Return the same error message from /login when password is incorrect and when account doesn't exist.

changelog.d/12738.misc

@@ -0,0 +1 @@
+Report login failures due to unknown third party identifiers in the same way as failures due to invalid passwords. This prevents an attacker from using the error response to determine if the identifier exists. Contributed by Daniel Aloni.

synapse/handlers/auth.py

@@ -80,6 +80,7 @@
 
 logger = logging.getLogger(__name__)
 
+INVALID_USERNAME_OR_PASSWORD = "Invalid username or password"
 
 def convert_client_dict_legacy_fields_to_identifier(
     submission: JsonDict,
@@ -1204,7 +1205,7 @@ async def validate_login(
                     await self._failed_login_attempts_ratelimiter.can_do_action(
                         None, (medium, address)
                     )
-                raise LoginError(403, "", errcode=Codes.FORBIDDEN)
+                raise LoginError(403, msg=INVALID_USERNAME_OR_PASSWORD, errcode=Codes.FORBIDDEN)
 
             identifier_dict = {"type": "m.id.user", "user": user_id}
 
@@ -1330,7 +1331,7 @@ async def _validate_userid_login(
 
         # We raise a 403 here, but note that if we're doing user-interactive
         # login, it turns all LoginErrors into a 401 anyway.
-        raise LoginError(403, "Invalid password", errcode=Codes.FORBIDDEN)
+        raise LoginError(403, msg=INVALID_USERNAME_OR_PASSWORD, errcode=Codes.FORBIDDEN)
 
     async def check_password_provider_3pid(
         self, medium: str, address: str, password: str