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Overview

In Kubernetes, authorization happens as a separate step from authentication. See the Accessing Control Overview for an overview of how authentication and authorization are applied to requests.

Authorization applies to all HTTP accesses on the main (secure) apiserver port.

The authorization check for any request compares attributes of the context of the request, (such as user, resource, and namespace) with access policies. An API call must be allowed by some policy in order to proceed.

The following implementations are available, and are selected by flag:

  • --authorization-mode=AlwaysDeny blocks all requests (used in tests).
  • --authorization-mode=AlwaysAllow allows all requests; use if you don't need authorization.
  • --authorization-mode=ABAC allows for a simple local-file-based user-configured authorization policy. ABAC stands for Attribute-Based Access Control.
  • --authorization-mode=RBAC allows for authorization to be driven by policy stored in the Kubernetes API. RBAC stands for Role-Based Access Control.
  • --authorization-mode=Webhook allows for authorization to be driven by a remote service using REST.

If multiple modes are provided the set is unioned, and only a single authorizer is required to admit the action. This means the flag:

--authorization-mode=AlwaysDeny,AlwaysAllow

will always allow.

Request Attributes

A request has the following attributes that can be considered for authorization:

  • user (the user-string which a user was authenticated as).
  • group (the list of group names the authenticated user is a member of).
  • "extra" (a map of arbitrary string keys to string values, provided by the authentication layer)
  • whether the request is for an API resource.
  • the request path.
    • allows authorizing access to miscellaneous non-resource endpoints like /api or /healthz (see kubectl).
  • the request verb.
    • API verbs get, list, create, update, patch, watch, proxy, redirect, delete, and deletecollection are used for resource requests
    • HTTP verbs get, post, put, and delete are used for non-resource requests
  • what resource is being accessed (for resource requests only)
  • what subresource is being accessed (for resource requests only)
  • the namespace of the object being accessed (for namespaced resource requests only)
  • the API group being accessed (for resource requests only); an empty string designates the core API group

The request verb for a resource API endpoint can be determined by the HTTP verb used and whether or not the request acts on an individual resource or a collection of resources:

HTTP verb request verb
POST create
GET, HEAD get (for individual resources), list (for collections)
PUT update
PATCH patch
DELETE delete (for individual resources), deletecollection (for collections)

Some components perform authorization checks for additional permissions using specialized verbs. For example:

  • PodSecurityPolicy checks for authorization of the use verb on podsecuritypolicies resources in the extensions API group.
  • RBAC checks for authorization of the bind verb on roles and clusterroles resources in the rbac.authorization.k8s.io API group.
  • Authentication layer checks for authorization of the impersonate verb on users, groups, and userextras in the authentication.k8s.io API group, and the serviceaccounts in the core API group.

ABAC Mode

Policy File Format

For mode ABAC, also specify --authorization-policy-file=SOME_FILENAME.

The file format is one JSON object per line. There should be no enclosing list or map, just one map per line.

Each line is a "policy object". A policy object is a map with the following properties:

  • Versioning properties:
    • apiVersion, type string; valid values are "abac.authorization.kubernetes.io/v1beta1". Allows versioning and conversion of the policy format.
    • kind, type string: valid values are "Policy". Allows versioning and conversion of the policy format.
  • spec property set to a map with the following properties:
    • Subject-matching properties:
      • user, type string; the user-string from --token-auth-file. If you specify user, it must match the username of the authenticated user.
      • group, type string; if you specify group, it must match one of the groups of the authenticated user. system:authenticated matches all authenticated requests. system:unauthenticated matches all unauthenticated requests.
    • Resource-matching properties:
      • apiGroup, type string; an API group.
        • Ex: extensions
        • Wildcard: * matches all API groups.
      • namespace, type string; a namespace.
        • Ex: kube-system
        • Wildcard: * matches all resource requests.
      • resource, type string; a resource type
        • Ex: pods
        • Wildcard: * matches all resource requests.
    • Non-resource-matching properties:
      • nonResourcePath, type string; non-resource request paths.
        • Ex: /version or /apis
        • Wildcard:
          • * matches all non-resource requests.
          • /foo/* matches /foo/ and all of its subpaths.
    • readonly, type boolean, when true, means that the policy only applies to get, list, and watch operations.

NOTES: An unset property is the same as a property set to the zero value for its type (e.g. empty string, 0, false). However, unset should be preferred for readability.

