Enabling cross-account access to an Amazon S3 bucket requires a "two-sided" permission configuration.

1. Resource-Based Policy (in Account A): The owner of the resource (the S3 bucket in Account A) must create a resource-based policy (a bucket policy) that explicitly grants access to the principal in the other account. Option C correctly grants permission to the root of Account B, effectively trusting Account B to delegate those permissions to its own IAM principals.

2. Identity-Based Policy (in Account B): The administrator of the principal's account (Account B) must attach an identity-based policy (an IAM policy) to the user (UserDataProcessor). This policy must explicitly allow the user to perform the intended actions on the resource in Account A. Option D correctly provides this permission.

Both policies are required for the access to succeed.

A. Cross-origin resource sharing (CORS) is a browser security mechanism for web applications and is not relevant for programmatic access by an IAM user.

B. While granting access directly to a user ARN is possible, the delegation model in option C (granting to the account) is a more scalable and common pattern.

E. This IAM policy grants access to a bucket within Account B (bucket-in-account-b), not the required bucket in the external Account A.

1. AWS Identity and Access Management User Guide: In the section on how policies are evaluated

it states: "For a cross-account request

the principal who makes the request must have permissions from an identity-based policy in their account and from the resource-based policy in the resource's account."

Source: AWS IAM User Guide

"Determining whether a request is allowed or denied within an account"

Policy evaluation logic.

2. Amazon S3 User Guide: The documentation provides a specific walkthrough for this scenario

called "Tutorial: Delegate access across AWS accounts using IAM roles". A more direct example is under "Bucket owner granting cross-account bucket permissions". It shows that the bucket policy in the source account must grant permissions to the destination account's root user

and an IAM policy in the destination account must grant permissions to the specific user or role.

Source: Amazon S3 User Guide

Identity and access management for Amazon S3

"Bucket owner granting cross-account bucket permissions". The guide explicitly shows a bucket policy with "Principal": {"AWS": "arn:aws:iam::AccountB-ID:root"} and a corresponding user policy allowing action on arn:aws:s3:::bucket-in-account-a/.

3. AWS Security Blog: The post "How to Use Bucket Policies and Apply Defense-in-Depth to Help Secure Your Amazon S3 Data" explains the delegation model: "You can write a bucket policy that grants another account access to your S3 bucket... The administrator of the trusted account must then grant their IAM users access to your bucket."

Source: AWS Security Blog

"How to Use Bucket Policies and Apply Defense-in-Depth to Help Secure Your Amazon S3 Data"

section "Granting cross-account access to your S3 bucket".