Jupyter Enterprise Gateway: ContainerProcessProxy._enforce_prohibited_ids Bypass

QFinch assessment

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=1L103-L107 https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/docs/source/operators/config-add-env.md?plain=1L88-L92 https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/docs/source/operators/deploy-kubernetes.md?plain=1L769

Potential 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.

Technical context

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.pyL29-L30 The checks https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprisegateway/services/processproxies/container.pyL113 and https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprisegateway/services/processproxies/container.pyL119 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.j2L35 and https://github.com/jupyter-server/enterprisegateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/etc/kernel-launchers/kubernetes/scripts/kernel-pod.yaml.j2L38 where they are parsed as an integer in the Jinja2 template - which will ignore the whitespace.