Demonstrating Kubernetes API Server Unauthorized Access Against K3s Clusters Through Authentication-Control Testing and Privilege-Boundary Validation

Modern containerized environments use Kubernetes API servers as the central control interface for managing workloads, services, configurations, namespaces, and cluster resources. K3s provides a lightweight Kubernetes distribution suitable for controlled laboratory deployments and security validation.

The Kubernetes API server is a security-sensitive control-plane component because authenticated requests can perform operations according to the permissions associated with the requesting identity. If authentication controls are incorrectly configured or an identity receives broader permissions than required, an unauthorized user may obtain access to Kubernetes resources beyond the intended security boundary.

In this use case, a controlled K3s Kubernetes cluster is deployed on Ubuntu Linux inside an isolated VirtualBox laboratory. A dedicated Kubernetes test identity is created with a deliberately restricted permission set, while a separate controlled security-testing identity is used to validate unauthorized-access conditions.

Kali Linux is used as the authorized penetration-testing platform. The testing focuses on the Kubernetes API server and evaluates whether unauthenticated requests are rejected, whether authenticated identities are correctly recognized, and whether identities are prevented from accessing resources outside their assigned namespace or permission boundary.

The penetration-testing workflow validates two related security controls: authentication control, which determines whether the Kubernetes API server requires and correctly validates an authorized identity before processing protected requests; and privilege-boundary control, which determines whether an authenticated identity can access only the Kubernetes resources permitted by its RBAC policy.

Kubernetes Role-Based Access Control (RBAC) is used to establish controlled permissions for laboratory identities. The test verifies whether namespace-scoped permissions remain within their intended boundary and whether unauthorized resource operations are rejected.

Kubernetes audit logging is enabled to provide evidence of API requests and authorization decisions. Wazuh monitors relevant K3s and Kubernetes security logs, while OpenSearch provides centralized investigation and correlation of authentication, authorization, and API activity.

The controlled penetration test is performed against the baseline configuration and then repeated after security controls are validated. The objective is to demonstrate that unauthorized API access is rejected, authorized operations remain functional, privilege boundaries are maintained, and security events can be investigated.

Complete Penetration Testing and Security Validation Workflow: K3s Kubernetes API Server → Authentication Request → Identity Validation → API Authorization → RBAC Evaluation → Unauthorized Access Attempt → Access Denial → Kubernetes Audit Logging → Wazuh Monitoring → OpenSearch Investigation → Security Validation → Remediation → Post-Remediation Testing.

K3s provides the controlled Kubernetes cluster environment for the penetration-testing exercise. The Kubernetes API server acts as the control-plane API through which authenticated clients interact with Kubernetes resources. Kubernetes RBAC determines which authenticated identities are permitted to perform specific operations against specific resources.

A Kubernetes API server can become exposed to unauthorized access when authentication requirements are weakened, credentials are improperly handled, or authenticated identities receive permissions beyond their intended role. An authenticated identity with excessive RBAC permissions may be able to read, create, modify, or delete resources outside its intended namespace or operational scope.

The security problem is therefore:

The proposed security-validation architecture verifies authentication requirements first and then validates the RBAC privilege boundary to ensure that authenticated identities cannot perform operations outside their assigned permissions.

The controlled attack attempts to access the K3s Kubernetes API server using an unauthorized or insufficiently privileged laboratory identity. The penetration test evaluates multiple access conditions rather than relying on a single authentication failure. The controlled testing includes unauthenticated API requests, authentication using a laboratory identity with restricted permissions, attempts to access resources belonging to another namespace, and attempts to perform operations outside the identity’s assigned RBAC permissions. Successful authentication should not automatically provide unrestricted Kubernetes access. The authenticated identity must still satisfy the RBAC policy associated with the requested resource and operation.

Kubernetes security separates the process of establishing who is making an API request from determining what that identity is allowed to do.

Authentication mechanisms establish the identity associated with an API request. Authorization then evaluates whether that authenticated identity has permission to perform the requested operation against the requested Kubernetes resource. For this use case, Kubernetes RBAC establishes the intended privilege boundary. A laboratory identity is given only the permissions required for its designated namespace and resource operations. The penetration test then attempts operations outside that boundary. A correctly enforced RBAC configuration should allow permitted operations while rejecting unauthorized operations. The security validation therefore focuses on both authentication and authorization rather than treating successful login as equivalent to full cluster access.

The secure processing flow is: