Fix `OAuth2` security scheme status code on "Not authenticated" error

fastapi/security/oauth2.py

@@ -10,7 +10,7 @@ from starlette.requests import Request
 from starlette.status import HTTP_401_UNAUTHORIZED, HTTP_403_FORBIDDEN
 
 # TODO: import from typing when deprecating Python 3.9
-from typing_extensions import Annotated, Doc
+from typing_extensions import Annotated, Doc, Literal, deprecated
 
 
 class OAuth2PasswordRequestForm:
@@ -369,20 +369,56 @@ class OAuth2(SecurityBase):
                 """
             ),
         ] = True,
+        not_authenticated_status_code: Annotated[
+            Literal[401, 403],
+            Doc(
+                """
+                By default, if no HTTP Authorization header provided and `auto_error`
+                is set to `True`, it will automatically raise an`HTTPException` with
+                the status code `401`.
+
+                If your client relies on the old (incorrect) behavior and expects the
+                status code to be `403`, you can set `not_authenticated_status_code` to
+                `403` to achieve it.
+
+                Keep in mind that this parameter is temporary and will be removed in
+                the near future.
+                """
+            ),
+            deprecated(
+                """
+                This parameter is temporary. It was introduced to give users time
+                to upgrade their clients to follow the new behavior and will eventually
+                be removed.
+
+                Use it as a short-term workaround, but consider updating your clients
+                to align with the new behavior.
+                """
+            ),
+        ] = 401,
     ):
         self.model = OAuth2Model(
             flows=cast(OAuthFlowsModel, flows), description=description
         )
         self.scheme_name = scheme_name or self.__class__.__name__
         self.auto_error = auto_error
+        self.not_authenticated_status_code = not_authenticated_status_code
 
     async def __call__(self, request: Request) -> Optional[str]:
         authorization = request.headers.get("Authorization")
         if not authorization:
             if self.auto_error:
+                if self.not_authenticated_status_code == HTTP_403_FORBIDDEN:
                     raise HTTPException(
                         status_code=HTTP_403_FORBIDDEN, detail="Not authenticated"
                     )
+                else:
+                    raise HTTPException(
+                        status_code=HTTP_401_UNAUTHORIZED,
+                        detail="Not authenticated",
+                        headers={"WWW-Authenticate": "Bearer"},
+                    )
+
             else:
                 return None
         return authorization

tests/test_security_oauth2.py

@@ -56,8 +56,9 @@ def test_security_oauth2_password_other_header():
 
 def test_security_oauth2_password_bearer_no_header():
     response = client.get("/users/me")
-    assert response.status_code == 403, response.text
+    assert response.status_code == 401, response.text
     assert response.json() == {"detail": "Not authenticated"}
+    assert response.headers["WWW-Authenticate"] == "Bearer"
 
 
 def test_strict_login_no_data():

tests/test_security_status_code_403_option.py

@@ -4,6 +4,7 @@ import pytest
 from fastapi import FastAPI, Security
 from fastapi.security.api_key import APIKeyBase, APIKeyCookie, APIKeyHeader, APIKeyQuery
 from fastapi.security.http import HTTPBase, HTTPBearer, HTTPDigest
+from fastapi.security.oauth2 import OAuth2
 from fastapi.security.open_id_connect_url import OpenIdConnect
 from fastapi.testclient import TestClient
 
@@ -65,6 +66,10 @@ def test_apikey_status_code_403_on_auth_error_no_auto_error(auth: APIKeyBase):
     [
         HTTPBearer(not_authenticated_status_code=403),
         OpenIdConnect(not_authenticated_status_code=403, openIdConnectUrl="/openid"),
+        OAuth2(
+            not_authenticated_status_code=403,
+            flows={"password": {"tokenUrl": "token", "scopes": {}}},
+        ),
     ],
 )
 def test_oauth2_status_code_403_on_auth_error(auth: Union[HTTPBase, OpenIdConnect]):
@@ -95,6 +100,12 @@ def test_oauth2_status_code_403_on_auth_error(auth: Union[HTTPBase, OpenIdConnec
             openIdConnectUrl="/openid",
             auto_error=False,
         ),
+        OAuth2(
+            not_authenticated_status_code=403,
+            flows={"password": {"tokenUrl": "token", "scopes": {}}},
+            auto_error=False,
+        ),
+
     ],
 )
 def test_oauth2_status_code_403_on_auth_error_no_auto_error(