AWS IAM: Identity and Access Control Explained (2026-09-01 23:38)

AWS IAM: Identity and Access Control

AWS Identity and Access Management (IAM) is the foundation of security in the AWS Cloud. It controls who (identity) can do what (permissions) on which resources (access control). Understanding IAM is essential for building secure, compliant, and well-architected systems.

Core Concepts

IAM is built around a few key building blocks. Getting comfortable with these terms will make everything else easier to reason about.

Principals

A principal is an entity that can make a request to AWS. This includes:

• Root user — the account owner with unrestricted access (avoid daily use).
• IAM users — long-lived identities for people or applications.
• IAM roles — temporary identities assumed by users, services, or federated identities.
• Federated identities — users authenticated via an external identity provider (SAML, OIDC).

Authentication vs. Authorization

• Authentication verifies who you are (credentials, access keys, tokens).
• Authorization determines what you're allowed to do (policies).

IAM Policies

Policies are JSON documents that define permissions. They are the heart of authorization in AWS.

Policy Types

Type	Attached To	Purpose
Identity-based	Users, groups, roles	Grant permissions to a principal
Resource-based	Resources (e.g., S3 buckets)	Grant cross-account or service access
Permissions boundaries	Users, roles	Set the maximum permissions
Service Control Policies (SCPs)	AWS Organizations	Guardrails across accounts

Anatomy of a Policy

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowReadOnlyS3",
      "Effect": "Allow",
      "Action": [
        "s3:GetObject",
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::my-app-bucket",
        "arn:aws:s3:::my-app-bucket/*"
      ],
      "Condition": {
        "IpAddress": {
          "aws:SourceIp": "203.0.113.0/24"
        }
      }
    }
  ]
}

Key elements:

• Effect — Allow or Deny.
• Action — the API operations being granted or denied.
• Resource — the ARNs the statement applies to.
• Condition — optional rules that refine when the policy is in effect.

How IAM Evaluates Requests

When a principal makes a request, AWS evaluates all applicable policies using this logic:

1. Default deny — every request is implicitly denied by default.
2. Explicit deny — an explicit Deny always wins and cannot be overridden.
3. Explicit allow — a matching Allow grants access if no Deny applies.

In practice: Explicit Deny > Explicit Allow > Implicit Deny.

IAM Roles: Temporary Credentials

Roles are the recommended way to grant permissions because they issue short-lived credentials instead of long-term access keys.

Common use cases:

• EC2 instance profiles — grant applications on an instance access to AWS services.
• Cross-account access — allow one account to assume a role in another.
• Service roles — let AWS services (e.g., Lambda) act on your behalf.

Example trust policy allowing Lambda to assume a role:

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

Best Practices

Following these guidelines dramatically reduces your security risk:

• Enforce least privilege — grant only the permissions required, then tighten over time.
• Use roles over users — prefer temporary credentials for workloads.
• Enable MFA — require multi-factor authentication for all human users.
• Rotate credentials — regularly rotate access keys, or eliminate them entirely.
• Avoid the root user — lock it down with MFA and use it only for account-level tasks.
• Use permissions boundaries and SCPs — set organizational guardrails.
• Audit with tools — leverage IAM Access Analyzer, Access Advisor, and CloudTrail.

Monitoring and Auditing

Visibility is critical for maintaining a secure posture:

• IAM Access Analyzer identifies resources shared externally and generates least-privilege policies.
• Access Advisor shows when services were last accessed, helping remove unused permissions.
• AWS CloudTrail logs every API call for forensic analysis and compliance.

Conclusion

IAM is the control plane for security across your entire AWS environment. By mastering principals, policies, and roles—and consistently applying least privilege—you build a strong foundation for a secure cloud architecture. Start with tight permissions, monitor continuously, and refine as your workloads evolve.