AWS S3 signed URLs handbook
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AWS S3 signed URLs handbook

How to handle files in a serverless environment

About the Book

Don't get fooled by the apparent simpleness of signed URLs. Whomever you want to protect your files from knows how they work under the hood. An implementation that "seems to work" is insecure and gives a false sense of security.

This book teaches you everything you need to know about S3 signed URLs. You'll learn what signed URLs are, why they are needed for serverless applications, how to implement them securely, and how they work with other AWS services. All this, with a special eye for security.

It contains the background knowledge so that you'll know the cases where signed URLs are the solution. It comes with almost a dozen deploy-to-try examples to allow easy experimentation with the different aspects.

When I initially started experimenting with signed URLs I quickly realized how easy it is to end up with a solution that is unreliable and insecure. I spent several months to figure out what is missing from the documentations so that you don't need to.

You'll learn:

  • How S3 signed URLs work and why they are essential for a serverless stack
  • How to solve common problems and how to secure the implementation
  • How to use them with other services, such as CloudFront and KMS

This book is written in a handbook style. It dives deep into a single technology and provides help when you need it. It features analyses how each choice or piece of technology affects the security of signed URLs.

Originally published in May 2020.

About the Author

Tamás Sallai
Tamás Sallai

Given a task that requires writing software, an expert provides better and more reliable solutions. I write articles and books to help you be that expert.

I'm a software developer focusing mostly on cloud computing and web technologies. I'm especially interested in how to handle edge cases to end up with dependable software. One of my main focus is security and how each part affects the whole system.

I co-author the advancedweb.hu blog where I've published more than a hundred technical articles.

Table of Contents

  • Introduction
  • Chapter 1: Overview
    • Use cases
    • From servers to serverless
      • 3-tier architecture
      • Serverless architecture
        • Why streaming is not an option
    • Signed URLs for S3
      • Credentials
      • URL structure
      • Expiration time
    • Security of S3 signed URLs
      • Algorithm
        • Signature versions 2 and 4
      • Bandwidth control
      • Implementation disclosure
      • Revocation
  • Chapter 2: Implementation
    • Sample code
      • Functionality
      • Sample code architecture
    • Infrastructure
      • Bucket
      • Object
      • Function
      • Execution role
    • Backend with Node.js
      • S3 service
      • Sign URLs
        • Expiration time
        • Synchronous or asynchronous
          • Which one to use?
        • Bucket region
    • Frontend
  • Chapter 3: Specific use-cases
    • Least privilege with dedicated roles
      • Architecture
      • Backend
        • Credentials
        • S3 service object
    • CORS
    • Using HTTP redirects
      • Redirect status code
      • CORS with HTTP redirects
    • How to check if a file exists before signing
    • How to set the filename
    • Integrate with CloudFront
      • Why use CloudFront
      • How to setup CloudFront
        • Rewriting the signed URL
        • Origin config
        • Cache behavior
        • Paths
      • CloudFront signed URLs
    • Caching
      • Validity
      • NodeJs implementation
      • Browser caching
      • Proxy caching
        • Pricing
        • Problems with proxy caching
          • Problem #1: No expiration checking
          • Problem #2: No signature checking
    • Uploading files
      • Permissions
      • CORS config for uploads
      • Backend
      • Frontend
      • Update the database on file upload
        • Implementation
          • Step #1: Signing the URL
          • Step #2: Frontend
          • Step #3: Update the database
          • Step #4: Infrastructure
      • PUT signed URLs
      • Security checklist for uploading files
    • Handling encrypted data
      • SSE-S3 and SSE-KMS with AWS Managed CMK
        • Uploads
      • SSE-KMS with Customer Managed CMK
        • Downloads
        • Uploads
      • SSE-C
        • Downloads
        • Uploads
    • Permanent URLs
      • Problems with permanent signed URLs
      • Filename-based solution
      • Token-based solution
        • Infrastructure
        • Create a token
        • Use a token
  • Troubleshooting
    • AccessDenied
      • There were headers present in the request which were not signed
      • SignatureDoesNotMatch
      • Request has expired
    • NoSuchKey
    • ExpiredToken
    • PermanentRedirect
    • InvalidRequest
  • Security checklist
  • Conclusion
  • About the author

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