Using Presigned URLs to Restrict File Upload Size on AWS S3

Amazon Simple Storage Service (S3) is widely used for storing and retrieving data across the cloud. One efficient method for securely uploading files to S3 without exposing your credentials is through presigned URLs. Presigned URLs allow you to grant temporary access to an object in your S3 bucket.

When working with presigned URLs, you might encounter scenarios where you want to restrict the size of files being uploaded. This can be particularly important in controlling storage costs and ensuring consistent performance. Below, we'll walk through how to use presigned URLs to achieve this.

How Presigned URLs Work

Presigned URLs are dynamically generated URLs that allow restricted access to S3 resources for a specified period of time. They are created using AWS SDKs and can be configured to allow actions like PUT or GET.

The basic idea is:

1. Generate a Presigned URL: Typically done server-side using the AWS SDK.
2. Use the Presigned URL: Client-side operations like uploading a file directly to S3.

Restricting File Size with Presigned URLs

To enforce a maximum file size for uploads via presigned URLs, AWS leverages the concept of conditions in the POST policy when the URL is generated. Here’s a step-by-step guide on how to accomplish this:

Step 1: Generate Presigned URL with Size Constraints

Using the AWS SDK for Python, boto3 , we can generate a presigned POST and include conditions to restrict file size.

• Security: Presigned URLs offer a secure method of granting users temporary access to S3 resources. Make sure that the expiration time is set appropriately.
• Performance: Restricting file size at the S3 level can help in reducing bandwidth and storage usage, potentially leading to cost savings.
• Validation: Always implement client-side checks to validate file size before upload to minimize unnecessary data transfer and failed uploads.
• Content Types: You can also restrict uploads by specifying conditions on content types in the POST policy.