AppRole Pull Authentication

Before a client can interact with Vault, it must authenticate against an auth method to acquire a token. This token has policies attached so that the behavior of the client can be governed.

Since tokens are the core method for authentication within Vault, there is a token auth method (often referred to as token store). This is a special auth method responsible for creating and storing tokens.

»Auth Methods

Auth methods perform authentication to verify the user or machine-supplied information. Some of the supported auth methods are targeted towards users while others are targeted toward machines or apps. For example, LDAP auth method enables user authentication using an existing LDAP server while AppRole auth method is recommended for machines or apps.

The Getting Started tutorial walks you through how to enable the GitHub auth method for user authentication.

This introductory tutorial focuses on generating tokens for machines or apps by enabling the AppRole auth method.

»Personas

The end-to-end scenario described in this tutorial involves two personas:

• admin with privileged permissions to configure an auth method
• app is the consumer of secrets stored in Vault

»Challenge

Think of a scenario where a DevOps team wants to configure Jenkins to read secrets from Vault so that it can inject the secrets to an app's environment variables (e.g. MYSQL_DB_HOST) at deployment time.

Instead of hardcoding secrets in each build script as plain text, Jenkins retrieves secrets from Vault.

As a user, you can authenticate with Vault using your LDAP credentials, and Vault generates a token. This token has policies granting you permission to perform the appropriate operations.

How can a Jenkins server programmatically request a token so that it can read secrets from Vault?

»Solution

Enable AppRole auth method so that the Jenkins server can obtain a Vault token with appropriate policies attached. Since each AppRole has attached policies, you can write fine-grained policies limiting which app can access which path.

»Prerequisites

To perform the tasks described in this tutorial, you need to have a Vault environment. Refer to the Getting Started tutorial to install Vault. Make sure that your Vault server has been initialized and unsealed.

NOTE: An interactive tutorial is also available if you do not have a Vault environment to perform the steps described in this tutorial. Click the Show Terminal button to start.

»Policy requirements

To perform all tasks demonstrated in this tutorial, your policy must include the following permissions:

# Mount the AppRole auth method
path "sys/auth/approle" {
  capabilities = [ "create", "read", "update", "delete", "sudo" ]
}

# Configure the AppRole auth method
path "sys/auth/approle/*" {
  capabilities = [ "create", "read", "update", "delete" ]
}

# Create and manage roles
path "auth/approle/*" {
  capabilities = [ "create", "read", "update", "delete", "list" ]
}

# Write ACL policies
path "sys/policies/acl/*" {
  capabilities = [ "create", "read", "update", "delete", "list" ]
}

# Write test data
# Set the path to "secret/data/mysql/*" if you are running `kv-v2`
path "secret/mysql/*" {
  capabilities = [ "create", "read", "update", "delete", "list" ]
}

If you are not familiar with policies, complete the policies tutorial.

»Scenario Introduction

AppRole is an authentication mechanism within Vault to allow machines or apps to acquire a token to interact with Vault. It uses RoleID and SecretID for login.

The basic workflow is:

In this tutorial, you are going to perform the following steps:

1. Enable AppRole auth method
2. Create a role with policy attached
3. Get RoleID and SecretID
4. Login with RoleID & SecretID
5. Read secrets using the AppRole token

Step 1 through 3 need to be performed by an admin user. Step 4 and 5 describe the commands that an app runs to get a token and read secrets from Vault.

»Step 1: Enable AppRole auth method

(Persona: admin)

Like many other auth methods, AppRole must be enabled before it can be used.

$ vault auth enable approle

»Step 2: Create a role with policy attached

(Persona: admin)

When you enabled the AppRole auth method, it gets mounted at the /auth/approle path. In this example, you are going to create a role for the app persona (jenkins in our scenario).

First, create a policy file named jenkins-pol.hcl with following policies to set appropriate permissions.

# Read-only permission on 'secret/data/mysql/*' path
path "secret/data/mysql/*" {
  capabilities = [ "read" ]
}

»Step 3: Get RoleID and SecretID

The RoleID and SecretID are like a username and password that a machine or app uses to authenticate.

Since the example created a jenkins role which operates in pull mode, Vault will generate the SecretID. You can set properties such as usage-limit, TTLs, and expirations on the SecretIDs to control its lifecycle.

To retrieve the RoleID, invoke the auth/approle/role/<ROLE_NAME>/role-id endpoint. To generate a new SecretID, invoke the auth/approle/role/<ROLE_NAME>/secret-id endpoint.

