aws-cloudformation-iam

AWS CloudFormation IAM Security

Overview

Use this skill to model IAM with CloudFormation in a way that stays secure, auditable, and maintainable.

The most important design concerns are:

• separating trust policies from permission policies
• preferring roles over long-lived users wherever possible
• keeping least-privilege boundaries readable and reusable

Do not treat SKILL.md as a full IAM encyclopedia. Use the bundled references for larger policy examples and service-specific variants.

When to Use

• Creating IAM roles for Lambda, ECS, EC2, Step Functions, or other AWS services
• Defining inline policies, managed policies, and permission boundaries in CloudFormation
• Modeling cross-account assume-role access with constrained trust policies
• Exporting IAM role ARNs or managed policy ARNs to downstream stacks
• Reviewing wildcard permissions, boundary drift, or role replacement risk
• Creating reusable IAM stacks for platform or application teams

Instructions

1. Define the trust boundary first

Identify who or what assumes the role (service principal, cross-account principal, or federated identity), then write the trust policy with explicit principals and conditions before adding permissions.

2. Grant the minimum permission set

Use inline policies for role-specific access; use managed policies for shared patterns across principals. Scope actions and resources tightly, and use conditions where possible.

3. Apply permission boundaries for delegated role creation

Use permission boundaries when teams create or extend roles in their own stacks, when guardrails are needed around privileged services (IAM, KMS, Organizations), or to separate maximum allowed permissions from application-specific policies.

Name roles and policies consistently so stack outputs and audits remain easy to trace.

4. Model cross-account access

For cross-account roles: trust only the exact source account or principal, add sts:ExternalId conditions when appropriate, keep permission and trust policies separate, and export only the ARNs that consuming accounts need.