diff --git a/changelog.d/12738.misc b/changelog.d/12738.misc
new file mode 100644
index 000000000..825222347
--- /dev/null
+++ b/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.
\ No newline at end of file
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index fbafbbee6..6e15028b0 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -81,6 +81,8 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+INVALID_USERNAME_OR_PASSWORD = "Invalid username or password"
+
 
 def convert_client_dict_legacy_fields_to_identifier(
     submission: JsonDict,
@@ -1215,7 +1217,9 @@ class AuthHandler:
                     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}
 
@@ -1341,7 +1345,7 @@ class AuthHandler:
 
         # 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