Why am I encountering a 403 error when trying to download a file from an S3 bucket in another account using S3FileDownloader in an ECS container with EC2?

The 403 error you're encountering when trying to download a file from an S3 bucket in another account using S3FileDownloader in an ECS container with EC2 is likely due to permissions issues. This error indicates that the request was understood, but access is denied. There are several potential reasons for this:

1. IAM Permissions: The IAM role associated with your ECS task may not have the necessary permissions to access the S3 bucket in the other account. Ensure that the task role has the appropriate S3 permissions, including s3:GetObject and possibly s3:ListBucket.

2. Cross-Account Access: When accessing an S3 bucket in another account, both accounts need to be properly configured. The bucket owner needs to grant permissions to your account, and your account needs the appropriate IAM permissions to access the bucket.

3. Bucket Policy: The S3 bucket in the other account may have a bucket policy that's restricting access. Check if there are any explicit Deny statements that could be preventing access from your ECS task.

4. KMS Encryption: If the S3 bucket uses AWS Key Management Service (KMS) for encryption, your IAM role may need additional permissions to use the KMS key.

5. VPC Configuration: If your ECS task is running in a VPC, ensure that it has proper network access to reach S3. This could involve setting up a VPC endpoint for S3 or ensuring that your VPC has internet access through a NAT gateway.

The reason it works in a container on your local machine but not in ECS could be due to different credentials being used. Your local machine might be using your personal AWS credentials, which may have broader permissions than the IAM role assigned to the ECS task.

To resolve this:

1. Review and update the IAM role permissions for your ECS task.
2. Verify the cross-account access settings.
3. Check the S3 bucket policy in the other account.
4. If KMS encryption is used, ensure your role has the necessary KMS permissions.
5. Verify the network configuration of your ECS task's VPC.

By systematically checking these areas, you should be able to identify and resolve the cause of the 403 error.
Sources
Troubleshoot 403 Access Denied error in Amazon S3 | AWS re:Post
Resolve S3 Access Denied error in CloudFormation | AWS re:Post
Why Does S3 Return 403 Instead of 404 When the Object Doesn’t Exist? | AWS re:Post

Compare your local IAM Permissions VS Ecs Task execution role permissions