• Prerequisites
• How it works
• Example
  • Limit token access to Vault secrets

Authenticating and reading secrets with HashiCorp Vault

This tutorial demonstrates how to authenticate, configure, and read secrets with HashiCorp’s Vault from GitLab CI/CD.

Prerequisites

This tutorial assumes you are familiar with GitLab CI/CD and Vault.

To follow along, you must have:

• An account on GitLab.
• Access to a running Vault server (at least v1.2.0) to configure authentication and to create roles and policies. For HashiCorp Vaults, this can be the Open Source or Enterprise version.

How it works

ID tokens are JSON Web Tokens (JWTs) used for OIDC authentication with third-party services. If a job has at least one ID token defined, the secrets keyword automatically uses that token to authenticate with Vault.

The following fields are included in the JWT:

Example JWT payload:

{
  "jti": "c82eeb0c-5c6f-4a33-abf5-4c474b92b558",
  "iss": "gitlab.example.com",
  "iat": 1585710286,
  "nbf": 1585798372,
  "exp": 1585713886,
  "sub": "job_1212",
  "namespace_id": "1",
  "namespace_path": "mygroup",
  "project_id": "22",
  "project_path": "mygroup/myproject",
  "user_id": "42",
  "user_login": "myuser",
  "user_email": "myuser@example.com",
  "pipeline_id": "1212",
  "pipeline_source": "web",
  "job_id": "1212",
  "ref": "auto-deploy-2020-04-01",
  "ref_type": "branch",
  "ref_path": "refs/heads/auto-deploy-2020-04-01",
  "ref_protected": "true",
  "environment": "production",
  "environment_protected": "true",
  "environment_action": "start"
}

The JWT is encoded by using RS256 and signed with a dedicated private key. The expire time for the token is set to job’s timeout, if specified, or 5 minutes if it is not. The key used to sign this token may change without any notice. In such case retrying the job generates new JWT using the current signing key.

You can use this JWT for authentication with a Vault server that is configured to allow the JWT authentication method. Provide your GitLab instance’s base URL (for example https://gitlab.example.com) to your Vault server as the oidc_discovery_url. The server can then retrieve the keys for validating the token from your instance.

When configuring roles in Vault, you can use bound claims to match against the JWT claims and restrict which secrets each CI/CD job has access to.

To communicate with Vault, you can use either its CLI client or perform API requests (using curl or another client).

Example

Let’s say you have the passwords for your staging and production databases stored in a Vault server that is running on http://vault.example.com:8200. Your staging password is pa$$w0rd and your production password is real-pa$$w0rd.

$ vault kv get -field=password secret/myproject/staging/db
pa$$w0rd

$ vault kv get -field=password secret/myproject/production/db
real-pa$$w0rd

To configure your Vault server, start by enabling the JWT Auth method:

$ vault auth enable jwt
Success! Enabled jwt auth method at: jwt/

Then create policies that allow you to read these secrets (one for each secret):

$ vault policy write myproject-staging - <<EOF
# Policy name: myproject-staging
#
# Read-only permission on 'secret/myproject/staging/*' path
path "secret/myproject/staging/*" {
  capabilities = [ "read" ]
}
EOF
Success! Uploaded policy: myproject-staging

$ vault policy write myproject-production - <<EOF
# Policy name: myproject-production
#
# Read-only permission on 'secret/myproject/production/*' path
path "secret/myproject/production/*" {
  capabilities = [ "read" ]
}
EOF
Success! Uploaded policy: myproject-production

You also need roles that link the JWT with these policies.

One for staging named myproject-staging:

$ vault write auth/jwt/role/myproject-staging - <<EOF
{
  "role_type": "jwt",
  "policies": ["myproject-staging"],
  "token_explicit_max_ttl": 60,
  "user_claim": "user_email",
  "bound_claims": {
    "project_id": "22",
    "ref": "master",
    "ref_type": "branch"
  }
}
EOF

And one for production named myproject-production:

$ vault write auth/jwt/role/myproject-production - <<EOF
{
  "role_type": "jwt",
  "policies": ["myproject-production"],
  "token_explicit_max_ttl": 60,
  "user_claim": "user_email",
  "bound_claims_type": "glob",
  "bound_claims": {
    "project_id": "22",
    "ref_protected": "true",
    "ref_type": "branch",
    "ref": "auto-deploy-*"
  }
}
EOF

This example uses bound claims to specify that only a JWT with matching values for the specified claims is allowed to authenticate.

