Vault Authentication with GitLab OpenID Connect

Introduced in GitLab 9.0

Vault is a secrets management application offered by HashiCorp. It allows you to store and manage sensitive information such as secret environment variables, encryption keys, and authentication tokens. Vault offers Identity-based Access, which means Vault users can authenticate through several of their preferred cloud providers.

This document explains how Vault users can authenticate themselves through GitLab by utilizing our OpenID authentication feature. The following assumes you already have Vault installed and running.

Get the OpenID Connect client ID and secret from GitLab:

First you must create a GitLab application to obtain an application ID and secret for authenticating into Vault. To do this, sign in to GitLab and follow these steps:
1. In the top-right corner, select your avatar.
2. Select Edit profile.
3. In the left sidebar, select Applications.
4. Fill out the application Name and Redirect URI.
5. Select the OpenID scope.
6. Select Save application.
7. Copy client ID and secret, or keep the page open for reference.
Enable OIDC auth on Vault:

OpenID Connect is not enabled in Vault by default. This needs to be enabled in the terminal.

Open a terminal session and run the following command to enable the OpenID Connect authentication provider in Vault:
```
vault auth enable oidc
```
You should see the following output in the terminal:
```
Success! Enabled oidc auth method at: oidc/
```
Write the OIDC configuration:

Next, Vault needs to be given the application ID and secret generated by GitLab.

In the terminal session, run the following command to give Vault access to the GitLab application you've just created with an OpenID scope. This allows Vault to authenticate through GitLab.

Replace your_application_id and your_secret in the example below with the application ID and secret generated for your app:
```
$ vault write auth/oidc/config \
    oidc_discovery_url="https://gitlab.com" \
    oidc_client_id="your_application_id" \
    oidc_client_secret="your_secret" \
    default_role="demo" \
    bound_issuer="localhost"
```
You should see the following output in the terminal:
```
Success! Data written to: auth/oidc/config
```
Write the OIDC Role Configuration:

Now that Vault has a GitLab application ID and secret, it needs to know the Redirect URIs and scopes given to GitLab during the application creation process. The redirect URIs need to match where your Vault instance is running. The oidc_scopes field needs to include the openid. Similarly to the previous step, replace your_application_id with the generated application ID from GitLab:

This configuration is saved under the name of the role you are creating. In this case, we are creating a demo role. Later, we show how you can access this role through the Vault CLI.

WARNING: If you're using a public GitLab instance (GitLab.com or any other instance publicly accessible), it's paramount to specify the bound_claims to allow access only to members of your group/project. Otherwise, anyone with a public account can access your Vault instance.
```
vault write auth/oidc/role/demo -<<EOF
{
   "user_claim": "sub",
   "allowed_redirect_uris": "your_vault_instance_redirect_uris",
   "bound_audiences": "your_application_id",
   "oidc_scopes": "openid",
   "role_type": "oidc",
   "policies": "demo",
   "ttl": "1h",
   "bound_claims": { "groups": ["yourGroup/yourSubgrup"] }
}
EOF
```
Sign in to Vault:
1. Go to your Vault UI (example: http://127.0.0.1:8200/ui/vault/auth?with=oidc).
2. If the OIDC method is not currently selected, open the dropdown and select it.
3. Click the Sign in With GitLab button, which opens a modal window:
4. Click Authorize on the modal to allow Vault to sign in through GitLab. This redirects you back to your Vault UI as a signed-in user.
Sign in using the Vault CLI (optional):

Vault also allows you to sign in via their CLI.

After writing the same configurations from above, you can run the command below in your terminal to sign in with the role configuration created in step 4 above:
```
vault login -method=oidc port=8250 role=demo
```
Here's a short explanation of what this command does:
1. In the Write the OIDC Role Configuration (step 4), we created a role called demo. We set role=demo so Vault knows which configuration we'd like to sign in with.
2. To set Vault to use the OIDC sign-in method, we set -method=oidc.
3. To set the port that GitLab should redirect to, we set port=8250 or another port number that matches the port given to GitLab when listing Redirect URIs.
After running the command, it presents a link in the terminal. Click the link in the terminal and a browser tab opens that confirms you're signed into Vault via OIDC:

The terminal outputs:
```
Success! You are now authenticated. The token information displayed below
is already stored in the token helper. You do NOT need to run "vault login"
again. Future Vault requests will automatically use this token.
```