proposal: bound service account tokens

Proposal to improve the utility of service account tokens by supporting
audience, time and key binding.

@kubernetes/sig-auth-api-reviews

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+# Bound Service Account Tokens
+
+Author: @mikedanese
+
+# Objective
+
+This document describes an API that would allow workloads running on Kubernetes
+to request JSON Web Tokens that are audience, time and eventually key bound.
+
+# Background
+
+Kubernetes already provisions JWTs to workloads. This functionality is on by
+default and thus widely deployed. The current workload JWT system has serious
+issues:
+
+1.  Security: JWTs are not audience bound. Any recipient of a JWT can masquerade
+    as the presenter to anyone else.
+1.  Security: The current model of storing the service account token in a Secret
+    and delivering it to nodes results in a broad attack surface for the
+    Kubernetes control plane when powerful components are run - giving a service
+    account a permission means that any component that can see that service
+    account's secrets is at least as powerful as the component.
+1.  Security: JWTs are not time bound. A JWT compromised via 1 or 2, is valid
+    for as long as the service account exists. This may be mitigated with
+    service account signing key rotation but is not supported by client-go and
+    not automated by the control plane and thus is not widely deployed.
+1.  Scalability: JWTs require a Kubernetes secret per service account.
+
+# Proposal
+
+Infrastructure to support on demand token requests will be implemented in the
+core apiserver. Once this API exists, a client of the apiserver will request an
+attenuated token for it's own use. The API will enforce required attenuations,
+e.g. audience and time binding.
+
+## Token attenuations
+
+### Audience binding
+
+Tokens issued from this API will be audience bound. Audience of requested tokens
+will be bound by the `aud` claim. The `aud` claim is an array of strings
+(usually URLs) that correspond to the intended audience of the token. A
+recipient of a token is responsible for verifying that it identifies as one of
+the values in the audience claim, and should otherwise reject the token. The
+TokenReview API will support this validation.
+
+### Time binding
+
+Tokens issued from this API will be time bound. Time validity of these tokens
+will be claimed in the following fields:
+
+* `exp`: expiration time
+* `nbf`: not before
+* `iat`: issued at
+
+A recipient of a token should verify that the token is valid at the time that
+the token is presented, and should otherwise reject the token. The TokenReview
+API will support this validation.
+
+Cluster administrators will be able to configure the maximum validity duration
+for expiring tokens. During the migration off of the old service account tokens,
+clients of this API may request tokens that are valid for many years. These
+tokens will be drop in replacements for the current service account tokens.
+
+### Object binding
+
+Tokens issued from this API may be bound to a Kubernetes object in the same
+namespace as the service account. The name, group, version, kind and uid of the
+object will be embedded as claims in the issued token. A token bound to an
+object will only be valid for as long as that object exists.
+
+Only a subset of object kinds will support object binding. Initially the only
+kinds that will be supported are:
+
+* v1/Pod
+* v1/Secret
+
+The TokenRequest API will validate this binding.
+
+## API Changes
+
+### Add `tokenrequests.authentication.k8s.io`
+
+We will add an imperative API (a la TokenReview) to the
+`authentication.k8s.io` API group:
+
+```golang
+type TokenRequest struct {
+  Spec   TokenRequestSpec
+  Status TokenRequestStatus
+}
+
+type TokenRequestSpec struct {
+  // Audiences are the intendend audiences of the token. A token issued
+  // for multiple audiences may be used to authenticate against any of
+  // the audiences listed. This implies a high degree of trust between
+  // the target audiences.
