Are presigned urls to s3 budget security wise flawed?

Understanding the Security Implications of Presigned URLs in Cloud Storage

In the rapidly evolving landscape of cloud storage solutions, security and cost management are critical considerations for developers and organizations alike. Recently, I delved into pricing models and operational costs associated with various platforms like Cloudflare R2, which charges per operation. This exploration led me to question some of the commonly accepted practices around presigned URLs—specifically, their security efficacy and cost implications.

The Use of Presigned URLs for User Uploads

Presigned URLs are a popular method for enabling users to upload files directly to cloud storage services such as Amazon S3. They offer a way to grant temporary, controlled access to upload or download objects without exposing sensitive credentials. Typically, a backend server generates a presigned URL, which is then provided to the user to perform the upload directly from their browser or application.

The Core Concern: Reusability and Cost Implications

While presigned URLs streamline the upload process and reduce server load, they introduce a potential security and cost-related vulnerability:

Unlimited Reuse: Presigned URLs are inherently reusable until they expire. A malicious user could write a script to repeatedly upload a small payload—say, a 1 KB image—thousands or even millions of times, effectively turning a single URL into an automated attack vector.
Operational Charging: Cloud providers typically charge for each upload operation, regardless of the object size. This means that even tiny uploads, when repeated excessively, can incur significant costs. For example, repeatedly uploading small files can multiply operational charges without necessarily increasing storage costs proportionally.
Bypassing Endpoint Controls: Protecting the backend endpoint that generates presigned URLs (such as through CAPTCHA or rate-limiting) might be ineffective against users who manually generate URLs or script their own requests. Once they have the URL, they can bypass many controls, potentially leading to abuse.

How Do Most Companies Mitigate These Risks?

Given these concerns, how do organizations that enable direct browser-to-cloud storage uploads manage security and cost worries?

Short-Lived URLs: Generating presigned URLs with very brief expiration times limits the window during which malicious users can exploit them.
Validation and Monitoring: Employing backend validation, logging, and anomaly detection helps identify suspicious activity and prevents abuse.
Access Policies and Permissions: Restricting permissions associated with presigned URLs ensures only the intended operations (e.g., upload