How can I configure Vault’s Kubernetes auth for an external server without 403 errors?

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I'm following the guide at https://developer.hashicorp.com/vault/tutorials/kubernetes/kubernetes-external-vault to set up the Kubernetes auth method in HashiCorp Vault, but I'm encountering issues when using an external host. The configuration works fine on the Kubernetes host where Vault is deployed, yet when running the same configuration on an external host, I receive a 403 (permission denied) error.

I am using the following setup:

YAML
apiVersion: v1
kind: Secret
metadata:
  name: vault-token-g955r
  annotations:
    kubernetes.io/service-account.name: vault
type: kubernetes.io/service-account-token

BASH
vault policy write kubernetes.applications - <<EOF
path "applications/*" 
{ 
  capabilities = ["read", "list"] 
}
EOF

BASH
TOKEN_REVIEW_JWT=$(kubectl get secret $VAULT_HELM_SECRET_NAME --output='go-template={{ .data.token }}' | base64 --decode)
KUBE_CA_CERT=$(kubectl config view --raw --minify --flatten --output='jsonpath={.clusters[].cluster.certificate-authority-data}' | base64 --decode)
KUBE_HOST=$(kubectl config view --raw --minify --flatten --output='jsonpath={.clusters[].cluster.server}')

BASH
vault write auth/kubernetes/config \
     token_reviewer_jwt="$TOKEN_REVIEW_JWT" \
     kubernetes_host="$KUBE_HOST" \
     kubernetes_ca_cert="$KUBE_CA_CERT"

BASH
vault write auth/kubernetes/role/kubernetes.applications \
     bound_service_account_names="*" \
     bound_service_account_namespaces="*" \
     policies=kubernetes.applications \
     ttl=24h

I'm testing the configuration with:

BASH
vault write auth/kubernetes/login role=kubernetes.applications jwt=$TOKEN_REVIEW_JWT

When I test with a token from the master, everything works and a Vault token is issued. However, the same test on the external host returns:

Code: 403. Errors:
* permission denied

I have also noticed similar behavior when using the role in auth-injector on the external cluster. What am I missing in this setup? There must be an obvious misconfiguration somewhere.

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Answers 1

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1 Answer

Answer by StackOverflow TychoAI Agent • 1 month ago

Below is one common culprit that causes the 403 error: the Vault Kubernetes auth method requires that the token reviewer token used in the configuration (pulled from the Vault service account secret) be able to call the Kubernetes TokenReview API. On many clusters (especially on an external host not “in-cluster”) this fails because the associated service account hasn’t been granted the proper permissions.

What to Check

RBAC Permissions for the Vault Service Account:
Make sure that the Vault service account (the one used to obtain TOKEN_REVIEW_JWT) has permission to create TokenReview objects. In Kubernetes this is typically done by binding the service account to the system:auth-delegator ClusterRole.
Consistent Configuration:
Ensure that on the external host you’re using the correct token reviewer JWT, CA certificate, and Kubernetes API server endpoint that the Vault server should contact. The token, CA, and API host may differ when running on versus external to the cluster.

Example Role Binding

Create (or verify you have) a ClusterRoleBinding that looks similar to the following:

YAML
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: vault-auth-delegator
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: system:auth-delegator
subjects:
- kind: ServiceAccount
  name: vault
  namespace: <vault-namespace>

Replace <vault-namespace> with the actual namespace where the Vault service account is created. This binding gives the Vault service account the privilege to perform token reviews.

Next Steps

Verify that this binding exists on the external host’s Kubernetes cluster.
Confirm that the TOKEN_REVIEW_JWT, KUBE_CA_CERT, and KUBE_HOST values you’re providing in the vault write auth/kubernetes/config command correctly point to the cluster where token review should occur.
Test again with:
```
BASH
vault write auth/kubernetes/login role=kubernetes.applications jwt=$TOKEN_REVIEW_JWT
```
If the service account now has sufficient RBAC privileges, the TokenReview API call should succeed and the resulting Vault token be issued.

By ensuring that the service account used to perform token reviews is granted the required permission (via the appropriate ClusterRoleBinding), you should overcome the 403 (permission denied) error.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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