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Certificate Signing Requests

FEATURE STATE: Kubernetes v1.18 [beta]

The Certificates API enables automation of X.509 credential provisioning by providing a programmatic interface for clients of the Kubernetes API to request and obtain X.509 certificatesA cryptographically secure file used to validate access to the Kubernetes cluster. from a Certificate Authority (CA).

A CertificateSigningRequest (CSR) resource is used to request that a certificate be signed by a denoted signer, after which the request may be approved or denied before finally being signed.

Request signing process

The CertificateSigningRequest resource type allows a client to ask for an X.509 certificate be issued, based on a signing request. The CertificateSigningRequest object includes a PEM-encoded PKCS#10 signing request in the spec.request field. The CertificateSigningRequest denotes the signer (the recipient that the request is being made to) using the spec.signerName field.

Once created, a CertificateSigningRequest must be approved before it can be signed. Depending on the signer selected, a CertificateSigningRequest may be automatically approved by a controllerA control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state. . Otherwise, a CertificateSigningRequest must be manually approved either via the REST API (or client-go) or by running kubectl certificate approve. Likewise, a CertificateSigningRequest may also be denied, which tells the configured signer that it must not sign the request.

For certificates that have been approved, the next step is signing. The relevant signing controller first validates that the signing conditions are met and then creates a certificate. The signing controller then updates the CertificateSigningRequest, storing the new certificate into the status.certificate field of the existing CertificateSigningRequest object. The status.certificate field is either empty or contains a X.509 certificate, encoded in PEM format. The CertificateSigningRequest status.certificate field is empty until the signer does this.

Once the status.certificate field has been populated, the request has been completed and clients can now fetch the signed certificate PEM data from the CertificateSigningRequest resource. Signers can instead deny certificate signing if the approval conditions are not met.

In order to reduce the number of old CertificateSigningRequest resources left in a cluster, a garbage collection controller runs periodically. The garbage collection removes CertificateSigningRequests that have not changed state for some duration:

  • Approved requests: automatically deleted after 1 hour
  • Denied requests: automatically deleted after 1 hour
  • Pending requests: automatically deleted after 1 hour

Signers

All signers should provide information about how they work so that clients can predict what will happen to their CSRs. This includes:

  1. Trust distribution: how trust (CA bundles) are distributed.
  2. Permitted subjects: any restrictions on and behavior when a disallowed subject is requested.
  3. Permitted x509 extensions: including IP subjectAltNames, DNS subjectAltNames, Email subjectAltNames, URI subjectAltNames etc, and behavior when a disallowed extension is requested.
  4. Permitted key usages / extended key usages: any restrictions on and behavior when usages different than the signer-determined usages are specified in the CSR.
  5. Expiration/certificate lifetime: whether it is fixed by the signer, configurable by the admin, determined by the CSR object etc and behavior if an expiration different than the signer-determined expiration is specified in the CSR.
  6. CA bit allowed/disallowed: and behavior if a CSR contains a request a for a CA certificate when the signer does not permit it.

Commonly, the status.certificate field contains a single PEM-encoded X.509 certificate once the CSR is approved and the certificate is issued. Some signers store multiple certificates into the status.certificate field. In that case, the documentation for the signer should specify the meaning of additional certificates; for example, this might be certificate plus intermediates to be presented during TLS handshakes.

Kubernetes signers

Kubernetes provides built-in signers that each have a well-known signerName:

  1. kubernetes.io/kube-apiserver-client: signs certificates that will be honored as client-certs by the kube-apiserver. Never auto-approved by kube-controller-managerControl Plane component that runs controller processes. .

    1. Trust distribution: signed certificates must be honored as client-certificates by the kube-apiserver. The CA bundle is not distributed by any other means.
    2. Permitted subjects - no subject restrictions, but approvers and signers may choose not to approve or sign. Certain subjects like cluster-admin level users or groups vary between distributions and installations, but deserve additional scrutiny before approval and signing. The CertificateSubjectRestriction admission plugin is available and enabled by default to restrict system:masters, but it is often not the only cluster-admin subject in a cluster.
    3. Permitted x509 extensions - honors subjectAltName and key usage extensions and discards other extensions.
    4. Permitted key usages - must include []string{"client auth"}. Must not include key usages beyond []string{"digital signature", "key encipherment", "client auth"}
    5. Expiration/certificate lifetime - minimum of CSR signer or request. The signer is responsible for checking that the certificate lifetime is valid and permissible.
    6. CA bit allowed/disallowed - not allowed.

