Identity and Access Management (IAM) is the heart of AWS security. It's also one of the most misunderstood services for beginners. Getting IAM right means your cloud environment is secure; getting it wrong can lead to devastating data breaches.

In this guide, we will break down the differences between IAM Users, Roles, and Policies, and when to use each for optimal security.

Root User Danger

Never use your AWS Root account for daily tasks. Create an IAM user with Administrator access for yourself and lock away the root credentials with MFA enabled.

1. IAM Users vs IAM Roles

A common misconception is that IAM Users and Roles are interchangeable. While both are identities in AWS, they serve fundamentally different purposes.

Identity Architecture
Figure 1: Mapping human identities to Users, and machine identities to Roles.
  • IAM Users: Represent a specific person or application. They have long-term credentials (passwords or access keys).
  • IAM Roles: Are intended to be assumable by anyone who needs them. They do not have long-term credentials. Instead, they provide temporary, short-lived session tokens.

2. Understanding Policies

Identities (Users/Roles) cannot do anything in AWS by default. You must attach Policies to grant them permissions. Policies are JSON documents that define what actions are allowed or denied on which resources.

json
{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": "s3:GetObject", "Resource": "arn:aws:s3:::my-secure-bucket/*" } ] }

The policy above allows the identity it is attached to read objects strictly from `my-secure-bucket`.

Principle of Least Privilege

Always grant the minimum permissions required for a task. Never use `*` (wildcard) for actions or resources unless absolutely necessary.

Conclusion

For humans, use IAM Identity Center (SSO) or IAM Users. For EC2 instances, Lambda functions, or external services, always use IAM Roles with least-privilege policies.