Securing Terraform Automation: Atlantis IAM Design and Implementation in AWS

Introduction

Infrastructure as Code (IaC) tools like Terraform enable organizations to manage cloud resources efficiently. However, managing permissions for Terraform operations is crucial to maintaining security and compliance. This article explores how to design and implement IAM roles and policies for Atlantis in an Amazon EKS environment.

Background

Atlantis is an open-source tool that automates Terraform workflows. It allows teams to collaborate on infrastructure changes via pull requests (PRs). Since Atlantis executes Terraform commands on behalf of users, it requires appropriate AWS IAM permissions to perform its tasks securely.

Challenges in IAM Design for Atlantis

1. Principle of Least Privilege (PoLP): Atlantis should have only the minimum permissions necessary to apply infrastructure changes.

2. Managing Multi-Account Access: Organizations often deploy infrastructure across multiple AWS accounts, requiring cross-account access management.

3. Securely Storing AWS Credentials: Storing and managing AWS credentials securely is essential to prevent unauthorized access.

4. Audit and Compliance: Tracking who initiated infrastructure changes and ensuring compliance with security policies is a key challenge.

Solution Approach

Using IAM Roles Instead of Static Credentials

• Instead of using AWS access keys, Atlantis should assume an IAM role with specific permissions.

Setting Up IAM Roles for Atlantis

• Create an IAM Role for Atlantis: Define an IAM role with trust policies allowing Atlantis (running in Amazon EKS) to assume it.
• Attach Least Privilege Policies: Assign policies granting only the necessary permissions for Terraform actions.

Enabling Cross-Account Access

• Create IAM roles in each AWS account that Atlantis needs to manage.
• Update trust policies to allow Atlantis to assume these roles.

Storing and Using IAM Credentials Securely

• Use Kubernetes Service Account IAM Roles (IRSA) to provide Atlantis with temporary credentials.
• Avoid storing long-term AWS credentials in configuration files.

Implementing Logging and Auditing

• Enable AWS CloudTrail to track Atlantis’s API calls.
• Use AWS IAM Access Analyzer to review granted permissions.

Step-by-Step Implementation

Step 1: Create an IAM Role for Atlantis

aws iam create-role --role-name AtlantisRole --assume-role-policy-document file://trust-policy.json

• The trust policy (trust-policy.json) should allow EKS to assume the role.

Step 2: Attach Required IAM Policies

Attach policies that grant only necessary permissions.

aws iam attach-role-policy --role-name AtlantisRole --policy-arn arn:aws:iam::aws:policy/TerraformApplyPolicy

Step 3: Enable IRSA for Atlantis in EKS

• Create a Kubernetes service account with IAM role annotations.
• Update the Atlantis deployment to use the service account.

Step 4: Configure Cross-Account Role Assumption

• Create a role in each target AWS account.
• Update the trust policy to allow AtlantisRole to assume it.

Step 5: Verify and Monitor Access

• Test Atlantis’s ability to assume roles and apply Terraform changes.
• Monitor API calls using AWS CloudTrail and IAM Access Analyzer.

Conclusion

By designing a secure IAM strategy for Atlantis, organizations can ensure Terraform automation runs safely while adhering to security best practices. This structured approach balances security with operational efficiency, enabling teams to manage infrastructure confidently.