Locking Vault Down with Policies

23 Jun 2018

The final part of my Vault miniseries focuses on permissioning, which is provided by Vault’s Policies.

As everything in Vault is represented as a path, the policies DSL (Domain Specific Language) just needs to apply permissions to paths to lock things down. For example, to allow all operations on the cubbyhole secret engine, we would define this policy:

path "cubbyhole/*" {
    capabilities = ["create", "read", "update", "delete", "list"]
}

Vault comes with a default policy which allows token operations (such as looking up its own token info, releasing and renewing tokens), and cubbyhole access.

Let’s combine the last two posts (Managing Postgres Connection Strings with Vault and Secure Communication with Vault) and create a Policy which will allow the use of generated database credentials. If you want more details on the how/why of the set up phase, see those two posts.

Setup

First, we’ll create two containers which will get removed on exit - a Postgres one and a Vault one. Vault is being started in dev mode, so we don’t need to worry about init and unsealing it.

docker run --rm -d -p 5432:5432 -e 'POSTGRES_PASSWORD=postgres' postgres:alpine
docker run --rm -d -p 8200:8200 --cap-add=IPC_LOCK -e VAULT_DEV_ROOT_TOKEN_ID=vault vault

Next, we’ll create our Postgres user account which Vault will use to create temporary credentials:

psql --username postgres --dbname postgres
psql> create role VaultAdmin with Login password 'vault' CreateRole;
psql> grant connect on database postgres to vaultadmin;

Let’s also configure the environment to talk to Vault as an administrator, and enable the two Vault plugins we’ll need:

export VAULT_ADDR="http://localhost:8200"
export VAULT_TOKEN="vault"

vault auth enable approle
vault secrets enable database

We’ll also set up our database secret engine, and configure database roll creation:

vault write database/config/postgres_demo \
    plugin_name=postgresql-database-plugin \
    allowed_roles="default" \
    connection_url="postgresql://:@10.0.75.1:5432/postgres?sslmode=disable" \
    username="VaultAdmin" \
    password="vault"

vault write database/roles/reader \
    db_name=postgres_demo \
    creation_statements="CREATE ROLE \"\" WITH LOGIN PASSWORD '' VALID UNTIL ''; \
        GRANT SELECT ON ALL TABLES IN SCHEMA public TO \"\";" \
    default_ttl="10m" \
    max_ttl="1h"

Creating a Policy

First, we need to create the policy. This can be supplied inline on the command line, but reading from a file means it can be source-controlled, and you something readable too!

While the filename doesn’t need to match the policy name, it helps make it a bit clearer if it does match, so we’ll call this file postgres-connector.hcl.

# vault read database/creds/reader
path "database/creds/reader" {
    capabilities = ["read"]
}

We can then register this policy into Vault. The write documentation indicates that you need to prefix the file path with @, but that doesn’t work for me:

vault policy write postgres-connector postgres-connector.hcl

Setup AppRoles

As before, we’ll create a demo_app role for our application to use to get a token. However this time, we’ll specify the policies field, and pass it in both default and our custom postgres-connector role.

vault write auth/approle/role/demo_app \
    policies="postgres-connector,default"

When we generate our client token using the secret_id and role_id, we’ll get a token which can create database credentials, as well as access the cubbyhole.

The final part of being an admin user for this is to generate and save the secret_id and role_id:

vault write -f -field=secret_id auth/approle/role/demo_app/secret-id
vault read -field=role_id auth/approle/role/demo_app/role-id

Creating a Token and Accessing the Database

Opening a new command line window, we need to generate our client token. Take the two id’s output from the admin window, and use them in the following code block:

export VAULT_ADDR="http://localhost:8200"
SECRET_ID="" # from the 'admin' window!
ROLE_ID="" # from the 'admin' window!

export VAULT_TOKEN=$(curl -X POST --data "{ \"role_id\":\"$ROLE_ID\", \"secret_id\":\"$SECRET_ID\" }" $VAULT_ADDR/v1/auth/approle/login | jq  -r .auth.client_token)

Now we have a client token, we can generate a database connection:

vault read database/creds/reader
# Key                Value
# ---                -----
# lease_id           database/creds/reader/dc2ae2b6-c709-0e2f-49a6-36b45aa84490
# lease_duration     10m
# lease_renewable    true
# password           A1a-1kAiN0gqU07BE39N
# username           v-approle-reader-incldNFPhixc1Kj25Rar-1529764057

Which can also be renewed:

vault lease renew database/creds/reader/dc2ae2b6-c709-0e2f-49a6-36b45aa84490
# Key                Value
# ---                -----
# lease_id           database/creds/reader/dc2ae2b6-c709-0e2f-49a6-36b45aa84490
# lease_duration     10m
# lease_renewable    true

However, if we try to write to the database roles, we get an error:

vault write database/roles/what dbname=postgres_demo
# Error writing data to database/roles/what: Error making API request.
#
# URL: PUT http://localhost:8200/v1/database/roles/what
# Code: 403. Errors:
#
# * permission denied

Summary

It is also a good idea to have separate fine-grained policies, which can then be grouped up against separate AppRoles, allowing each AppRole to have just the permissions it needs. For example, you could have the following Policies:

  • postgres-connection
  • postgres-admin
  • rabbitmq-connection
  • kafka-consumer

You would then have several AppRoles defined which could use different Policies:

  • App1: rabbitmq-connection, postgres-connection
  • App2: kafka-consumer, rabbitmq-connection
  • App3: postgres-admin

Which helps encourage you to have separate AppRoles for each of your applications!

Finally, the Vault website has a guide on how to do this too…which I only found after writing this! At least what I wrote seems to match up with their guide pretty well, other than I also use AppRole authentication (and so should you!)

vault, security, microservices

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