Exploiting Kubernetes' Image Pull Implementation to Deny Node Availability

Kubernetes (K8s) has grown in popularity over the past few years to become the de-facto standard for container orchestration in cloud-native environments. While research is not new to topics such as containerization and access control security, the Application Programming Interface (API) interactions between K8s and its runtime interfaces have not been studied thoroughly. In particular, the CRI-API is responsible for abstracting the container runtime, managing the creation and lifecycle of containers along with the downloads of the respective images. However, this decoupling of concerns and the abstraction of the container runtime renders K8s unaware of the status of the downloading process of the container images, obstructing the monitoring of the resources allocated to such process. In this paper, we discuss how this lack of status information can be exploited as a Denial of Service attack in a K8s cluster. We show that such attacks can generate up to 95% average CPU usage, prevent downloading new container images, and increase I/O and network usage for a potentially unlimited amount of time. Finally, we propose two possible mitigation strategies: one, implemented as a stopgap solution, and another, requiring more radical architectural changes in the relationship between K8s and the CRI-API.