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k-rail is a workload policy enforcement tool for Kubernetes. It can help you secure a multi tenant cluster with minimal disruption and maximum velocity.

Why k-rail?

By default, the Kubernetes APIs allow for a variety of easy privilege escalation routes. When operating a multi-tenant cluster, many features can be dangerous or introduce instability and must be used judiciously. k-rail attempts to make workload policy enforcement easy in Kubernetes, even if you already have a large number of diverse workloads. Several features enable you to roll out policy enforcement safely without breaking existing workloads:

  • Passive report-only mode of running policies
  • Structured violation data logged, ready for analysis and dashboards
      "enforced": true,
      "kind": "PodExec",
      "namespace": "ecommerce",
      "policy": "pod_no_exec",
      "resource": "payment-processor",
      "user": "",
      "time": "2019-11-03T06:28:07Z"
  • Flexible and powerful policy exemptions by resource name, namespace, groups, and users
    - resource_name: "*"
      namespace: "cluster-conformance-testing"
      username: ""
      group: "*"
        - "pod_no_privileged_containers"
        - "pod_no_bind_mounts"
        - "pod_no_host_network"
        - "pod_default_seccomp_policy"
        - "pod_no_host_pid"
        - "pod_no_exec"
  • Realtime interactive feedback for engineers and systems that apply resources
    $ kubectl apply --namespace default -f deploy/non-compliant-ingress.yaml
    Error from server (InternalError): error when creating "deploy/non-compliant-ingress.yaml":
    Internal error occurred: admission webhook "" denied the request:
    Ingress bad-ingress had violation: Require Ingress Exemption: Using the 'public' Ingress class requires an exemption

By leveraging the first three features you can quickly and easily roll out enforcement to deployments without breaking them and monitor violations with confidence. The interactive feedback informs and educates engineers during future policy violations.

Cruise was able to utilize this software to apply enforcement to more than a dozen clusters with thousands of existing diverse workloads in all environments in about a week without breaking existing deployments. Now you can too.

Suggested usage

If you have a new cluster without existing workloads, just run k-rail in enforcement mode for the policies you desire and add exemptions as needed.

If you have a cluster with existing workloads, run it in monitor mode for a few weeks or until you have collected enough data. The violation events are emmitted in the logs in JSON, so it is suggested that you analyze that data collected to make exemptions as needed. Once the exemptions are applied, you can safely turn on enforcement mode without breaking existing workloads.


You can install or update k-rail using the helm chart in deploy/helm.

For the Helm deployment, all configuration for policies and exemptions are contained in deploy/helm/values.yaml.

For Helm 2 and below, it is recommended to use helm template render the YAML for applying rather than using Helm Tiller:

helm template --namespace k-rail deploy/helm | kubectl apply -f - 

By default all policies are enforced (report_only: false).

Test the default configuration by applying the provided non-compliant deployment:

kubectl apply --namespace default -f deploy/non-compliant-deployment.yaml


Removing k-rail is similar to the installation, but you use delete instead of apply:

helm template --namespace k-rail deploy/helm | kubectl delete -f -

Viewing policy violations

There are a few ways of viewing violations. Violations from realtime feedback and the Events API is most useful for users, but violations from logs is most useful for presentation and analysis.

Violations from realtime feedback

You may see violations when applying your resources:

$ kubectl apply -f deploy/non-compliant-deployment.yaml

Error from server (k-rail): error when creating "deploy/non-compliant-deployment.yaml": admission webhook "" denied the request:
Deployment bad-deployment had violation: Host Bind Mounts: host bind mounts are forbidden
Deployment bad-deployment had violation: Docker Sock Mount: mounting the Docker socket is forbidden
Deployment bad-deployment had violation: Immutable Image Reference: image tag must include its sha256 digest
Deployment bad-deployment had violation: No Host Network: Using the host network is forbidden
Deployment bad-deployment had violation: No Privileged Container: Using privileged containers is forbidden
Deployment bad-deployment had violation: No New Capabilities: Adding additional capabilities is forbidden
Deployment bad-deployment had violation: No Host PID: Using the host PID namespace is forbidden
Deployment bad-deployment had violation: Safe to evict: annotation is required for Pods that use emptyDir or hostPath mounts to enable cluster autoscaling

Violations from the Events API

You can also see violations that have occurred recently with the events API:

$ kubectl get events --namespace default
3m41s       Warning   FailedCreate   ReplicaSet   Error creating: admission webhook "" denied the request:
bad-pod-5f7cd9bf45-rbhsb had violation: Docker Sock Mount: mounting the Docker socket is forbidden

Violations from logs

Violations are also emitted as structured data in the logs:

$ kubectl logs --namespace k-rail --selector name=k-rail | jq '.'

