Building a Secure Read-Only REST API for AWS S3

The endpoint responded instantly. Seconds later, the file’s metadata was in your hands, untouched and safe, thanks to a read-only role built for AWS S3.

When you design a REST API that connects to AWS S3, role permissions define the line between secure access and open risk. A read-only IAM role is the strongest safeguard for APIs that need to pull objects without the ability to change or delete them. This setup isolates your API logic from write operations, protecting your buckets from accidents or exploits.

AWS S3 Read-Only Role Basics

To create a read-only role for S3 in AWS, you:

1. Open the IAM console.
2. Create a new role with trusted entity set to your application’s execution environment (such as an EC2 instance, Lambda function, or ECS task).
3. Attach the built-in policy AmazonS3ReadOnlyAccess.
4. Restrict the role’s scope with explicit bucket ARNs if needed, adding a condition block for tighter control.

This means your REST API can run GET requests against S3 objects without risk of overwriting or deleting them.

Integrating the Role with a REST API

Once the role exists, attach it to the compute resource hosting your API. Your code will use the AWS SDK with temporary credentials from the role to execute read calls, such as:

const AWS = require('aws-sdk');
const s3 = new AWS.S3();

async function getObject(bucket, key) {
 const params = { Bucket: bucket, Key: key };
 const result = await s3.getObject(params).promise();
 return result.Body;
}

The AWS SDK honors your IAM permissions. Attempting a write operation will return an AccessDenied error. That’s exactly what you want in a read-only workflow.

Securing Endpoints and Monitoring Usage

Pair IAM role restrictions with API Gateway authorizers or token-based access control to prevent unauthorized requests. Use CloudTrail to log every S3 request from the role so you can audit access patterns over time. Even read-only calls can become a vector for leaked data if exposed publicly.

Performance Considerations

When dealing with large numbers of objects, minimize calls by using S3 ListObjectsV2 to batch metadata retrieval. Combine this with caching inside your API layer to reduce load on S3 and lower costs.

Why Read-Only Roles Matter

They are simple to configure and drastically reduce attack surface. In multi-tenant systems, they enforce strong separation of concerns, letting each API endpoint serve its purpose without crossing into dangerous territory.

Lock down your S3 buckets, keep your REST API lean, and give your team confidence that read access will always mean read access.

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