github.com/dallasmarlow/eks_cert_fingerprint_indexer

eks_cert_fingerprint_indexer

This repository contains a utility program which distributes TLS certificate SHA1 fingerprints for AWS EKS cluster identity OIDC issuer endpoints via AWS SSM parameters.

Why is this a thing?

AWS EKS clusters use OpenID Connect Providers when granting AWS IAM credentials to K8S entities (namespaces, service accounts). When creating a new OIDC provider to use with an existing EKS cluster you must supply a certificate fingerprint of the OIDC issuer that was used when the EKS cluster was created.

AWS provides the following documentation with instructions on how to obtain the certificate fingerprint manually: https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_providers_create_oidc_verify-thumbprint.html.

Typically when managing EKS clusters using Terraform you can use the TLS provider to fetch the certificate fingerprint of the OIDC issuer associated with a given EKS cluster directly. Example:

data "tls_certificate" "cluster_issuer" {
  url = aws_eks_cluster.example.identity[0].oidc[0].issuer
}

resource "aws_iam_openid_connect_provider" "eks_cluster" {
  client_id_list = [
    "sts.amazonaws.com",
  ]
  thumbprint_list = [
    data.tls_certificate.cluster_issuer.certificates[0].sha1_fingerprint,
  ]
  url = aws_eks_cluster.example.identity[0].oidc[0].issuer
}

The Terraform code above provides the same functionality as the utility program within this repo in a more direct manner, but this approach will only work if the Terraform process has direct access to AWS IP addresses used with the OIDC hosted services.

In the event that the Terraform process uses a forward proxy (e.g.: Squid) or other systems which limit internet egress access to specific DNS zones, the approach above will fail due to the Terraform TLS provider tls_certificate data source implementation which will attempt to open a TCP connection to the host IP address directly.

The eks_cert_fingerprint_indexer program is designed to provide an alternative solution for people who find themselves in the situation described above.

What is this thing?

Simply put, the eks_cert_fingerprint_indexer program reads EKS OIDC issuer certificates, generates SHA1 fingerprints and stores them in AWS SSM parameters. This allows other provisioning systems (e.g.: Terraform, CloudFormation) to fetch the fingerprint values and use them when creating new AWS OIDC providers. Terraform example:

data "aws_ssm_parameter" "eks_cluster_oidc_fingerprint" {
  name = "/eks_cluster_oidc_fingerprints/example"
}

resource "aws_iam_openid_connect_provider" "eks_cluster" {
  client_id_list = [
    "sts.amazonaws.com",
  ]
  thumbprint_list = [
    data.aws_ssm_parameter.eks_cluster_oidc_fingerprint.value,
  ]
  url = aws_eks_cluster.example.identity[0].oidc[0].issuer
}

The eks_cert_fingerprint_indexer program performs the following actions when executed:

• List EKS clusters within a configured AWS region.
• Describe each EKS cluster to fetch the OIDC issuer URL associated.
• Establish TLS connection to each OIDC issuer URL to read certificates.
• Generate SHA1 "fingerprint" of the configured certificate index (default is last certificate defined) within the certificate chain.
• Create a SSM parameter using the configured key prefix suffixed by /<EKS cluster name>.

---

Building an executable

The eks_cert_fingerprint_indexer program is written in go which will need to be installed in the build env. There are two ways to build the eks_cert_fingerprint_indexer program:

• As a standalone program configured via CLI flags: env GO111MODULE=on go build -o eks_cert_fingerprint_indexer cmd/*.
• As an AWS Lambda function configured via environment_variables: env GO111MODULE=on go build -o eks_cert_fingerprint_indexer lambda/*

Lambda environment_variables

• CERT_INDEX - Reverse numeric index of the certificate to fingerprint within chain, defaults to last cert defined within chain.
• SSM_KEY_PFX - SSM parameter name prefix, values must start and end with /. Default: /eks_cluster_oidc_fingerprints/
• SSM_OVERWRITE - Overwrite existing SSM parameters. Default: false
• VERIFY_CERTS - Verify TLS certificates. Default: true

AWS IAM requirements

The following IAM policy can be used to grant an IAM entity (group, role, user) the required access for the eks_cert_fingerprint_indexer program to run. The SsmReadWrite IAM statement restricts SSM access to the default SSM key prefix path and will need to be changed if using a non-default SSM key prefix.

{
    "Statement": [
        {
            "Action": [
                "eks:ListClusters",
                "eks:DescribeCluster"
            ],
            "Effect": "Allow",
            "Resource": "*",
            "Sid": "EksReadOnly"
        },
        {
            "Action": [
                "ssm:PutParameter",
                "ssm:GetParameter"
            ],
            "Effect": "Allow",
            "Resource": "arn:aws:ssm:<AWS region>:<AWS account>:parameter/eks_cluster_oidc_fingerprints/*",
            "Sid": "SsmReadWrite"
        }
    ],
    "Version": "2012-10-17"
}

When running the eks_cert_fingerprint_indexer program as a lambda function additional access is required to submit CloudWatch logs and create ENIs within VPCs if configured to run within VPC subnets.

AWS lambda managed policies (choose one of the following, not both):

• arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole
• arn:aws:iam::aws:policy/service-role/AWSLambdaVPCAccessExecutionRole

When running the eks_cert_fingerprint_indexer program as a lambda a new IAM role will need to be created with the policies above attached. The role will also need a trust policy attached to allow the lambda service to assume it:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "",
            "Effect": "Allow",
            "Principal": {
                "Service": "lambda.amazonaws.com"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}

Questions

If you have questions you can message me via email or twitter (contact info in github profile).