Certificate Signing Requests

FEATURE STATE: Kubernetes v1.19 [stable]

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 certificates 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. Note that spec.signerName is a required key after API version certificates.k8s.io/v1.

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 controller. 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. The 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 the behavior when an expiration is different than the signer-determined expiration that 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 the certificate plus intermediates to be presented during TLS handshakes.

The PKCS#10 signing request format doesn't allow to specify a certificate expiration or lifetime. The expiration or lifetime therefore has to be set through e.g. an annotation on the CSR object. While it's theoretically possible for a signer to use that expiration date, there is currently no known implementation that does. (The built-in signers all use the same ClusterSigningDuration configuration option, which defaults to 1 year, and can be changed with the --cluster-signing-duration command-line flag of the kube-controller-manager.)

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 certificates by the API server. Never auto-approved by kube-controller-manager.

    1. Trust distribution: signed certificates must be honored as client certificates by the API server. 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 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 ["client auth"]. Must not include key usages beyond ["digital signature", "key encipherment", "client auth"].
    5. Expiration/certificate lifetime - set by the --cluster-signing-duration option for the kube-controller-manager implementation of this signer.
    6. CA bit allowed/disallowed - not allowed.
  2. kubernetes.io/kube-apiserver-client-kubelet: signs client certificates that will be honored as client certificates by the API server. May be auto-approved by kube-controller-manager.

    1. Trust distribution: signed certificates must be honored as client certificates by the API server. The CA bundle is not distributed by any other means.
    2. Permitted subjects - organizations are exactly ["system:nodes"], common name starts with "system:node:".
    3. Permitted x509 extensions - honors key usage extensions, forbids subjectAltName extensions and drops other extensions.
    4. Permitted key usages - exactly ["key encipherment", "digital signature", "client auth"].
    5. Expiration/certificate lifetime - set by the --cluster-signing-duration option for the kube-controller-manager implementation of this 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 API server, but has no other guarantees. Never auto-approved by kube-controller-manager.

    1. Trust distribution: signed certificates must be honored by the API server as valid to terminate connections to a kubelet. The CA bundle is not distributed by any other means.
    2. Permitted subjects - organizations are exactly ["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 ["key encipherment", "digital signature", "server auth"].
    5. Expiration/certificate lifetime - set by the --cluster-signing-duration option for the kube-controller-manager implementation of this signer.
    6. CA bit allowed/disallowed - not allowed.
  4. kubernetes.io/legacy-unknown: has no guarantees for trust at all. Some third-party distributions of Kubernetes may honor client certificates signed by it. The stable CertificateSigningRequest API (version certificates.k8s.io/v1 and later) does not allow to set the signerName as kubernetes.io/legacy-unknown. Never auto-approved by kube-controller-manager.

    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 - set by the --cluster-signing-duration option for the kube-controller-manager implementation of this 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 API server 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

Normal user

A few steps are required in order to get a normal user to be able to authenticate and invoke an API. First, this user must have certificate issued by the Kubernetes cluster, and then present that Certificate to the API call as the Certificate Header or through the kubectl.

Create private key

The following scripts show how to generate PKI private key and CSR. It is important to set CN and O attribute of the CSR. CN is the name of the user and O is the group that this user will belong to. You can refer to RBAC for standard groups.

openssl genrsa -out john.key 2048
openssl req -new -key john.key -out john.csr

Create CertificateSigningRequest

Create a CertificateSigningRequest and submit it to a Kubernetes Cluster via kubectl. Below is a script to generate the CertificateSigningRequest.

cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: john
spec:
  groups:
  - system:authenticated
  request: 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
  signerName: kubernetes.io/kube-apiserver-client
  usages:
  - client auth
EOF

Some points to note:

  • usages has to be 'client auth'
  • request is the base64 encoded value of the CSR file content. You can get the content using this command: cat john.csr | base64 | tr -d "\n"

Approve certificate signing request

Use kubectl to create a CSR and approve it.

Get the list of CSRs:

kubectl get csr

Approve the CSR:

kubectl certificate approve john

Get the certificate

Retrieve the certificate from the CSR:

kubectl get csr/john -o yaml

The certificate value is in Base64-encoded format under status.certificate.

Create Role and RoleBinding

With the certificate created. it is time to define the Role and RoleBinding for this user to access Kubernetes cluster resources.

This is a sample script to create a Role for this new user:

kubectl create role developer --verb=create --verb=get --verb=list --verb=update --verb=delete --resource=pods

This is a sample command to create a RoleBinding for this new user:

kubectl create rolebinding developer-binding-john --role=developer --user=john

Add to kubeconfig

The last step is to add this user into the kubeconfig file. This example assumes the key and certificate files are located at "/home/vagrant/work/".

First, you need to add new credentials:

kubectl config set-credentials john --client-key=/home/vagrant/work/john.key --client-certificate=/home/vagrant/work/john.crt --embed-certs=true

Then, you need to add the context:

kubectl config set-context john --cluster=kubernetes --user=john

To test it, change the context to john:

kubectl config use-context john

Approval or 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 or 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 or 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 operator 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/v1
kind: CertificateSigningRequest
...
status:
  conditions:
  - lastUpdateTime: "2020-02-08T11:37:35Z"
    lastTransitionTime: "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/v1
kind: CertificateSigningRequest
...
status:
  conditions:
  - lastUpdateTime: "2020-02-08T11:37:35Z"
    lastTransitionTime: "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. This field contains one or more PEM-encoded certificates.

All PEM blocks must have the "CERTIFICATE" label, contain no headers, and the encoded data must be a BER-encoded ASN.1 Certificate structure as described in section 4 of RFC5280.

Example certificate content:

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Non-PEM content may appear before or after the CERTIFICATE PEM blocks and is unvalidated, to allow for explanatory text as described in section 5.2 of RFC7468.

When encoded in JSON or YAML, this field is base-64 encoded. A CertificateSigningRequest containing the example certificate above would look like this:

apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
...
status:
  certificate: "LS0tLS1CRUdJTiBDRVJUSUZJQ0FURS0tLS0tCk1JS..."

What's next