fix(framework) Avoid processing requests from non-FleetStub (#4900)

Co-authored-by: Javier <[email protected]>

src/py/flwr/server/superlink/fleet/grpc_rere/server_interceptor.py

@@ -43,9 +43,11 @@
 MAX_TIMESTAMP_DIFF = TIMESTAMP_TOLERANCE + SYSTEM_TIME_TOLERANCE
 
 
-def _unary_unary_rpc_terminator(message: str) -> grpc.RpcMethodHandler:
+def _unary_unary_rpc_terminator(
+    message: str, code: Any = grpc.StatusCode.UNAUTHENTICATED
+) -> grpc.RpcMethodHandler:
     def terminate(_request: GrpcMessage, context: grpc.ServicerContext) -> GrpcMessage:
-        context.abort(grpc.StatusCode.UNAUTHENTICATED, message)
+        context.abort(code, message)
         raise RuntimeError("Should not reach this point")  # Make mypy happy
 
     return grpc.unary_unary_rpc_method_handler(terminate)
@@ -68,7 +70,7 @@ def __init__(self, state_factory: LinkStateFactory, auto_auth: bool = False):
         self.state_factory = state_factory
         self.auto_auth = auto_auth
 
-    def intercept_service(
+    def intercept_service(  # pylint: disable=too-many-return-statements
         self,
         continuation: Callable[[Any], Any],
         handler_call_details: grpc.HandlerCallDetails,
@@ -79,6 +81,13 @@ def intercept_service(
         metadata sent by the node. Continue RPC call if node is authenticated, else,
         terminate RPC call by setting context to abort.
         """
+        # Filter out non-Fleet service calls
+        if not handler_call_details.method.startswith("/flwr.proto.Fleet/"):
+            return _unary_unary_rpc_terminator(
+                "This request should be sent to a different service.",
+                grpc.StatusCode.FAILED_PRECONDITION,
+            )
+
         state = self.state_factory.state()
         metadata_dict = dict(handler_call_details.invocation_metadata)