AWS S3 Access Control: IAM vs Bucket Policies

TL;DR

Use IAM policies for controlling access to S3 based on who is accessing the resource (users, groups, roles). Use Bucket Policies when you need to control access based on where the request is coming from or specific conditions related to the object itself. Often, a combination of both provides the most secure and flexible solution.

1. Understanding IAM Policies

IAM (Identity and Access Management) policies are attached to users, groups, or roles. They define what actions those identities can perform on AWS resources, including S3 buckets and objects. They’re great for general access control.

• Granularity: Control permissions at a very detailed level (e.g., specific operations like s3:GetObject or s3:PutObject).
• Centralised Management: Manage all your AWS permissions in one place.
• Best for: Controlling access based on user identity and group membership.

Example IAM Policy allowing read-only access to a specific bucket:

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

2. Understanding Bucket Policies

Bucket policies are attached directly to S3 buckets. They define who can access the bucket and its objects, and under what conditions.

• Resource-Specific: Apply only to the specific bucket they’re attached to.
• Conditions: You can specify conditions like IP address ranges, referrer headers, or multi-factor authentication requirements.
• Best for: Controlling access based on request origin, object attributes, and cross-account access.

Example Bucket Policy allowing public read access to objects:

{ "Version": "2012-10-17", "Statement": [ { "Sid": "PublicReadGetObject", "Effect": "Allow", "Principal": "*", "Action": "s3:GetObject", "Resource": "arn:aws:s3:::your-bucket-name/*" } ] }

3. IAM vs Bucket Policies: A Comparison

Here’s a table summarising the key differences:

Feature	IAM Policy	Bucket Policy
Attachment	Users, Groups, Roles	S3 Bucket
Scope	AWS-wide	Specific Bucket
Focus	Who is accessing?	Where is the request coming from/Object attributes?
Conditions	Limited	Extensive

4. When to Use Which (and Combining Them)

1. Simple User Access: If you just need to control which users can read/write files, use IAM policies.
2. Public Access: For making objects publicly accessible, a Bucket Policy is the standard approach. Be very careful when granting public access!
3. Cross-Account Access: When allowing access from another AWS account, use a Bucket Policy to specify the allowed account ID.
4. IP Address Restrictions: Use a Bucket Policy to restrict access based on the requester’s IP address.
5. Complex Scenarios: Often, you’ll need both! For example:
   • IAM policy grants general S3 permissions to users.
   • Bucket policy restricts access to specific objects or requires MFA for certain operations.

5. Best Practices

• Principle of Least Privilege: Grant only the minimum permissions necessary.
• Regularly Review Policies: Ensure your policies are still appropriate and haven’t become overly permissive.
• Use AWS Policy Generator: The AWS Policy Generator can help you create basic IAM and Bucket Policies.
• Test Thoroughly: Before deploying any changes, test your policies to ensure they work as expected.