onthelambda provides a streamlined way to authenticate your Golang AWS Lambda function to Hashicorp Vault.

package main

import (
    "github.com/aws/aws-lambda-go/lambda"
    "github.com/daveadams/onthelambda"
    "log"
)

func MyLambdaHandler() {
    client, err := onthelambda.VaultClient()
    if err != nil {
        log.Fatalf("ERROR: %s", err)
    }

    resp, _ := client.Logical().Read("secret/message")
    log.Printf("The secret message is '%s'", resp.Data["value"].(string))
}

func main() {
    lambda.Start(MyLambdaHandler)
}

First, you'll need Vault up and running somewhere network-accessible to your Lambda function. That's out of scope for this README, but please see the Vault documentation for more.

Then you'll need to set up an AWS authentication provider. You may already have one configured. If so, you can use that one or you can set up a new one just for this purpose. You don't need to worry about backend credentials for this authentication method. It works without any AWS credentials needing to be loaded into Vault. Or if you do have credentials loaded they don't need to have access to the AWS account your Lambda is running in.

To establish a new AWS authentication provider, run:

$ vault auth enable -path lambda -description "IAM auth for Lambdas" aws
Success! Enabled aws auth method at: lambda/

You will also need to set the iam_server_id_header_value if you wish to use the extra layer of security (as described below):

$ vault write auth/lambda/config/client \
      iam_server_id_header_value=vault.example.com

Next, you'll need to establish whatever Vault policies your Lambda will need. See the Vault Policies documentation for details.

Now you'll need to know the ARN of your Lambda execution role. You can create it with the Lambda web console or by hand. Either way it should look something like:

arn:aws:iam::987654321098:role/service-role/MyLambdaRole

IMPORTANT: You must remove any non-essential path from the role ARN unless you have configured your AWS auth provider with IAM permissions to look up roles. In this example, service-role/ is the path segment. So the principal ARN you will be specifying to Vault in the next step will be:

arn:aws:iam::987654321098:role/MyLambdaRole

Now it's time to create the Vault authentication role. It can be named anything you wish. In this case, we'll call it my-vault-role and make it periodic since onthelambda will handle renewal automatically:

$ vault write auth/lambda/role/my-vault-role \
      auth_type=iam \
      period=14400 \
      policies=list-of,vault-policies,separated-by-commas \
      resolve_aws_unique_ids=false \
      bound_iam_principal_arn=arn:aws:iam::987654321098:role/MyLambdaRole

Now you are ready to configure your Lambda.

All configuration is done with environment variables:

• VAULT_ADDR (Required) The URL of the Vault instance, eg https://myvault.example.com.
• VAULT_AUTH_PROVIDER (Required) The relative path of the AWS authentication provider, eg lambda for auth/lambda in the example above.
• VAULT_AUTH_ROLE (Required) The name of the Vault role to authenticate to, eg my-vault-role in the example above.
• VAULT_AUTH_HEADER (Optional, but recommended) The value of the X-Vault-AWS-IAM-Server-ID HTTP header to be included in the signed STS request this code uses to authenticate. This value is often set to the URL or DNS name of the Vault server to prevent potential replay attacks.

That should be all that is required to get up and running.

This software is public domain. No rights are reserved. See LICENSE for more information.