  "enforced": true,
  "kind": "Deployment",
  "namespace": "default",
  "policy": "pod_no_host_network",
  "resource": "evil-deployment",
  "time": "2019-10-23T19:54:24Z",
  "user": ""
  "enforced": true,
  "kind": "Deployment",
  "namespace": "default",
  "policy": "pod_no_privileged_container",
  "resource": "evil-deployment",
  "time": "2019-10-23T19:54:24Z",
  "user": ""
  "enforced": true,
  "kind": "Deployment",
  "namespace": "default",
  "policy": "pod_no_new_capabilities",
  "resource": "evil-deployment",
  "time": "2019-10-23T19:54:24Z",
  "user": ""

Since the violations are outputted as structured data, you are encouraged to aggregate and display that information. GCP BigQuery + Data Studio, Sumologic, Elasticsearch + Kibana, Splunk, etc are all capable of this.

Supported policies

No Exec

The No Exec policy prevents users from execing into running pods unless they have an exemption. This policy is typically enforced within a production environment, but run in report-only mode in dev and staging environments to facilitate debugging.

Execing into a pod can enable someone to do many nefarious things to that workload. Eventually this policy will also apply a taint label to the Pod to indicate that it should no longer be trusted and can be evicted.

No Bind Mounts

Host bind mounts (also called hostPath mounts) can be used to exfiltrate data from or escalate privileges on the host system. Using host bind mounts can cause unreliability of the node if it causes a partition to fill up.

No Docker Sock Mount

The Docker socket bind mount provides API access to the host Docker daemon, which can be used for privilege escalation or otherwise control the container host. Using Docker sock mounts can cause unreliability of the node because of the extra workloads that the Kubernetes schedulers are not aware of.

Note: It is recommended to use the No Bind Mounts policy to disable all hostPath mounts rather than only this policy.

Mutate Default Seccomp Profile

Sets a default seccomp profile (runtime/default or a configured one) for Pods if they have no existing seccomp configuration. The default seccomp policy for Docker and Containerd both block over 40 syscalls, many of which are potentially dangerous. The default policies are usually very compatible with applications, too.

Policy configuration

The Mutate Default Seccomp Profile policy can be configured in the k-rail configuration file.


  policy_default_seccomp_policy: "runtime/default"

Immutable Image Reference

Docker image tags in a registry are mutable, so if you reference a tag without specifying the image digest someone or something could change the image you were using without you knowing.

You can obtain the immutable reference for an image with this command:

$ docker inspect --format='{{index .RepoDigests 0}}' alpine:3.8  
You can also add the tag back in for it to be more human readable:


No Host Network

Host networking enables packet capture of host network interfaces and a bypass to some cloud meta data APIs, such as the GKE metadata API. The metadata API can be used to escalate access.

No Host PID

The host PID namespace can be used to inspect process environment variables (which often contain secrets). It can also potentially be used to dump process memory, modify kernel parameters, and insert kprobes+uprobes into the kernel to exfiltrate information.

No New Capabilities

Kernel Capabilities can be used to escalate to level of kernel API access available to the process. Some can enable loading kernel modules, changing namespace, load eBPF byte code in the kernel and other potentially dangerous things.

No Privileged Container

Privileged containers have all capabilities and also removes cgroup resource accounting.

No Helm Tiller

Helm Tiller installations often have an unauthenticated API open to the cluster which provides a privilege escalation route to ClusterAdmin or NamespaceEditor.

Note: This policy only blocks images that /tiller in their name from being deployed. It is not a robust policy and serves more as a reminder for engineers to seek an alternate route of deployment, such as using helm template or isopod.

Trusted Image Repository

There are many malicious, poorly configured, and outdated and vulnerable images available in public Docker image repositories. Images must be sourced from configured trusted internal repositories or from an official Docker Hub repository.

Policy configuration

The Trusted Image Repository policy can be configured in the k-rail configuration file.


    - '^*'   # private GCR repo
    - '^*'             # official k8s GCR repo
    - '^[A-Za-z0-9\-:@]+$'         # official docker hub images

Safe to Evict (DEPRECATED)

DEPRECATED - See Mutate Safe to Evict below

The Kubernetes autoscaler will not evict pods using hostPath or emptyDir mounts unless they have this annotation:

This policy validates that Pods have this annotation. You'll probably find the mutation policy below more useful.

Mutate Safe to Evict

The Kubernetes autoscaler will not evict pods using hostPath or emptyDir mounts unless they have this annotation:

This policy mutates Pods that do not have the annotation specfied to be true. It will not override existing annotations with false.

You can also set the annoation on existing Pods with this one-liner:

$ kubectl get po --all-namespaces -o json | jq -r '.items[] | select(.spec.volumes[].hostPath or .spec.volumes[].emptyDir) | [ .metadata.namespace, ] | @tsv' | while IFS=$'\t' read -r namespace pod; do echo "\n NAMESPACE: $namespace \n POD: $pod \n"; kubectl annotate pod -n $namespace $pod ""; done

Require Ingress Exemption

The Require Ingress Exemption policy requires the configured ingress classes to have an a Policy exemption to be used. This is typically useful if you want to gate the usage of public ingress.

