Kubernetes 1.30: Multi-Webhook and Modular Authorization Made Much Easier

By Rita Zhang (Microsoft), Jordan Liggitt (Google), Nabarun Pal (VMware), Leigh Capili (VMware) | Friday, April 26, 2024

With Kubernetes 1.30, we (SIG Auth) are moving Structured Authorization Configuration to beta.

Today's article is about authorization: deciding what someone can and cannot access. Check a previous article from yesterday to find about what's new in Kubernetes v1.30 around authentication (finding out who's performing a task, and checking that they are who they say they are).

Introduction

Kubernetes continues to evolve to meet the intricate requirements of system administrators and developers alike. A critical aspect of Kubernetes that ensures the security and integrity of the cluster is the API server authorization. Until recently, the configuration of the authorization chain in kube-apiserver was somewhat rigid, limited to a set of command-line flags and allowing only a single webhook in the authorization chain. This approach, while functional, restricted the flexibility needed by cluster administrators to define complex, fine-grained authorization policies. The latest Structured Authorization Configuration feature (KEP-3221) aims to revolutionize this aspect by introducing a more structured and versatile way to configure the authorization chain, focusing on enabling multiple webhooks and providing explicit control mechanisms.

The Need for Improvement

Cluster administrators have long sought the ability to specify multiple authorization webhooks within the API Server handler chain and have control over detailed behavior like timeout and failure policy for each webhook. This need arises from the desire to create layered security policies, where requests can be validated against multiple criteria or sets of rules in a specific order. The previous limitations also made it difficult to dynamically configure the authorizer chain, leaving no room to manage complex authorization scenarios efficiently.

The Structured Authorization Configuration feature addresses these limitations by introducing a configuration file format to configure the Kubernetes API Server Authorization chain. This format allows specifying multiple webhooks in the authorization chain (all other authorization types are specified no more than once). Each webhook authorizer has well-defined parameters, including timeout settings, failure policies, and conditions for invocation with CEL rules to pre-filter requests before they are dispatched to webhooks, helping you prevent unnecessary invocations. The configuration also supports automatic reloading, ensuring changes can be applied dynamically without restarting the kube-apiserver. This feature addresses current limitations and opens up new possibilities for securing and managing Kubernetes clusters more effectively.

Sample Configurations

Here is a sample structured authorization configuration along with descriptions for all fields, their defaults, and possible values.

