Threat Model

The Kubernetes Security Special Interest Group (SIG) has defined an Admission Control Threat Model for Kubernetes. The Kubewarden team continuously evaluates Kubewarden against this threat model, and works to provide secure defaults. It’s recommended that Kubewarden administrators read and understand the threat model, and use it to devise their own circumstance specific threat model as needed.

Details about each threat is provided in the document published by SIG Security.

Kubernetes threats

Threat 1 - Attacker floods webhook with traffic preventing its operation

Scenario

An attacker who has access to the Webhook endpoint, at the network level, could send large quantities of traffic, causing an effective denial of service to the admission controller.

Mitigation

Webhook fails closed. if the webhook doesn’t respond in time, for any reason, the API server should reject the request. This is Kubewarden’s default behavior.

Failing closed means that if, for any reason, Kubewarden stops responding or crashes, the API server rejects the request by default. This is even if the request would normally be accepted by Kubewarden.

Threat 2 - Attacker passes workloads which require complex processing causing timeouts

Scenario

An attacker, who can access the admission controller at a network level, passes requests to the admission controller requiring complex processing and causing timeouts as the admission controller uses compute power to process the workloads.

Mitigation

Webhook fails closed and authenticate callers. This is Kubewarden’s default behavior.

Threat 3 - Attacker exploits mis-configuration of webhook to bypass

Scenario

An attacker, who has rights to create workloads in the cluster, is able to exploit a mis-configuration to bypass the intended security control.

Mitigation

Regular reviews of webhook configuration can help catch issues.

Threat 4 - Attacker has rights to delete or modify the Kubernetes webhook object

Scenario

An attacker who has Kubernetes API access, has sufficient privileges to delete the webhook object in the cluster.

Mitigation

RBAC rights should be strictly controlled.

To-do

Most of RBAC isn’t within the scope of the current discussion. However, the following will be provided in due course for helping Kubewarden users:

  • Directions around minimum RBAC to be implemented.

  • Provision & documentation of a policy that detects and could block RBAC changes.

Threat 5 - Attacker gets access to valid credentials for the webhook

Scenario

An attacker gains access to valid client credentials for the admission controller webhook.

Mitigation

Webhook fails closed. This is Kubewarden’s default behavior.

Threat 6 - Attacker gains access to a cluster admin credential

Scenario

An attacker gains access to a cluster-admin level credential in the Kubernetes cluster.

Mitigation

N/A

Threat 7 - Attacker sniffs traffic on the container network

Scenario

An attacker who has access to the container network is able to sniff traffic between the API server and the admission controller webhook.

Mitigation

Since the webhook uses TLS encryption for all traffic, Kubewarden is safe.

Threat 8 - Attacker carries out a MITM attack on the webhook

Scenario

An attacker on the container network, who has access to the NET_RAW capability can try to use MITM tooling to intercept traffic between the API server and admission controller webhook.

Mitigation

Webhook mTLS authentication should be used. You should also use capabilities-psp and configure it to drop NET_RAW capabilities.

To-do

Implement mutual TLS authentication. Additionally, it would be possible to add a policy within the recommended policies section in the kubewarden-defaults which drops the NET_RAW capability.

Threat 9 - Attacker steals traffic from the webhook via spoofing

Scenario

An attacker is able to redirect traffic from the API server which is intended for the admission controller webhook by spoofing.

Mitigation

Webhook mTLS authentication is used.

To-do

Kubewarden should implement mutual TLS authentication

Threat 10 - Abusing a mutation rule to create a privileged container

Scenario

An attacker is able to cause a mutating admission controller to modify a workload, such that it allows for privileged container creation.

Mitigation

All rules should be reviewed and tested.

Threat 11 - Attacker deploys workloads to namespaces that are exempt from admission control

Scenario

An attacker is able to deploy workloads to Kubernetes namespaces that are exempt from the admission controller configuration.

Mitigation

RBAC rights are strictly controlled

To-do

Most of the RBAC is out of scope with respect to this decision. However, the Kubewarden team aims to:

  • Warn users via our docs and suggest some minimum RBAC to be used.

  • Provide a policy which detects RBAC changes and maybe block them.

Threat 12 - Block rule can be bypassed due to missing match (for example, missing initcontainers)

Scenario

An attacker created a workload manifest which uses a feature of the Kubernetes API which is not covered by the admission controller

Mitigation

All rules should be reviewed and tested. You should review PRs changing any rules in policies deployment.

Threat 13 - Attacker exploits bad string matching on a blocklist to bypass rules

Scenario

An attacker, who has rights to create workloads, bypasses a rule by exploiting bad string matching.

Mitigation

All rules should be reviewed and tested.

To-do

Introduce tests to cover this rule. As always, you should review PRs changing the rules in the policies deployment.

Threat 14 - Attacker uses new/old features of the Kubernetes API which have no rules

Scenario

An attacker, with rights to create workloads, uses new features of the Kubernetes API (for example, a changed API version) to bypass a rule.

Mitigation

All rules should be reviewed and tested. There is a policy that tests for the use of deprecated resources. It’s available from the deprecated-api-versions-policy.

deprecated-api-versions-policy only deals with Custom Resources known to it. The threat is both deprecated resource versions, and new unknown ones that are misused, hence the policy only covers part of the problem.

Threat 15 - Attacker deploys privileged container to node running Webhook controller

Scenario

An attacker, who has rights to deploy privileged containers to the cluster, creates a privileged container on the cluster node where the admission controller webhook operates.

Mitigation

Admission controller uses restrictive policies to prevent privileged workloads.

Threat 16 - Attacker mounts a privileged node hostpath allowing modification of Webhook controller configuration

Scenario

An attacker, who has rights to deploy hostPath volumes with workloads, creates a volume that allows for access to the admission controller pod’s files.

Mitigation

Admission controller uses restrictive policies to prevent privileged workloads

To-do

Add a recommended policy in the kubewarden-default Helm chart to prevent this.

Threat 17 - Attacker has privileged SSH access to cluster node running admission webhook

Scenario

An attacker is able to log into cluster nodes as a privileged user via SSH.

Mitigation

N/A

Threat 18 - Attacker uses policies to send confidential data from admission requests to external systems

Scenario

An attacker is able to configure a policy that listens to admission requests and sends sensitive data to an external system.

Mitigation

Strictly control external access for webhook Kubewarden policies run in a restrictive environment. They don’t have network access.

Kubewarden threats

Kubewarden threat 1 - Bootstrapping of trust for admission controller

Scenario

Assuming a trusted but new Kubernetes cluster, an attacker is able to compromise the Kubewarden stack before any of the policies securing it are deployed and enforced.

For example, by:

  • using unsigned and malicious images for:

    • Kubewarden-controller

    • policy-server

    • any of the Kubewarden dependencies

    • any optional dependencies (Grafana, Prometheus, and others)

  • by compromising the Helm charts payload

Mitigation

  1. Kubewarden provides a Software Bill Of Materials, which lists all images needed. This aids with Zero-Trust. The Kubernetes Administrator must verify the Kubewarden images, its dependencies' images, and charts out of the Kubernetes cluster, in a trusted environment. This can be done with cosign, for example. Incidentally, this is part of the implementation needed for air-gapped installations.

  2. Use signed Helm charts, and verified digests instead of tags for Kubewarden images in those Helm charts. This doesn’t secure dependencies though.