins")
@@ -82,8 +80,8 @@ def _get_env(remote, user):
             ConanOutput().info("Got password '******' from environment")
         return user, passwd
 
+
 def _load_auth_remote_plugin(auth_remote_plugin_path):
     if os.path.exists(auth_remote_plugin_path):
         mod, _ = load_python_file(auth_remote_plugin_path)
-        if hasattr(mod, "auth_remote_plugin"):
-            return mod.auth_remote_plugin
+        return getattr(mod, "auth_remote_plugin", None)