0006-threat-model.md

Feature Name	Kubernetes admission control threat model
Start Date	04/18/2022
Category	Security
RFC PR	#2
State	ACCEPTED

Summary

Describe the threat model for the admissions control, how it can effect Kubewarden and the possible mitigations.

Motivation

The goal of this is turn Kubewarden more secure to operate.

Examples / User Stories

As Kubewarden developer, I want to know which are the weak point of the Kubernetes admission control feature, so I can mitigate this problem.

As Kubewarden user, I want to know which are the weak points of the Kubernetes admission control, so I can apply the mitigation in my cluster.

Detailed design

The threat model of the Kubernetes admission control list 18 possible threats. All of them will be discussed sharing a brief description of it, possible mitigation and what we, as Kubewarden team, should do about it (if necessary). As well as help our users to be aware of the risks.

If more details about each threat is needed, go to the original document here: https://github.com/kubernetes/sig-security/tree/main/sig-security-docs/papers/admission-control

Threat 1 - Attacker floods webhook with traffic preventing its operation

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. In other words, if the webhook does not respond in time, for any reasion, API server should reject the request. We are safe on this. Kubewarden default behavior already does that.

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

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

Mitigation Webhook fail closed and authenticate callers. We are safe, Kubewarden already does that.

Threat 3 - Attacker exploits misconfiguration of webhook to bypass

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 catch issues.

Threat 4 - Attacker has rights to delete or modify the k8s webhook object

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

Mitigation RBAC rights are strictly controlled.

To do I think most of the RBAC is not Kubewarden responsibility. But we can help our users if we:

• Warning them in our docs and suggest some minimum RBAC to be used.
• Provide a policy which detect RBAC changes and maybe block them.

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

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

Mitigation Webhook fails closed.

Kubewarden is failed closed. Thus, we should be fine.

Threat 6 - Attacker gains access to a cluster admin credential

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

Mitigation N/A

I cannot see how Kubewarden can help here.

Threat 7 - Attacker sniffs traffic on the container network

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

Mitigation Webhook uses TLS encryption for all traffic

Kubewarden are safe. Because the webhook connections are encrypted.

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

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

Mitigation Webhook mTLS authentication is used.

To do Kubewarden should implement mutual TLS authentication We can add in the recommended policies from the kubewarden-defaults Helm chart a policy to drop the NET_RAW capability.

Threat 9 - Attacker steals traffic from the webhook via spoofing

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

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 are reviewed and tested.

To do We may came up with some tests to cover this rules reviews We should carefully review a PR changing the rules in the policies deployment

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

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 I think most of the RBAC is not Kubewarden responsability. But we can help our users if we:

• Warning them in our docs and suggest some minimum RBAC to be used.
• Provide a policy which detect RBAC changes and maybe block them. Is this possible?

Threat ID 12 - Block rule can be bypassed due to missing match (e.g. missing initcontainers)

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 are reviewed and tested.

To do We may came up with some tests to cover this rules reviews We should carefully review a PR changing the rules in the policies deployment

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

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

Mitigation All rules are reviewed and tested.

To do We may came up with some tests to cover this rules reviews We should carefully review a PR changing the rules in the policies deployment

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

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 are reviewed and tested.

To do We may came up with some tests to cover this rules reviews and API versions. We can create a configuration to reject by default requests where the API version not cover by the policy. We should warning policies developers to cover all the supported API version in theirs tests and reject all of others.

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

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

To do I do not know if Kuberwarden can help on this. Kubewarden does not have access to containers running in the cluster node.

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

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

Mitigation Admission controller uses restrictive policies to prevent privileged workloads

To do We can add a recommended policy in the kubewarden-default Helm chart to prevent this.

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

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

I don't think Kubewarden can help on this

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

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 do not have network access.

Threat Kubewarden ID 1 - Bootstrapping of trust for admission controller

Assuming a trusted but fresh Kubernetes cluster, an attacker is able to compromise the Kubewarden stack before any of the policies securing it is deployed and enforcing. For example, by using unsigned and malicious images for kubewarden-controller, policy-server, or any of the Kubewarden dependencies (cert-manager) or optional dependencies (grafana, prometheus..), or 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 (and 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.

Drawbacks

Alternatives

Unresolved questions