+  Audiences []string
+
+  // ValidityDuration is the requested duration of validity of the request. The
+  // token issuer may return a token with a different validity duration so a
+  // client needs to check the 'expiration' field in a response.
+  ValidityDuration metav1.Duration
+
+  // BoundObjectRef is a reference to an object that the token will be bound to.
+  // The token will only be valid for as long as the bound object exists.
+  BoundObjectRef *BoundObjectReference
+}
+
+type BoundObjectReference struct {
+  // Kind of the referent. Valid kinds are 'Pod' and 'Secret'.
+  Kind string
+  // API version of the referent.
+  APIVersion string
+
+  // Name of the referent.
+  Name string
+  // UID of the referent.
+  UID types.UID
+}
+
+type TokenRequestStatus struct {
+  // Token is the token data
+  Token string
+
+  // Expiration is the time of expiration of the returned token. Empty means the
+  // token does not expire.
+  Expiration metav1.Time
+}
+
+```
+
+This API will be exposed as a subresource under a serviceacccount object. A
+requestor for a token for a specific service account will `POST` a
+`TokenRequest` to the `/token` subresource of that service account object.
+
+### Modify `tokenreviews.authentication.k8s.io`
+
+The TokenReview API will be extended to support passing an additional audience
+field which the service account authenticator will validate.
+
+```golang
+type TokenReviewSpec struct {
+  // Token is the opaque bearer token.
+  Token string
+  // Audiences is the identifier that the client identifies as.
+  Audiences []string
+}
+```
+
+### Example Flow
+
+```
+> POST /apis/v1/namespaces/default/serviceaccounts/default/token
+> {
+>   "kind": "TokenRequest",
+>   "apiVersion": "authentication.k8s.io/v1",
+>   "spec": {
+>     "audience": [
+>       "https://kubernetes.default.svc"
+>     ],
+>     "validityDuration": "99999h",
+>     "boundObjectRef": {
+>       "kind": "Pod",
+>       "apiVersion": "v1",
+>       "name": "pod-foo-346acf"
+>     }
+>   }
+> }
+{
+  "kind": "TokenRequest",
+  "apiVersion": "authentication.k8s.io/v1",
+  "spec": {
+    "audience": [
+      "https://kubernetes.default.svc"
+    ],
+    "validityDuration": "99999h",
+    "boundObjectRef": {
+      "kind": "Pod",
+      "apiVersion": "v1",
+      "name": "pod-foo-346acf"
+    }
+  },
+  "status": {
+    "token":
+    "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJz[payload omitted].EkN-[signature omitted]",
+    "expiration": "Jan 24 16:36:00 PST 3018"
+  }
+}
+```
+
+The token payload will be:
+
+```
+{
+  "iss": "https://example.com/some/path",
+  "sub": "system:serviceaccount:default:default,
+  "aud": [
+    "https://kubernetes.default.svc"
+  ],
+  "exp": 24412841114,
+  "iat": 1516841043,
+  "nbf": 1516841043,
+  "kubernetes.io": {
+    "serviceAccountUID": "c0c98eab-0168-11e8-92e5-42010af00002",
+    "boundObjectRef": {
+      "kind": "Pod",
+      "apiVersion": "v1",
+      "uid": "a4bb8aa4-0168-11e8-92e5-42010af00002",
+      "name": "pod-foo-346acf"
+    }
+  }
+}
+```
+
+## Service Account Authenticator Modification
+
+The service account token authenticator will be extended to support validation
+of time and audience binding claims.
+
+## ACLs for TokenRequest
+
+The NodeAuthorizer will allow the kubelet to use its credentials to request a
+service account token on behalf of pods running on that node. The
+NodeRestriction admission controller will require that these tokens are pod
+bound.
+
+## Footnotes
+
+* New apiserver flags:
+  * --service-account-issuer: Identifier of the issuer.
+  * --service-account-signing-key: Path to issuer private key used for signing.
+  * --service-account-api-audience: Identifier of the API. Used to validate
+    tokens authenticating to the Kubernetes API.
+* The Kubernetes apiserver will identify itself as `kubernetes.default.svc`
+  which is the DNS name of the Kubernetes apiserver. When no audience is
+  requested, the audience is defaulted the audience is defaulted to an array
+  containing only this identifier.
+