S3 Policy vs IAM Policy: here Should Your Cloud Access Live?
Both use JSON policies, but putting them in the wrong place creates major security blind spots. Here is when to use IAM versus S3 bucket policies.
When setting up access permissions in AWS, almost every developer hits a familiar crossroads: Should this permission be handled in an IAM user/role policy, or should it go directly on the Amazon S3 Bucket Policy?
Both use standard AWS JSON policy syntax, both evaluate
Allow and Explicit Deny, and both can achieve similar outcomes on the surface. But placing permissions in the wrong place can create security blind spots or make scaling your architecture a nightmare.Here is a simple breakdown of how they differ and a rule of thumb for when to use which.
1. Identity-Based Policies (IAM)
Identity-based policies attach directly to users, groups, or IAM roles.
- Focus: "What can this specific entity do across AWS?"
- Best For: Centralized user management and service-to-service roles (like an Amazon EC2 instance or AWS Lambda execution role needing temporary S3 access).
- Limitations: IAM policies alone cannot grant access to identities from an external AWS account unless paired with cross-account trust configurations or a resource policy.
2. Resource-Based Policies (S3 Bucket Policies)
S3 bucket policies attach directly to the bucket itself and must explicitly specify a
Principal field.- Focus: "Who is allowed (or denied) from accessing this specific storage resource?"
- Best For:
- Cross-Account Access: Granting secure, scoped access to accounts outside your AWS organization without provisioning individual IAM roles for them.
- Enforcing Global Guards: Mandating that all incoming traffic uses HTTPS via an explicit
aws:SecureTransport: falsedeny statement. - Origin Access Control (OAC): Restricting bucket reads exclusively to an Amazon CloudFront distribution.
The Decision Rule: Which One Should You Pick?
- Default to IAM Policies if you are managing internal application workloads (e.g., Lambda functions reading data) or onboarding developers into your team with defined role permissions.
- Default to S3 Bucket Policies when you need strict perimeter control over the bucket itself—such as blocking unencrypted uploads, enforcing TLS, or bridging access across different AWS accounts.
Remember: in AWS policy evaluation, an Explicit Deny in either policy always wins, regardless of what the other allows.
Question for the Community
When you're locking down production S3 buckets, do you prefer managing permissions centrally via IAM roles, or do you enforce security guardrails directly at the bucket policy level? Drop your thoughts below!
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