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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:

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:

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)

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:

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 user to do something, write a policy with the user property set to "*".

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):

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": {"user": "*", "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 1.3 RBAC mode is in alpha and considered experimental.

To use RBAC, you must both enable the authorization module with --authorization-mode=RBAC, and enable the API version, with a --runtime-config= that includes rbac.authorization.k8s.io/v1alpha1.

Privilege Escalation Prevention and Bootstrapping

The rbac.authorization.k8s.io API group inherently attempts to prevent users from escalating privileges. Simply put, a user can’t grant permissions they don’t already have even when the RBAC authorizer it disabled. If “user-1” does not have the ability to read secrets in “namespace-a”, they cannot create a binding that would grant that permission to themselves or any other user.

For bootstrapping the first roles, it becomes necessary for someone to get around these limitations. For the alpha release of RBAC, an API Server flag was added to allow one user to step around all RBAC authorization and privilege escalation checks. NOTE: This is subject to change with future releases.

--authorization-rbac-super-user=admin

Once set the specified super user, in this case “admin”, can be used to create the roles and role bindings to initialize the system.

This flag is optional and once the initial bootstrapping is performed can be unset.

Roles, RolesBindings, ClusterRoles, and ClusterRoleBindings

The RBAC API Group declares four top level types which will be covered in this section. Users can interact with these resources as they would with any other API resource. Through kubectl, direct calls to the API, etc. For instance, kubectl create -f (resource).yml can be used with any of these examples, though readers who wish to follow along should review the following section on bootstrapping first.

In the RBAC API Group, roles hold a logical grouping of permissions. These permissions map very closely to ABAC policies, but only contain information about requests being made. Permission are purely additive, rules may only omit permissions they do not wish to grant.

Here’s an example of a role which grants read access to pods within the “default” namespace.

kind: Role
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  namespace: default
  name: pod-reader
rules:
  - apiGroups: [""] # The API group "" indicates the core API Group.
    resources: ["pods"]
    verbs: ["get", "watch", "list"]
    nonResourceURLs: []

ClusterRoles hold the same information as a Role but can apply to any namespace as well as non-namespaced resources (such as Nodes, PersistentVolume, etc.). The following ClusterRole can grant permissions to read secrets in any namespace.

kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  # "namespace" omitted since ClusterRoles are not namespaced.
  name: secret-reader
rules:
  - apiGroups: [""]
    resources: ["secrets"]
    verbs: ["get", "watch", "list"]
    nonResourceURLs: []

RoleBindings perform the task of granting the permission to a user or set of users. They hold a list of subjects which they apply to, and a reference to the Role being assigned.

The following RoleBinding assigns the “pod-reader” role to the user “jane” within the “default” namespace, and allows jane to read pods.

# This role binding allows "jane" to read pods in the namespace "default"
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  name: read-pods
  namespace: default
subjects:
  - kind: User # May be "User", "Group" or "ServiceAccount"
    name: jane
roleRef:
  kind: Role
  namespace: default
  name: pod-reader
  apiVersion: rbac.authorization.k8s.io/v1alpha1

RoleBindings may also refer to a ClusterRole. However, a RoleBinding that refers to a ClusterRole only applies in the RoleBinding’s namespace, not at the cluster level. This allows admins to define a set of common roles for the entire cluster, then reuse them in multiple namespaces.

For instance, even though the following RoleBinding refers to a ClusterRole, “dave” (the subject) will only be able read secrets in the “development” namespace, the namespace of the RoleBinding.

# This role binding allows "dave" to read secrets in the namespace "development"
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  name: read-secrets
  namespace: development # This binding only applies in the "development" namespace
subjects:
  - kind: User # May be "User", "Group" or "ServiceAccount"
    name: dave
roleRef:
  kind: ClusterRole
  name: secret-reader
  apiVersion: rbac.authorization.k8s.io/v1alpha1

Finally a ClusterRoleBinding may be used to grant permissions in all namespaces. The following ClusterRoleBinding allows any user in the group “manager” to read secrets in any namepsace.

# This cluster role binding allows anyone in the "manager" group to read secrets in any namespace.
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  name: read-secrets
subjects:
  - kind: Group # May be "User", "Group" or "ServiceAccount"
    name: manager
roleRef:
  kind: ClusterRole
  name: secret-reader
  apiVersion: rbac.authorization.k8s.io/v1alpha1

Referring to Resources

Most resources are represented by a string representation of their name, such as “pods”, just as it appears in the URL for the relevant API endpoint. However, some Kubernetes APIs involve a “subresource” such as the logs for a pod. The URL for the pods logs endpoint is:

GET /api/v1/namespaces/{namespace}/pods/{name}/log

In this case, “pods” is the namespaced resource, and “log” is a subresource of pods. To represent this in an RBAC role, use a slash to delimit the resource and subresource names. To allow a subject to read both pods and pod logs, you would write:

kind: Role
apiVersion: rbac.authorization.k8s.io/v1alpha1
metadata:
  namespace: default
  name: pod-and-pod-logs-reader
rules:
  - apiGroups: [""]
    resources: ["pods", "pods/log"]
    verbs: ["get", "list"]

Referring to Subjects

RoleBindings and ClusterRoleBindings bind “subjects” to “roles”. Subjects can be groups, users or service accounts.

Users are represented by strings. These can be plain usernames, like “alice”, or email style names, like “bob@example.com”, or numeric ids as string. It is up to the Kubernetes admin to configure the authentication modules to produce usernames in the desired format. The RBAC authorization system does not require any particular format. However, the prefix system: is reserved for Kubernetes system use, and so the admin should ensure usernames should not contain this prefix by accident.

Groups information in Kubernetes is currently provided by the Authenticator modules. (In the future we may add a separate way for the RBAC Authorizer to query groups information for users.) Groups, like users, are represented by a string, and that string has no format requirements, other than that the prefix system: is reserved.

Service Accounts have usernames with the system: prefix and belong to groups with the system: prefix.

Role Binding Examples

Only the subjects section of a RoleBinding object shown in the following examples.

For a user called alice@example.com, specify

subjects:
  - kind: User
    name: "alice@example.com"

For a group called frontend-admins, specify:

subjects:
  - kind: Group
    name: "frontend-admins"

For the default service account in the kube-system namespace:

subjects:
 - kind: ServiceAccount
   name: default
   namespace: kube-system

For all service accounts in the qa namespace:

subjects:
- kind: Group
  name: system:serviceaccounts:qa

For all service accounts everywhere:

subjects:
- kind: Group
  name: system:serviceaccounts

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-sever
  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 loser 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": "*",
      "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/*, /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.v1beta1.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/v1beta1/subjectaccessreviews endpoint and get back a response. For instance:

kubectl create --v=8 -f -  << __EOF__
{
  "apiVersion": "authorization.k8s.io/v1beta1",
  "kind": "SubjectAccessReview",
  "spec": {
    "resourceAttributes": {
      "namespace": "kittensandponies",
      "verb": "GET",
      "group": "unicorn.example.org",
      "resource": "pods"
    },
    "user": "jane",
    "group": [
      "group1",
      "group2"
    ]
  }
}
__EOF__

--- snip lots of output ---

I0913 08:12:31.362873   27425 request.go:908] Response Body: {"kind":"SubjectAccessReview","apiVersion":"authorization.k8s.io/v1beta1","metadata":{"creationTimestamp":null},"spec":{"resourceAttributes":{"namespace":"kittensandponies","verb":"GET","group":"*","resource":"pods"},"user":"jane","group":["group1","group2"]},"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.

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