Jupyter Server is the backend for Jupyter web applications. From 2.0.0 until 2.6.0, enabling GatewayClient.url replaced the normal KernelWebsocketHandler with WebSocketChannelsHandler, which authenticated the user but did not call authorizer.is_authorized(...) for the execute action on the kernels resource. A restricted authenticated user could therefore open an upstream kernel channel and obtain kernel execution despite the configured Authorizer denying that action, exposing data and credentials available to the kernel and permitting modification or disruption of the kernel environment. The default gateway configuration is fixed in version 2.6.0. In 2.6.0 and later, the same authorization bypass remains possible when a deployment manually registers the deprecated jupyter_server.gateway.handlers module, because its handler still omits the configured kernel-execute authorization decision.

Affected: PyPI: jupyter-server

### Summary

Jupyter Server's gateway WebSocket handler checks that a user is authenticated, but it does not ask the configured Authorizer whether that user may execute kernels.

In Jupyter Server 2.0.0 through 2.5.0, enabling GatewayClient.url automatically replaced the normal kernel channel handler with this gateway-specific handler. An authenticated user who is denied the `execute` action on the `kernels` resource can therefore still open a kernel channel and send messages to the upstream kernel.

### Details

The normal handler performs the expected authorization check before creating the WebSocket connection:

https://github.com/jupyter-server/jupyter_server/blob/v2.5.0/jupyter_server/services/kernels/websocket.py#L39-L47

The gateway handler only checks `current_user` and then opens the upstream channel. It never calls `authorizer.is_authorized(...)`:

https://github.com/jupyter-server/jupyter_server/blob/v2.5.0/jupyter_server/gateway/handlers.py#L49-L75 https://github.com/jupyter-server/jupyter_server/blob/v2.5.0/jupyter_server/gateway/handlers.py#L87-L98

Gateway mode selected that handler automatically by replacing handlers with matching routes:

https://github.com/jupyter-server/jupyter_server/blob/v2.5.0/jupyter_server/serverapp.py#L437-L446

PR #1261 stopped replacing the normal handler and routed gateway traffic through `GatewayWebSocketConnection` instead. This appears to have fixed the default gateway configuration in 2.6.0, although the deprecated handler is…