Combined with protected branches, you can restrict who is able to authenticate and read the secrets.

Any of the claims included in the JWT can be matched against a list of values in the bound claims. For example:

"bound_claims": {
  "user_login": ["alice", "bob", "mallory"]
}

"bound_claims": {
  "ref": ["main", "develop", "test"]
}

"bound_claims": {
  "namespace_id": ["10", "20", "30"]
}

"bound_claims": {
  "project_id": ["12", "22", "37"]
}

• If only namespace_id is used, all projects in the namespace are allowed.
• If both namespace_id and project_id are used, Vault first checks if the project’s namespace is in namespace_id. If not, it then checks if the project is in project_id.

token_explicit_max_ttl specifies that the token issued by Vault, upon successful authentication, has a hard lifetime limit of 60 seconds.

user_claim specifies the name for the Identity alias created by Vault upon a successful login.

bound_claims_type configures the interpretation of the bound_claims values. If set to glob, the values are interpreted as globs, with * matching any number of characters.

The claim fields listed in the table above can also be accessed for Vault’s policy path templating purposes by using the accessor name of the JWT auth within Vault. The mount accessor name (ACCESSOR_NAME in the example below) can be retrieved by running vault auth list.

Policy template example making use of a named metadata field named project_path:

path "secret/data/{{identity.entity.aliases.ACCESSOR_NAME.metadata.project_path}}/staging/*" {
  capabilities = [ "read" ]
}

Role example to support the templated policy above, mapping the claim field project_path as a metadata field through use of claim_mappings configuration:

{
  "role_type": "jwt",
  ...
  "claim_mappings": {
    "project_path": "project_path"
  }
}

For the full list of options, see Vault’s Create Role documentation.

Now, configure the JWT Authentication method:

$ vault write auth/jwt/config \
    oidc_discovery_url="https://gitlab.example.com" \
    bound_issuer="https://gitlab.example.com"

bound_issuer specifies that only a JWT with the issuer (that is, the iss claim) set to gitlab.example.com can use this method to authenticate, and that the oidc_discovery_url (https://gitlab.example.com) should be used to validate the token.

For the full list of available configuration options, see Vault’s API documentation.

In GitLab, create the following CI/CD variables to provide details about your Vault server:

• VAULT_SERVER_URL - The URL of your Vault server, for example https://vault.example.com:8200.
• VAULT_AUTH_ROLE - Optional. The role to use when attempting to authenticate. If no role is specified, Vault uses the default role specified when the authentication method was configured.
• VAULT_AUTH_PATH - Optional. The path where the authentication method is mounted. Default is jwt.
• VAULT_NAMESPACE - Optional. The Vault Enterprise namespace to use for reading secrets and authentication. If no namespace is specified, Vault uses the root (/) namespace. The setting is ignored by Vault Open Source.

The following job, when run for the default branch, can read secrets under secret/myproject/staging/, but not the secrets under secret/myproject/production/:

job_with_secrets:
  id_tokens:
    VAULT_ID_TOKEN:
      aud: https://example.vault.com
  secrets:
    STAGING_DB_PASSWORD:
      vault: secret/myproject/staging/db/password@secrets # authenticates using $VAULT_ID_TOKEN
  script:
    - access-staging-db.sh --token $STAGING_DB_PASSWORD

In this example:

• @secrets - The vault name, where your Secrets Engines are enabled.
• secret/myproject/staging/db - The path location of the secret in Vault.
• password The field to be fetched within the referenced secret.

Limit token access to Vault secrets

You can control ID token access to Vault secrets by using Vault protections and GitLab features. For example, restrict the token by:

• Using Vault bound claims for specific groups using group_claim.
• Hard coding values for Vault bound claims based on the user_login and user_email of specific users.
• Setting Vault time limits for TTL of the token as specified in token_explicit_max_ttl, where the token expires after authentication.
• Scoping the JWT to GitLab protected branches that are restricted to a subset of project users.
• Scoping the JWT to GitLab protected tags, that are restricted to a subset of project users.