  2. kubernetes.io/kube-apiserver-client-kubelet: signs client certificates that will be honored as client-certs by the kube-apiserver. May be auto-approved by kube-controller-managerControl Plane component that runs controller processes. .

    1. Trust distribution: signed certificates must be honored as client-certificates by the kube-apiserver. The CA bundle is not distributed by any other means.
    2. Permitted subjects - organizations are exactly []string{"system:nodes"}, common name starts with "system:node:"
    3. Permitted x509 extensions - honors key usage extensions, forbids subjectAltName extensions, drops other extensions.
    4. Permitted key usages - exactly []string{"key encipherment", "digital signature", "client auth"}
    5. Expiration/certificate lifetime - minimum of CSR signer or request. Sanity of the time is the concern of the signer.
    6. CA bit allowed/disallowed - not allowed.

  3. kubernetes.io/kubelet-serving: signs serving certificates that are honored as a valid kubelet serving certificate by the kube-apiserver, but has no other guarantees. Never auto-approved by kube-controller-managerControl Plane component that runs controller processes. .

    1. Trust distribution: signed certificates must be honored by the kube-apiserver as valid to terminate connections to a kubelet. The CA bundle is not distributed by any other means.
    2. Permitted subjects - organizations are exactly []string{"system:nodes"}, common name starts with "system:node:"
    3. Permitted x509 extensions - honors key usage and DNSName/IPAddress subjectAltName extensions, forbids EmailAddress and URI subjectAltName extensions, drops other extensions. At least one DNS or IP subjectAltName must be present.
    4. Permitted key usages - exactly []string{"key encipherment", "digital signature", "server auth"}
    5. Expiration/certificate lifetime - minimum of CSR signer or request.
    6. CA bit allowed/disallowed - not allowed.

  4. kubernetes.io/legacy-unknown: has no guarantees for trust at all. Some distributions may honor these as client certs, but that behavior is not standard Kubernetes behavior. Never auto-approved by kube-controller-managerControl Plane component that runs controller processes. .

    1. Trust distribution: None. There is no standard trust or distribution for this signer in a Kubernetes cluster.
    2. Permitted subjects - any
    3. Permitted x509 extensions - honors subjectAltName and key usage extensions and discards other extensions.
    4. Permitted key usages - any
    5. Expiration/certificate lifetime - minimum of CSR signer or request. Sanity of the time is the concern of the signer.
    6. CA bit allowed/disallowed - not allowed.
Note: Failures for all of these are only reported in kube-controller-manager logs.

Distribution of trust happens out of band for these signers. Any trust outside of those described above are strictly coincidental. For instance, some distributions may honor kubernetes.io/legacy-unknown as client certificates for the kube-apiserver, but this is not a standard. None of these usages are related to ServiceAccount token secrets .data[ca.crt] in any way. That CA bundle is only guaranteed to verify a connection to the kube-apiserver using the default service (kubernetes.default.svc).

Authorization

To allow creating a CertificateSigningRequest and retrieving any CertificateSigningRequest:

  • Verbs: create, get, list, watch, group: certificates.k8s.io, resource: certificatesigningrequests

For example:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: csr-creator
rules:
- apiGroups:
  - certificates.k8s.io
  resources:
  - certificatesigningrequests
  verbs:
  - create
  - get
  - list
  - watch

To allow approving a CertificateSigningRequest:

  • Verbs: get, list, watch, group: certificates.k8s.io, resource: certificatesigningrequests
  • Verbs: update, group: certificates.k8s.io, resource: certificatesigningrequests/approval
  • Verbs: approve, group: certificates.k8s.io, resource: signers, resourceName: <signerNameDomain>/<signerNamePath> or <signerNameDomain>/*