»Step 4: Login with RoleID & SecretID

(Persona: app)

The client (in this case, Jenkins) uses the RoleID and SecretID passed by the admin to authenticate with Vault. If Jenkins did not receive the RoleID and/or SecretID, the admin needs to investigate.

$ vault write auth/approle/login role_id="675a50e7-cfe0-be76-e35f-49ec009731ea" \
  secret_id="ed0a642f-2acf-c2da-232f-1b21300d5f29"

  Key                     Value
  ---                     -----
  token                   s.ncEw5bAZJqvGJgl8pBDM0C5h
  token_accessor          gIQFfVhUd8fDsZjC7gLBMnQu
  token_duration          1h
  token_renewable         true
  token_policies          ["default" "jenkins"]
  identity_policies       []
  policies                ["default" "jenkins"]
  token_meta_role_name    jenkins

»Step 5: Read secrets using the AppRole token

(Persona: app)

Once receiving a token from Vault, the client can make future requests using this token.

$ VAULT_TOKEN=s.ncEw5bAZJqvGJgl8pBDM0C5h vault kv get secret/mysql/webapp
No value found at secret/mysql/webapp

$ vault login s.ncEw5bAZJqvGJgl8pBDM0C5h
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.

Key                     Value
---                     -----
token                   s.ncEw5bAZJqvGJgl8pBDM0C5h
token_accessor          gIQFfVhUd8fDsZjC7gLBMnQu
token_duration          55m17s
token_renewable         true
token_policies          ["default" "jenkins"]
identity_policies       []
policies                ["default" "jenkins"]
token_meta_role_name    jenkins

$ vault kv get secret/mysql/webapp
No value found at secret/mysql/webapp

$ tee mysqldb.json <<"EOF"
{
  "url": "foo.example.com:35533",
  "db_name": "users",
  "username": "admin",
  "password": "pa$$w0rd"
}
EOF

$ vault kv put secret/mysql/webapp @mysqldb.json

»Response Wrap the SecretID

The RoleID is equivalent to a username, and SecretID is the corresponding password. The app needs both to log in with Vault. Naturally, the next question becomes how to deliver those values to the client securely.

A common solution involves three personas instead of two: admin, app, and trusted entity. The trusted entity delivers the RoleID and SecretID to the client by separate means.

For example, Terraform as a trusted entity can deliver the RoleID onto the virtual machine. When the app runs on the virtual machine, the RoleID already exists on the virtual machine.

SecretID is like a password. To keep the SecretID confidential, use response wrapping so that only the expecting client can unwrap the SecretID.

In Step 3, you executed the following command to retrieve the Secret ID.

$ vault write -f auth/approle/role/jenkins/secret-id

Instead, use response wrapping by passing the -wrap-ttl parameter.

$ vault write -wrap-ttl=60s -f auth/approle/role/jenkins/secret-id

Key                              Value
---                              -----
wrapping_token:                  s.2kAzCgg1kN7vdpE1xxZxzpug
wrapping_accessor:               1N3YCs02iuZ75OCTi4eN0tuA
wrapping_token_ttl:              1m
wrapping_token_creation_time:    2019-12-16 17:17:13.956126 -0800 PST
wrapping_token_creation_path:    auth/approle/role/jenkins/secret-id

Send this wrapping_token to the client so that the response can be unwrapped and obtain the SecretID.

$ VAULT_TOKEN=s.2kAzCgg1kN7vdpE1xxZxzpug vault unwrap

Key                   Value
---                   -----
secret_id             7673bcf6-bbba-0fa6-a54c-51a6a3219c92
secret_id_accessor    e0104ca1-0afd-5d90-3b99-646bbcb5c179

To learn more about the wrapping token, read the Cubbyhole Response Wrapping tutorial.

»Limit the SecretID Usages

Treat the SecretID like a password and force it to be regenerated after a number of use.

To do so, update the role definition with secret_id_num_uses set.

$ vault write auth/approle/role/jenkins token_policies="jenkins" \
        token_ttl=1h token_max_ttl=4h \
        secret_id_num_uses=10

In this example, a SecretID of the jenkins role can be used for up to 10 times to authenticate and fetch a client token. After the number of uses is reached, the SecretID expires and you would need to generat a new one. This is similar to forcing a password rotation.

»Help and Reference

• AppRole Auth Method
• AppRole Auth Method (API)
• Authenticating Applications with HashiCorp Vault AppRole