apiVersion: apiserver.config.k8s.io/v1beta1
kind: AuthorizationConfiguration
authorizers:
  - type: Webhook
    # Name used to describe the authorizer
    # This is explicitly used in monitoring machinery for metrics
    # Note:
    #   - Validation for this field is similar to how K8s labels are validated today.
    # Required, with no default
    name: webhook
    webhook:
      # The duration to cache 'authorized' responses from the webhook
      # authorizer.
      # Same as setting `--authorization-webhook-cache-authorized-ttl` flag
      # Default: 5m0s
      authorizedTTL: 30s
      # The duration to cache 'unauthorized' responses from the webhook
      # authorizer.
      # Same as setting `--authorization-webhook-cache-unauthorized-ttl` flag
      # Default: 30s
      unauthorizedTTL: 30s
      # Timeout for the webhook request
      # Maximum allowed is 30s.
      # Required, with no default.
      timeout: 3s
      # The API version of the authorization.k8s.io SubjectAccessReview to
      # send to and expect from the webhook.
      # Same as setting `--authorization-webhook-version` flag
      # Required, with no default
      # Valid values: v1beta1, v1
      subjectAccessReviewVersion: v1
      # MatchConditionSubjectAccessReviewVersion specifies the SubjectAccessReview
      # version the CEL expressions are evaluated against
      # Valid values: v1
      # Required, no default value
      matchConditionSubjectAccessReviewVersion: v1
      # Controls the authorization decision when a webhook request fails to
      # complete or returns a malformed response or errors evaluating
      # matchConditions.
      # Valid values:
      #   - NoOpinion: continue to subsequent authorizers to see if one of
      #     them allows the request
      #   - Deny: reject the request without consulting subsequent authorizers
      # Required, with no default.
      failurePolicy: Deny
      connectionInfo:
        # Controls how the webhook should communicate with the server.
        # Valid values:
        # - KubeConfigFile: use the file specified in kubeConfigFile to locate the
        #   server.
        # - InClusterConfig: use the in-cluster configuration to call the
        #   SubjectAccessReview API hosted by kube-apiserver. This mode is not
        #   allowed for kube-apiserver.
        type: KubeConfigFile
        # Path to KubeConfigFile for connection info
        # Required, if connectionInfo.Type is KubeConfigFile
        kubeConfigFile: /kube-system-authz-webhook.yaml
        # matchConditions is a list of conditions that must be met for a request to be sent to this
        # webhook. An empty list of matchConditions matches all requests.
        # There are a maximum of 64 match conditions allowed.
        #
        # The exact matching logic is (in order):
        #   1. If at least one matchCondition evaluates to FALSE, then the webhook is skipped.
        #   2. If ALL matchConditions evaluate to TRUE, then the webhook is called.
        #   3. If at least one matchCondition evaluates to an error (but none are FALSE):
        #      - If failurePolicy=Deny, then the webhook rejects the request
        #      - If failurePolicy=NoOpinion, then the error is ignored and the webhook is skipped
      matchConditions:
      # expression represents the expression which will be evaluated by CEL. Must evaluate to bool.
      # CEL expressions have access to the contents of the SubjectAccessReview in v1 version.
      # If version specified by subjectAccessReviewVersion in the request variable is v1beta1,
      # the contents would be converted to the v1 version before evaluating the CEL expression.
      #
      # Documentation on CEL: https://kubernetes.io/docs/reference/using-api/cel/
      #
      # only send resource requests to the webhook
      - expression: has(request.resourceAttributes)
      # only intercept requests to kube-system
      - expression: request.resourceAttributes.namespace == 'kube-system'
      # don't intercept requests from kube-system service accounts
      - expression: "!('system:serviceaccounts:kube-system' in request.groups)"
  - type: Node
    name: node
  - type: RBAC
    name: rbac
  - type: Webhook
    name: in-cluster-authorizer
    webhook:
      authorizedTTL: 5m
      unauthorizedTTL: 30s
      timeout: 3s
      subjectAccessReviewVersion: v1
      failurePolicy: NoOpinion
      connectionInfo:
        type: InClusterConfig

The following configuration examples illustrate real-world scenarios that need the ability to specify multiple webhooks with distinct settings, precedence order, and failure modes.

Protecting Installed CRDs

Ensuring of Custom Resource Definitions (CRDs) availability at cluster startup has been a key demand. One of the blockers of having a controller reconcile those CRDs is having a protection mechanism for them, which can be achieved through multiple authorization webhooks. This was not possible before as specifying multiple authorization webhooks in the Kubernetes API Server authorization chain was simply not possible. Now, with the Structured Authorization Configuration feature, administrators can specify multiple webhooks, offering a solution where RBAC falls short, especially when denying permissions to 'non-system' users for certain CRDs.

Assuming the following for this scenario:

The "protected" CRDs are installed.
They can only be modified by users in the group admin.

apiVersion: apiserver.config.k8s.io/v1beta1
kind: AuthorizationConfiguration
authorizers:
  - type: Webhook
    name: system-crd-protector
    webhook:
      unauthorizedTTL: 30s
      timeout: 3s
      subjectAccessReviewVersion: v1
      matchConditionSubjectAccessReviewVersion: v1
      failurePolicy: Deny
      connectionInfo:
        type: KubeConfigFile
        kubeConfigFile: /files/kube-system-authz-webhook.yaml
      matchConditions:
      # only send resource requests to the webhook
      - expression: has(request.resourceAttributes)
      # only intercept requests for CRDs
      - expression: request.resourceAttributes.resource.resource = "customresourcedefinitions"
      - expression: request.resourceAttributes.resource.group = ""
      # only intercept update, patch, delete, or deletecollection requests
      - expression: request.resourceAttributes.verb in ['update', 'patch', 'delete','deletecollection']
  - type: Node
  - type: RBAC

