| Package | Affected versions | Patched versions |
|---|---|---|
| jupyter-server | ≥ 2.0.0, < 2.6.0 | 2.6.0 |
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.
The normal handler performs the expected authorization check before creating the WebSocket connection:
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 still present in current releases and can still be registered directly.
https://github.com/jupyter-server/jupyter_server/pull/1261
execute on kernels for a regular authenticated user.GatewayClient.url on Jupyter Server 2.0.0-2.5.0.Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeKernelWebsocketHandler would reject that user with 403 in pre_get.WebSocketChannelsHandler. Its authenticate method only rejects anonymous users, and get proceeds to the upstream gateway without an authorization decision.This is a missing authorization check, not an authentication bypass. It matters in deployments that use a restrictive custom Authorizer and Jupyter Gateway/Enterprise Gateway. A read-only or otherwise limited authenticated user can cross the intended authorization boundary and obtain kernel execution, with access to data and credentials available to that kernel.
jupyter_server.gateway.handlers module still exposes the old behavior.