What is the principle of least privilege and how does it apply to IAM roles in AWS?

technical-conceptual · Junior level · cloud-devops-security

What the interviewer is really asking

Gauge foundational security thinking and whether the candidate can translate an abstract security principle into concrete cloud IAM practice.

What to say

• Least privilege means granting only the permissions required to perform a specific task — no more.
• In AWS IAM, this means attaching policies that name specific actions (e.g., s3:GetObject) and resources (specific bucket ARN) rather than using wildcards like * or managed policies like AdministratorAccess.
• It reduces blast radius: if a Lambda role is compromised, an attacker can only reach the S3 bucket the Lambda needs, not the entire account.

What to avoid

• Saying least privilege means 'don't give everyone admin access' without going deeper.
• Suggesting that broad policies are fine for development environments.
• Confusing authentication (who you are) with authorisation (what you can do).

Example answers

Strong: For a Lambda that reads from one DynamoDB table, I'd write an inline policy allowing only dynamodb:GetItem and dynamodb:Query on that specific table ARN — not dynamodb:* on *.

Weak: Just attach the ReadOnlyAccess managed policy — it's safer than AdminAccess and good enough.