In the future, policies may be expressed in a JSON format, and managed via a REST interface.

Authorization Algorithm

A request has attributes which correspond to the properties of a policy object.

When a request is received, the attributes are determined. Unknown attributes are set to the zero value of its type (e.g. empty string, 0, false).

A property set to "*" will match any value of the corresponding attribute.

The tuple of attributes is checked for a match against every policy in the policy file. If at least one line matches the request attributes, then the request is authorized (but may fail later validation).

To permit any authenticated user to do something, write a policy with the group property set to "system:authenticated".

To permit any unauthenticated user to do something, write a policy with the group property set to "system:unauthenticated".

To permit a user to do anything, write a policy with the apiGroup, namespace, resource, and nonResourcePath properties set to "*".

Kubectl

Kubectl uses the /api and /apis endpoints of api-server to negotiate client/server versions. To validate objects sent to the API by create/update operations, kubectl queries certain swagger resources. For API version v1 those would be /swaggerapi/api/v1 & /swaggerapi/experimental/v1.

When using ABAC authorization, those special resources have to be explicitly exposed via the nonResourcePath property in a policy (see examples below):

  • /api, /api/*, /apis, and /apis/* for API version negotiation.
  • /version for retrieving the server version via kubectl version.
  • /swaggerapi/* for create/update operations.

To inspect the HTTP calls involved in a specific kubectl operation you can turn up the verbosity:

kubectl --v=8 version

Examples

  1. Alice can do anything to all resources:

    {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "alice", "namespace": "*", "resource": "*", "apiGroup": "*"}}

  2. Kubelet can read any pods:

    {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "kubelet", "namespace": "*", "resource": "pods", "readonly": true}}

  3. Kubelet can read and write events:

    {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "kubelet", "namespace": "*", "resource": "events"}}

  4. Bob can just read pods in namespace "projectCaribou":

    {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "bob", "namespace": "projectCaribou", "resource": "pods", "readonly": true}}

  5. Anyone can make read-only requests to all non-resource paths:

    {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"group": "system:authenticated", "readonly": true, "nonResourcePath": "*"}}
{"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"group": "system:unauthenticated", "readonly": true, "nonResourcePath": "*"}}

Complete file example

A quick note on service accounts

A service account automatically generates a user. The user's name is generated according to the naming convention:

system:serviceaccount:<namespace>:<serviceaccountname>

Creating a new namespace also causes a new service account to be created, of this form:

system:serviceaccount:<namespace>:default

For example, if you wanted to grant the default service account in the kube-system full privilege to the API, you would add this line to your policy file:

{"apiVersion":"abac.authorization.kubernetes.io/v1beta1","kind":"Policy","spec":{"user":"system:serviceaccount:kube-system:default","namespace":"*","resource":"*","apiGroup":"*"}}

The apiserver will need to be restarted to pickup the new policy lines.

RBAC Mode

When specified "RBAC" (Role-Based Access Control) uses the "rbac.authorization.k8s.io" API group to drive authorization decisions, allowing admins to dynamically configure permission policies through the Kubernetes API.

As of 1.6 RBAC mode is in beta.

To enable RBAC, start the apiserver with --authorization-mode=RBAC.

For details about administrating and using RBAC, view the RBAC guide.

Webhook Mode

When specified, mode Webhook causes Kubernetes to query an outside REST service when determining user privileges.

Configuration File Format

Mode Webhook requires a file for HTTP configuration, specify by the --authorization-webhook-config-file=SOME_FILENAME flag.

The configuration file uses the kubeconfig file format. Within the file "users" refers to the API Server webhook and "clusters" refers to the remote service.

A configuration example which uses HTTPS client auth:

# clusters refers to the remote service.
clusters:
  - name: name-of-remote-authz-service
    cluster:
      certificate-authority: /path/to/ca.pem      # CA for verifying the remote service.
      server: https://authz.example.com/authorize # URL of remote service to query. Must use 'https'.

# users refers to the API Server's webhook configuration.
users:
  - name: name-of-api-server
    user:
      client-certificate: /path/to/cert.pem # cert for the webhook plugin to use
      client-key: /path/to/key.pem          # key matching the cert

# kubeconfig files require a context. Provide one for the API Server.
current-context: webhook
contexts:
- context:
    cluster: name-of-remote-authz-service
    user: name-of-api-server
  name: webhook

Request Payloads

When faced with an authorization decision, the API Server POSTs a JSON serialized api.authorization.v1beta1.SubjectAccessReview object describing the action. This object contains fields describing the user attempting to make the request, and either details about the resource being accessed or requests attributes.