Preventing unnecessarily nested webhooks

A system administrator wants to apply specific validations to requests before handing them off to webhooks using frameworks like Open Policy Agent. In the past, this would require running nested webhooks within the one added to the authorization chain to achieve the desired result. The Structured Authorization Configuration feature simplifies this process, offering a structured API to selectively trigger additional webhooks when needed. It also enables administrators to set distinct failure policies for each webhook, ensuring more consistent and predictable responses.

apiVersion: apiserver.config.k8s.io/v1beta1
kind: AuthorizationConfiguration
authorizers:
  - type: Webhook
    name: system-crd-protector
    webhook:
      unauthorizedTTL: 30s
      timeout: 3s
      subjectAccessReviewVersion: v1
      matchConditionSubjectAccessReviewVersion: v1
      failurePolicy: Deny
      connectionInfo:
        type: KubeConfigFile
        kubeConfigFile: /files/kube-system-authz-webhook.yaml
      matchConditions:
      # only send resource requests to the webhook
      - expression: has(request.resourceAttributes)
      # only intercept requests for CRDs
      - expression: request.resourceAttributes.resource.resource = "customresourcedefinitions"
      - expression: request.resourceAttributes.resource.group = ""
      # only intercept update, patch, delete, or deletecollection requests
      - expression: request.resourceAttributes.verb in ['update', 'patch', 'delete','deletecollection']
  - type: Node
  - type: RBAC
  - name: opa
    type: Webhook
    webhook:
      unauthorizedTTL: 30s
      timeout: 3s
      subjectAccessReviewVersion: v1
      matchConditionSubjectAccessReviewVersion: v1
      failurePolicy: Deny
      connectionInfo:
        type: KubeConfigFile
        kubeConfigFile: /files/opa-default-authz-webhook.yaml
      matchConditions:
      # only send resource requests to the webhook
      - expression: has(request.resourceAttributes)
      # only intercept requests to default namespace
      - expression: request.resourceAttributes.namespace == 'default'
      # don't intercept requests from default service accounts
      - expression: "!('system:serviceaccounts:default' in request.groups)"

What's next?

From Kubernetes 1.30, the feature is in beta and enabled by default. For Kubernetes v1.31, we expect the feature to stay in beta while we get more feedback from users. Once it is ready for GA, the feature flag will be removed, and the configuration file version will be promoted to v1.

Learn more about this feature on the structured authorization configuration Kubernetes doc website. You can also follow along with KEP-3221 to track progress in coming Kubernetes releases.

Call to action

In this post, we have covered the benefits of the Structured Authorization Configuration feature in Kubernetes v1.30 and a few sample configurations for real-world scenarios. To use this feature, you must specify the path to the authorization configuration using the --authorization-config command line argument. From Kubernetes 1.30, the feature is in beta and enabled by default. If you want to keep using command line flags instead of a configuration file, those will continue to work as-is. Specifying both --authorization-config and --authorization-modes/--authorization-webhook-* won't work. You need to drop the older flags from your kube-apiserver command.

The following kind Cluster configuration sets that command argument on the APIserver to load an AuthorizationConfiguration from a file (authorization_config.yaml) in the files folder. Any needed kubeconfig and certificate files can also be put in the files directory.

kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
featureGates:
  StructuredAuthorizationConfiguration: true  # enabled by default in v1.30
kubeadmConfigPatches:
  - |
    kind: ClusterConfiguration
    metadata:
      name: config
    apiServer:
      extraArgs:
        authorization-config: "/files/authorization_config.yaml"
      extraVolumes:
      - name: files
        hostPath: "/files"
        mountPath: "/files"
        readOnly: true    
nodes:
- role: control-plane
  extraMounts:
  - hostPath: files
    containerPath: /files

We would love to hear your feedback on this feature. In particular, we would like feedback from Kubernetes cluster administrators and authorization webhook implementors as they build their integrations with this new API. Please reach out to us on the #sig-auth-authorizers-dev channel on Kubernetes Slack.

How to get involved

If you are interested in helping develop this feature, sharing feedback, or participating in any other ongoing SIG Auth projects, please reach out on the #sig-auth channel on Kubernetes Slack.

You are also welcome to join the bi-weekly SIG Auth meetings held every other Wednesday.

Acknowledgments

This feature was driven by contributors from several different companies. We would like to extend a huge thank you to everyone who contributed their time and effort to make this possible.

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