For example:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: csr-approver
rules:
- apiGroups:
  - certificates.k8s.io
  resources:
  - certificatesigningrequests
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - certificates.k8s.io
  resources:
  - certificatesigningrequests/approval
  verbs:
  - update
- apiGroups:
  - certificates.k8s.io
  resources:
  - signers
  resourceNames:
  - example.com/my-signer-name # example.com/* can be used to authorize for all signers in the 'example.com' domain
  verbs:
  - approve

To allow signing a CertificateSigningRequest:

  • Verbs: get, list, watch, group: certificates.k8s.io, resource: certificatesigningrequests
  • Verbs: update, group: certificates.k8s.io, resource: certificatesigningrequests/status
  • Verbs: sign, group: certificates.k8s.io, resource: signers, resourceName: <signerNameDomain>/<signerNamePath> or <signerNameDomain>/*
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: csr-signer
rules:
- apiGroups:
  - certificates.k8s.io
  resources:
  - certificatesigningrequests
  verbs:
  - get
  - list
  - watch
- apiGroups:
  - certificates.k8s.io
  resources:
  - certificatesigningrequests/status
  verbs:
  - update
- apiGroups:
  - certificates.k8s.io
  resources:
  - signers
  resourceName:
  - example.com/my-signer-name # example.com/* can be used to authorize for all signers in the 'example.com' domain
  verbs:
  - sign

Approval & rejection

Control plane automated approval

The kube-controller-manager ships with a built-in approver for certificates with a signerName of kubernetes.io/kube-apiserver-client-kubelet that delegates various permissions on CSRs for node credentials to authorization. The kube-controller-manager POSTs SubjectAccessReview resources to the API server in order to check authorization for certificate approval.

Approval & rejection using kubectl

A Kubernetes administrator (with appropriate permissions) can manually approve (or deny) CertificateSigningRequests by using the kubectl certificate approve and kubectl certificate deny commands.

To approve a CSR with kubectl:

kubectl certificate approve <certificate-signing-request-name>

Likewise, to deny a CSR:

kubectl certificate deny <certificate-signing-request-name>

Approval & rejection using the Kubernetes API

Users of the REST API can approve CSRs by submitting an UPDATE request to the approval subresource of the CSR to be approved. For example, you could write an operatorA specialized controller used to manage a custom resource that watches for a particular kind of CSR and then sends an UPDATE to approve them.

When you make an approval or rejection request, set either the Approved or Denied status condition based on the state you determine:

For Approved CSRs:

apiVersion: certificates.k8s.io/v1beta1
kind: CertificateSigningRequest
...
status:
  conditions:
  - lastUpdateTime: "2020-02-08T11:37:35Z"
    message: Approved by my custom approver controller
    reason: ApprovedByMyPolicy # You can set this to any string
    type: Approved

For Denied CSRs:

apiVersion: certificates.k8s.io/v1beta1
kind: CertificateSigningRequest
...
status:
  conditions:
  - lastUpdateTime: "2020-02-08T11:37:35Z"
    message: Denied by my custom approver controller
    reason: DeniedByMyPolicy # You can set this to any string
    type: Denied

It's usual to set status.conditions.reason to a machine-friendly reason code using TitleCase; this is a convention but you can set it to anything you like. If you want to add a note just for human consumption, use the status.conditions.message field.

Signing

Control plane signer

The Kubernetes control plane implements each of the Kubernetes signers, as part of the kube-controller-manager.

Note: Prior to Kubernetes v1.18, the kube-controller-manager would sign any CSRs that were marked as approved.

API-based signers

Users of the REST API can sign CSRs by submitting an UPDATE request to the status subresource of the CSR to be signed.

As part of this request, the status.certificate field should be set to contain the signed certificate.

What's next

  • Read Manage TLS Certificates in a Cluster
  • View the source code for the kube-controller-manager built in signer
  • View the source code for the kube-controller-manager built in approver
  • For details of X.509 itself, refer to RFC 5280 section 3.1
  • For information on the syntax of PKCS#10 certificate signing requests, refer to RFC 2986