CKS Minimize Microservice Vulnerabilities Practice Question
You want to run a workload in a sandboxed container using gVisor. You have created a RuntimeClass named 'gvisor' that references the 'runsc' handler. Which of the following Pod specs correctly uses this RuntimeClass?
⚠ Common exam trap
The CNCF-CKS exam often tests the distinction between `runtimeClassName` (a flat string field) and incorrect nested or annotation-based approaches, as candidates may confuse it with other Pod spec fields like `nodeSelector` or `annotations`.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
apiVersion: v1 kind: Pod metadata: name: sandbox-pod spec: runtimeClassName: gvisor containers: - name: app image: nginx
The `runtimeClassName` field in the Pod spec is the standard Kubernetes API field used to specify a RuntimeClass for a Pod. The RuntimeClass named 'gvisor' must be defined in the cluster with a handler 'runsc', and setting `runtimeClassName: gvisor` in the Pod spec instructs the kubelet to use the gVisor runtime (runsc) to run the containers in a sandboxed environment, minimizing microservice vulnerabilities.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
apiVersion: v1 kind: Pod metadata: name: sandbox-pod annotations: runtimeClass: gvisor spec: containers: - name: app image: nginx
Why it's wrong here
Annotations are arbitrary metadata and do not influence runtime selection, so the Pod runs under the default runtime. It is tempting because annotations configure some Kubernetes features, and they are correct for non-scheduling metadata such as Prometheus scrape hints — but RuntimeClass selection requires the dedicated spec field.
- ✓
apiVersion: v1 kind: Pod metadata: name: sandbox-pod spec: runtimeClassName: gvisor containers: - name: app image: nginx
Why this is correct
Setting runtimeClassName: gvisor in the pod spec instructs the kubelet to select the gvisor RuntimeClass, which maps to the runsc handler and runs containers through gVisor's user-space kernel for sandboxing. This is the correct field; nodeName or annotations would not engage the sandbox runtime.
- ✗
apiVersion: v1 kind: Pod metadata: name: sandbox-pod spec: nodeSelector: runtime: gvisor containers: - name: app image: nginx
Why it's wrong here
nodeSelector matches node labels, not RuntimeClass names, so it schedules the Pod onto nodes labelled runtime: gvisor without invoking runsc. It is tempting because node selection is a legitimate scheduling mechanism, and it is correct when pinning workloads to nodes with specific hardware or taints — but it cannot select a container runtime handler.
- ✗
apiVersion: v1 kind: Pod metadata: name: sandbox-pod spec: runtimeClass: name: gvisor containers: - name: app image: nginx
Why it's wrong here
Kubernetes has no spec.runtimeClass field; RuntimeClass is selected via spec.runtimeClassName, so this manifest is rejected by the API server. It is tempting because the nested name: gvisor mirrors the RuntimeClass object's own metadata, and that structure is correct inside a RuntimeClass resource — but not within a Pod spec.
Go deeper
Related to this question
Learn chapter
Monitoring and Runtime: Threat Detection and Incident Response
Key term
Container Runtime Sandbox
A container runtime sandbox is a security boundary that isolates a container from the host system and other containers, preventing malicious or broken processes from escaping and causing harm.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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