Note that webhook API objects are subject to the same versioning compatibility rules as other Kubernetes API objects. Implementers should be aware of looser compatibility promises for beta objects and check the "apiVersion" field of the request to ensure correct deserialization. Additionally, the API Server must enable the authorization.k8s.io/v1beta1 API extensions group (--runtime-config=authorization.k8s.io/v1beta1=true).

An example request body:

{
  "apiVersion": "authorization.k8s.io/v1beta1",
  "kind": "SubjectAccessReview",
  "spec": {
    "resourceAttributes": {
      "namespace": "kittensandponies",
      "verb": "get",
      "group": "unicorn.example.org",
      "resource": "pods"
    },
    "user": "jane",
    "group": [
      "group1",
      "group2"
    ]
  }
}

The remote service is expected to fill the SubjectAccessReviewStatus field of the request and respond to either allow or disallow access. The response body's "spec" field is ignored and may be omitted. A permissive response would return:

{
  "apiVersion": "authorization.k8s.io/v1beta1",
  "kind": "SubjectAccessReview",
  "status": {
    "allowed": true
  }
}

To disallow access, the remote service would return:

{
  "apiVersion": "authorization.k8s.io/v1beta1",
  "kind": "SubjectAccessReview",
  "status": {
    "allowed": false,
    "reason": "user does not have read access to the namespace"
  }
}

Access to non-resource paths are sent as:

{
  "apiVersion": "authorization.k8s.io/v1beta1",
  "kind": "SubjectAccessReview",
  "spec": {
    "nonResourceAttributes": {
      "path": "/debug",
      "verb": "get"
    },
    "user": "jane",
    "group": [
      "group1",
      "group2"
    ]
  }
}

Non-resource paths include: /api, /apis, /metrics, /resetMetrics, /logs, /debug, /healthz, /swagger-ui/, /swaggerapi/, /ui, and /version. Clients require access to /api, /api/*, /apis, /apis/*, and /version to discover what resources and versions are present on the server. Access to other non-resource paths can be disallowed without restricting access to the REST api.

For further documentation refer to the authorization.v1beta1 API objects and plugin/pkg/auth/authorizer/webhook/webhook.go.

Module Development

Other implementations can be developed fairly easily. The APIserver calls the Authorizer interface:

type Authorizer interface {
  Authorize(a Attributes) error
}

to determine whether or not to allow each API action.

An authorization plugin is a module that implements this interface. Authorization plugin code goes in pkg/auth/authorizer/$MODULENAME.

An authorization module can be completely implemented in go, or can call out to a remote authorization service. Authorization modules can implement their own caching to reduce the cost of repeated authorization calls with the same or similar arguments. Developers should then consider the interaction between caching and revocation of permissions.

Checking API Access

Kubernetes exposes the subjectaccessreviews.v1.authorization.k8s.io resource as a normal resource that allows external access to API authorizer decisions. No matter which authorizer you choose to use, you can issue a POST with a SubjectAccessReview just like the webhook authorizer to the apis/authorization.k8s.io/v1/subjectaccessreviews endpoint and get back a response. For instance:

kubectl create --v=8 -f -  << __EOF__
{
  "apiVersion": "authorization.k8s.io/v1",
  "kind": "SubjectAccessReview",
  "spec": {
    "resourceAttributes": {
      "namespace": "kittensandponies",
      "verb": "get",
      "group": "unicorn.example.org",
      "resource": "pods"
    },
    "user": "jane",
    "group": [
      "group1",
      "group2"
    ],
    "extra": {
      "scopes": [
        "openid",
        "profile"
      ]
    }
  }
}
__EOF__

--- snip lots of output ---

I0913 08:12:31.362873   27425 request.go:908] Response Body: {"kind":"SubjectAccessReview","apiVersion":"authorization.k8s.io/v1","metadata":{"creationTimestamp":null},"spec":{"resourceAttributes":{"namespace":"kittensandponies","verb":"GET","group":"unicorn.example.org","resource":"pods"},"user":"jane","group":["group1","group2"],"extra":{"scopes":["openid","profile"]}},"status":{"allowed":true}}
subjectaccessreview "" created

This is useful for debugging access problems, in that you can use this resource to determine what access an authorizer is granting.