What is an IAM permissions boundary, and when would you use one?
AWS · Basic level
Answer
A permissions boundary sets the maximum permissions a user or role can receive. It does not grant access by itself; it limits delegated administrators or automation from creating identities with broader permissions than allowed.
Technical explanation
Boundaries are useful when platform teams delegate role creation to application teams or CI/CD pipelines.
IAM evaluation is layered: identity policies, resource policies, trust policies, boundaries, SCPs, session policies, and explicit denies all contribute to the final decision.
Prefer temporary credentials through STS, roles, IAM Identity Center, instance profiles, IRSA, or OIDC federation instead of long-lived access keys.
Use conditions, resource ARNs, tags, MFA requirements, external IDs, source account/source ARN constraints, and Access Analyzer to reduce blast radius.
Hands-on example
1. Create a least-privilege IAM role for a small workload, including trust policy, permission policy, tags, and CloudTrail visibility.
2. Test the role with aws sts get-caller-identity and one allowed action, then deliberately test one denied action.
3. Run IAM Access Analyzer or policy simulation and refine broad actions/resources before production.
4. Record the access pattern in IaC and require review for future policy changes.
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