# Requesting a client certificate from a remote Vault server with Microsoft Entra ID authentication

This guide shows how an administrator configures the Vault OIDC authentication method with Microsoft Entra ID as the identity provider, and how a user logs in with the Entra ID account to a remote Vault server and gets a client certificate signed by the Vault CA.

## Prerequisites

- The Vault PKI configured as described in step 1a of the main `README.md`: the `pki_int` mount and the `wif-client` role (`vault_configure.sh`).
- A Vault server reachable over HTTPS. The examples use `https://vault.example.com:8200`.
- Permissions to register applications in the Entra ID tenant.
- `vault` and `openssl` on the workstation of the administrator and of the user, and a web browser on the workstation of the user.

The commands are written for bash.

## 1. Register the application in Entra ID

Role: **Administrator**

1. Register an application for Vault in the tenant.
2. Add the redirect URIs of the type "Web":

    - `http://localhost:8250/oidc/callback` for the CLI login.
    - `https://vault.example.com:8200/ui/vault/auth/oidc/oidc/callback` for the Vault UI login.

3. Record the **Application (client) ID** and the **Directory (tenant) ID**.
4. Under **Certificates & secrets**, create a client secret and record its value.
5. Under **Token configuration**, add a groups claim and select the ID token.
6. Create a security group of the users who may request certificates, add the users to it, and record the **Object ID** of the group.
7. Under **API permissions**, add the Microsoft Graph delegated permission `User.Read` and grant the admin consent. Entra ID omits the groups from the token of users who are members of more than 200 groups, and Vault then reads them through Graph.

## 2. Configure Vault

Role: **Administrator**

Point the CLI at the server and authenticate with an administrator token:

```bash
export VAULT_ADDR=https://vault.example.com:8200
export VAULT_TOKEN=<admin-token>
```

1. Create a policy that allows signing certificate requests with the `wif-client` role and nothing else.
2. Enable the OIDC authentication method and configure Entra ID as the provider.
3. Create a role that accepts only members of the group and attaches the policy. The token is valid for 15 minutes and can be renewed up to 1 hour.

`pki-client.hcl`:

```hcl
path "pki_int/sign/wif-client" {
  capabilities = ["update"]
}
```

```bash
vault policy write pki-client pki-client.hcl

vault auth enable oidc

vault write auth/oidc/config - <<'EOF'
{
  "oidc_discovery_url": "https://login.microsoftonline.com/<tenant-id>/v2.0",
  "oidc_client_id": "<client-id>",
  "oidc_client_secret": "<client-secret>",
  "default_role": "entra-pki",
  "provider_config": {
    "provider": "azure"
  }
}
EOF

vault write auth/oidc/role/entra-pki - <<'EOF'
{
  "role_type": "oidc",
  "user_claim": "sub",
  "allowed_redirect_uris": [
    "http://localhost:8250/oidc/callback",
    "https://vault.example.com:8200/ui/vault/auth/oidc/oidc/callback"
  ],
  "groups_claim": "groups",
  "oidc_scopes": ["profile"],
  "bound_claims": {"groups": ["<group-object-id>"]},
  "token_policies": ["pki-client"],
  "token_ttl": "15m",
  "token_max_ttl": "1h"
}
EOF
```

The redirect URIs in the role must match the ones registered in Entra ID exactly, including the scheme, the port and the trailing path. The `bound_claims` entry limits the login to the members of the group, the other Entra ID users get an error at login.

The commands above pass JSON on standard input, because the CLI cannot express the `bound_claims` map as `key=value` arguments.

> **Note:** The `wif-client` role accepts any common name (`allow_any_name=true`). Every member of the group can therefore request a certificate with any subject CN, including the CN that other users or workloads rely on as the WiF subject. Restrict the role (`allowed_domains`, `allow_bare_domains`, `enforce_hostnames`) when the users must not choose their CN freely.

## 3. Request a certificate

Role: **User**

1. Set the address of the Vault server. Set `VAULT_CACERT` as well when the server certificate is issued by a private CA.

    ```bash
    export VAULT_ADDR=https://vault.example.com:8200
    export VAULT_CACERT=/path/to/server-ca.pem
    ```

2. Log in with the Entra ID account. The CLI starts a listener on `localhost:8250` and opens the browser with the Entra ID sign-in page. After the sign-in the CLI stores the token in the token helper file (`~/.vault-token`).

    ```bash
    vault login -method=oidc role=entra-pki
    ```

3. Generate the client key and the CSR locally. The key is never sent to Vault.

    ```bash
    openssl req \
        -newkey rsa:4096 \
        -keyout client_key.pem \
        -noenc \
        -subj "/CN=test-client" \
        -out client.csr
    ```

4. Sign the CSR with the Vault CA.

    ```bash
    vault write -field=certificate pki_int/sign/wif-client \
        csr=@client.csr \
        common_name=test-client \
        ttl=8760h > client_cert.pem
    ```

5. Check the certificate.

    ```bash
    openssl x509 -in client_cert.pem -noout -subject -issuer -dates -ext extendedKeyUsage
    ```

6. Revoke the token when the work is done.

    ```bash
    vault token revoke -self
    ```

The user token can only sign with the `wif-client` role. Reading the role or any other path returns `permission denied`.