Policy configuration

The Require Ingress Exemption policy can be configured in the k-rail configuration file.


    - nginx-public


For the Helm deployment, all configuration is contained in deploy/helm/values.yaml.


Log levels can be set in the k-rail configuration file. Acceptable values are debug, warn, and info. The default log level is info.

All reporting and enforcement operations are logged in a structured json blob per event. It is useful to run policies in report-only mode, analyze your state in with the structured logs, and flip on enforcement mode when appropriate.

Modes of operation

Global report-only mode

When global_report_only_mode is toggled in the config, ALL policies run in report_only mode, even if configured otherwise. This mode must be false to have any policies in enforcement mode.

Policy modes

Policies can be enabled/disabled, and run in report-only or enforcement mode as specified in the config.

Policy exemptions

A folder to load policy exemptions from can be specified from config. Load exemptions by specifying the -exemptions-path-glob parameter, and specify a path glob that includes the exemptions, such as /config/policies/*.yaml.

For the Helm deployment, all policy and exemption configuration is contained in deploy/helm/values.yaml.

The format of an exemption config is YAML, and looks like this:

# exempt all kube-system pods since they are largely provided by GKE
- resource_name: "*"
  namespace: "kube-system"
  - "*"

# malicious-pod needs host network to escalate access via GCE metadata API
- resource_name: malicious-pod
  namespace: malicious
  exempt_policies: ["pod_no_host_network"]

# allow everything
# - resource_name: "*"
#   namespace: "*"
#   username: "*"
#   group: "*"
#   exempt_policies: ["*"]

Policy configuration

Some policies are configurable. Policy configuration is contained in the k-rail configuration file, and documentation for a policy's configuration can be found in the Supported policies heading above.

For the Helm deployment, all policy and exemption configuration is contained in deploy/helm/values.yaml.

Adding new policies

Policies must satisfy this interface:

// Policy specifies how a Policy is implemented
// Returns an optional slice of violations and an optional slice of patch operations if mutation is desired.
type Policy interface {
	Name() string
	Validate(ctx context.Context,
		config policies.Config,
		ar *admissionv1beta1.AdmissionRequest,
	) ([]policies.ResourceViolation, []policies.PatchOperation)

Name() must return a string that matches a policy name that is provided in configuration. Validate accepts an AdmissionRequest, and the resource of interest must be extracted from it. See resource/pod.go for an example of extracting PodSpecs from an AdmissionRequest. If mutation on a resource is desired, you can return a slice of JSONPatch operations and nil for the violations.

Policies can be registered in internal/policies.go. Any policies that are registered but do not have configuration provided get enabled in report-only mode.


Resources are having timeout events

If you see timeout events on resources, this may be because the k-rail service is unreachable from the Kubernetes apiserver. Newer versions (1.14+) of Kubernetes are not likely to have this issue if the MutatingWebhookConfiguration failurePolicy is set to Ignore and timeoutSeconds is set to a lower number (such as 5 or less).

To determine if this is occuring because the service is unreachable, check the kube-apiserver logs. You will see logs similar to,

E0911 04:57:22.686526       1 dispatcher.go:68] failed calling webhook "": Post https://k-rail.k-rail.svc:443/?timeout=30s: dial tcp connect: connection refused

Checking kube-apiserver logs

Checking kube-apiserver logs depends on what Kubernetes distribution you use.

  • For minikube and other self hosted (meaning Kubernetes runs its infra on itself) clusters, you can typically just view the logs for the apiserver pod in the kube-system namespace.
  • For some non self-hosted clusters, such as GKE, you can download the apiserver logs through the Kubernetes proxy:
kubectl proxy --port 8001 &
curl http://localhost:8001/logs/kube-apiserver.log > /tmp/out.log

Viewing webhook latency and status code metrics

The apiserver tracks latency and status code metrics for webhooks. This may be useful for debugging timeouts or assurance of performance.

kubectl proxy --port 8001 &
curl -s http://localhost:8001/metrics | grep k-rail

Policies are enabled, but are not triggering when they should

This may be caused by the k-rail service being unreachable. To determine this, see Resources are having timeout events.

Policies are enabled, but a deployment is blocked and an exemption is needed

If you need to make an exemption to a policy, see Policy exemptions.

Checking the mTLS certificate expiration

By default, a 10 year certificate is generated during each apply of k-rail. Re-applying will renew it.

You can also check the expiration with this command:

$ kubectl get secret --namespace k-rail k-rail-cert -o json | jq -r '.data["cert.pem"]' | base64 -d | openssl x509 -noout -text | grep -A 3 "Validity"

            Not Before: Oct 24 05:40:16 2019 GMT
            Not After : Oct 21 05:40:16 2029 GMT
        Subject: CN = k-rail.k-rail.svc


Copyright (c) 2019-present, Cruise LLC

This source code is licensed under the Apache License, Version 2.0, found in the LICENSE file in the root directory of this source tree. You may not use this file except in compliance with the License.

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