AWS S3 is a robust storage service; securing access to your S3 buckets is critical. One common issue users face is configuring bucket policies with incorrect VPC or VPC Endpoint IDs, leading to frustrating "Access Denied" errors. This guide will walk you through troubleshooting and resolving these issues, ensuring smooth and secure access to your S3 resources.

Understanding the Issue

When you restrict S3 bucket access to specific VPCs or VPC Endpoints, even a tiny mistake in the policy can block access. These errors often occur due to:

  • Typographical mistakes in VPC or VPC Endpoint IDs.
  • Changes in your VPC setup that weren't reflected in the policy.
  • Misconfigured bucket policies.

Solution

If your application is facing unexpected access issues to the S3 bucket, this guide provides a structured approach to help you identify and resolve the problem through the following steps.

  1. Start by verifying your VPC and VPC Endpoint IDs; to ensure you have the correct IDs for your setup.
    1. Using AWS Management Console:
      1. VPC ID: Navigate to the VPC Dashboard, select "Your VPCs", and note the VPC ID.
      2. VPC Endpoint ID: In the VPC Dashboard, go to "Endpoints" and note the VPC Endpoint ID.
    2. Or if you like to use the AWS CloudShell, you can copy the following commands, and it will give you both available VPCs and VPC Endpoint IDs:
# List all VPCs
aws ec2 describe-vpcs --query "Vpcs[*].VpcId"

# List all VPC Endpoints
aws ec2 describe-vpc-endpoints --query "VpcEndpoints[*].VpcEndpointId"
  1. Review and correct the S3 Bucket Policy: You can access your S3 bucket policy by navigating to Amazon S3 > Buckets > [bucket name > Permissions > Bucket Policy, and look for discrepancies in the VPC or VPC Endpoint ID. A typical policy restricting access might look like this:
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Access-from-specific-VPC",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": "arn:aws:s3:::your-bucket-name/*",
            "Condition": {
                "StringEquals": {
                    "aws:SourceVpc": "vpc-incorrect",
                    "aws:SourceVpce": "vpce-incorrect"
                }
            }
        }
    ]
}
  1. If you find the SourceVPC and VPCE is not matching with the desired VPC and VPCE IDs:
    1. You may Update "aws:SourceVPC" and "aws:SourceVPCE" within the Bucket Policy script.
    2. Or by using the AWS CloudShell commands:
aws s3api put-bucket-policy --bucket your-bucket-name --policy file://updated-policy.json
  1. Test Access: After updating the policy, test accessing your S3 bucket. Use the AWS CLI or SDKs for a simple operation like listing the bucket contents:
aws s3 ls s3://your-bucket-name

Final Thoughts

Incorrect VPC or VPC Endpoint IDs in S3 bucket policies can lead to access issues that disrupt your workflows. You can resolve these issues by systematically verifying and updating your IDs and maintaining secure access to your S3 resources.