SelfSigned

The SelfSigned issuer doesn't represent a certificate authority as such, but instead denotes that certificates will "sign themselves" using a given private key. In other words, the private key of the certificate will be used to sign the certificate itself.

This Issuer type is useful for bootstrapping a root certificate for a custom PKI (Public Key Infrastructure), or for otherwise creating simple ad-hoc certificates.

There are important caveats - including security issues - to consider with SelfSigned issuers; in general you'd likely want to use a CA issuer rather than a SelfSigned issuer. That said, SelfSigned issuers are really useful for initially bootstrapping a CA issuer.

Note: a CertificateRequest that references a self-signed certificate must also contain the cert-manager.io/private-key-secret-name annotation since the private key corresponding to the CertificateRequest is required to sign the certificate. This annotation is added automatically by the Certificate controller.

Deployment

Since the SelfSigned issuer has no dependency on any other resource, it is the simplest to configure. Only the SelfSigned stanza is required to be present in the issuer spec, with no other configuration required:

apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: selfsigned-issuer
  namespace: sandbox
spec:
  selfSigned: {}
---
apiVersion: cert-manager.io/v1
kind: ClusterIssuer
metadata:
  name: selfsigned-cluster-issuer
spec:
  selfSigned: {}

Once deployed, you should be able to see immediately that the issuer is ready for signing:

$ kubectl get issuers  -n sandbox -o wide selfsigned-issuer
NAME                READY   STATUS                AGE
selfsigned-issuer   True                          2m

$ kubectl get clusterissuers -o wide selfsigned-cluster-issuer
NAME                        READY   STATUS   AGE
selfsigned-cluster-issuer   True             3m

Bootstrapping CA Issuers

One of the ideal use cases for SelfSigned issuers is to bootstrap a custom root certificate for a private PKI, including with the cert-manager CA issuer.

The YAML below will create a SelfSigned issuer, issue a root certificate and use that root as a CA issuer:

apiVersion: cert-manager.io/v1
kind: ClusterIssuer
metadata:
  name: selfsigned-issuer
spec:
  selfSigned: {}
---
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: osm-ca
  namespace: sandbox
spec:
  isCA: true
  commonName: osm-system
  secretName: osm-ca
  issuerRef:
    name: selfsigned-issuer
    kind: ClusterIssuer
    group: cert-manager.io
---
apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: osm-ca
  namespace: sandbox
spec:
  ca:
    secretName: osm-ca

CRL Distribution Points

You may also optionally specify CRL Distribution Points as an array of strings, each of which identifies the location of a CRL in which the revocation status of issued certificates can be checked:

...
spec:
  selfSigned:
    crlDistributionPoints:
    - "http://example.com"

Caveats

Trust

Clients consuming SelfSigned certificates have no way to trust them without already having the certificates beforehand. As such, most clients are likely to be forced to "TOFU" (trust on first use), which has security implications in the event of a man-in-the-middle attack.

Certificate Validity

One side-effect of a certificate being self-signed is that its Subject DN and its Issuer DN are identical. The X.509 RFC 5280, section 4.1.2.4 requires that:

The issuer field MUST contain a non-empty distinguished name (DN).

However, self-signed certs don't have a subject DN set by default. Unless you manually set a certificate's Subject DN, the Issuer DN will be empty and the certificate will technically be invalid.

Validation of this specific area of the spec is patchy and varies between TLS libraries, but there's always the risk that a library will improve its validation - entirely within spec - in the future and break your app if you're using a certificate with an empty Issuer DN.

To avoid this, be sure to set a Subject for SelfSigned certs. This can be done by setting the spec.subject on a cert-manager Certificate object which will be issued by a SelfSigned issuer.

Starting in version 1.3, cert-manager will emit a Kubernetes warning event of type BadConfig if it detects that a certificate is being created by a SelfSigned issuer which has an empty Issuer DN.