A mutating admission webhook rewrites objects before they are stored. Pointed at an attacker endpoint and scoped to pods, it injects into every new workload: an extra sidecar container, a hostPath volume, or a modified command. The injection is automatic and cluster-wide, so legitimate deployments carry the attacker's code.

# Registration (requires admissionregistration write): a MutatingWebhookConfiguration
# matching pods, pointed at the attacker's webhook service, that returns a JSONPatch
# adding a privileged sidecar to spec.containers.
kubectl get mutatingwebhookconfigurations

Exploitation notes#

• Injecting a privileged or hostPath sidecar re-establishes node access on every new pod, so the foothold regenerates faster than defenders remove it.
• Scope the webhook narrowly (namespaces, labels) to limit noise while still covering high-value workloads.
• It needs write to mutatingwebhookconfigurations and a reachable webhook endpoint; a benign configuration name helps it blend in.

References#

• Kubernetes: dynamic admission control
• Kubernetes: admission webhook good practices