Sohail Jafri

Posted on Mar 18

How to use AWS S3 pre-signed URLs to upload and download files

#aws #javascript #tutorial #node

So, this month we landed a new client for whom we must keep track of distributor's and doctor's orders and sales. The client had a requirement to keep the S3 bucket private and only allow access to the files using pre-signed URLs. So in this blog, I will show you how to use pre-signed URLs to upload and download files from an AWS S3 bucket while keeping the bucket private.

Prerequisites
Let's break down the task into smaller steps.
Use Cases
Bonus Code Snippet
Additional Resource
Conclusion

Prerequisites

Basic knowledge of JavaScript
Basic knowledge of AWS S3 bucket
Basic knowledge of HTTP requests
Basic knowledge of Node.js and Express.js

Let's break down the task into smaller steps.

Setting up the backend
Develop a function to generate an AWS S3 pre-signed URL
Configuring AWS S3 bucket
Connecting function to an API endpoint
Setting up the frontend
Connecting frontend to the API

Step 1: Setting up the backend

mkdir backend
cd backend
npm init -y
npm install express aws-sdk
touch index.js

You can use type nul > index.js for Windows users to create a new file.

// index.js
const express = require('express')
const app = express()
const AWS = require('aws-sdk')

app.listen(3000, () => {
  console.log('Server is running on port 3000')
})

Step 2: Develop a function to generate an AWS S3 pre-signed URL

// index.js
const express = require('express')
const app = express()
const AWS = require('aws-sdk')

const s3 = new AWS.S3({
    accessKeyId: process.env.AWS_ACCESS_KEY_ID, // Your AWS Access Key ID
    secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY, // Your AWS Secret Access Key
    region: process.env.AWS_REGION // Your AWS region
    signatureVersion: 'v4', // This is the default value
})

const awsS3GeneratePresignedUrl = async (
  path,
  operation = 'putObject', // Default value is putObject, for get use getObject
  expires = 60
): Promise<string> => {
  const params = {
    Bucket: bucketName, // Bucket name
    Key: path, // File name you want to save as in S3
    Expires: expires, // 60 seconds is the default value, change if you want
  }
  const uploadURL = await s3.getSignedUrlPromise(operation, params)
  return uploadURL
}

app.listen(3000, () => {
    console.log('Server is running on port 3000')
})

Step 3: Configuring AWS S3 bucket

If we try to connect API to our function, we will get a CROS (Cross-Origin Resource Sharing) error. To fix this, we need to configure our S3 bucket to allow access from our API. We want access to PUT and GET requests from our API. To do this, we need to add a CORS configuration to our S3 bucket in the following way:

Open the Amazon S3 console at https://console.aws.amazon.com/s3/
Search for the bucket you want to configure and click on it
Click on the Permissions tab
Scroll down to the Cross-origin resource sharing (CORS) section
Click on Edit and add the following policy

[
    {
        "AllowedHeaders": [
            "*"
        ],
        "AllowedMethods": [
            "PUT",
            "GET",
            "HEAD"
        ],
        "AllowedOrigins": [
            "*"
        ],
        "ExposeHeaders": []
    }
]

Click on Save changes
Also make sure that Block public access (bucket settings) is turned on to keep your bucket private (optional)

Change the AllowedOrigins to your API URL to make it more secure. Or you can also use wildcard * to allow access from any origin.

Step 4: Connecting function to an API endpoint

We will have 2 endpoints, one to generate a pre-signed URL for uploading a file and the other to generate a pre-signed URL for downloading a file.

// index.js
const express = require('express')
const app = express()
const AWS = require('aws-sdk')

const s3 = new AWS.S3({
    accessKeyId: process.env.AWS_ACCESS_KEY_ID, // Your AWS Access Key ID
    secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY, // Your AWS Secret Access Key
    region: process.env.AWS_REGION // Your AWS region
    signatureVersion: 'v4', // This is the default value
})

const awsS3GeneratePresignedUrl = async (
  path,
  operation = 'putObject', // Default value is putObject, for get use getObject
  expires = 60
): Promise<string> => {
  const params = {
    Bucket: bucketName, // Bucket name
    Key: path, // File name you want to save as in S3
    Expires: expires, // 60 seconds is the default value, change if you want
  }
  const uploadURL = await s3.getSignedUrlPromise(operation, params)
  return uploadURL
}

app.get('/generate-presigned-url', async (req, res) => {
  const { path } = req.query
  const uploadURL = await awsS3GeneratePresignedUrl(path, 'putObject', 60)
  res.send({ path, uploadURL })
})

app.get('/download-presigned-url', async (req, res) => {
  const { path } = req.query
  const downloadURL = await awsS3GeneratePresignedUrl(path, 'getObject', 60)
  res.send({ downloadURL })
})

app.listen(3000, () => {
    console.log('Server is running on port 3000')
})

Step 5: Setting up the frontend

For the front end, I am using React.js. You can use any front-end framework of your choice. and install axios to make HTTP requests.

npx create-react-app frontend
cd frontend
npm install axios

Step 6: Connecting frontend to the API

// App.js
import React, { useState } from 'react'
import axios from 'axios'

export default function App() {
  const [uploadURL, setUploadURL] = useState('')
  const [downloadURL, setDownloadURL] = useState('')

  const generatePresignedURL = async (path, type) => {
    const response = await axios.get(
      `http://localhost:3000/generate-presigned-url?path=${path}`,
    )
    if (type === 'upload') {
      setUploadURL(response.data.uploadURL)
    } else {
      setDownloadURL(response.data.downloadURL)
    }
  }

  return (
    <div>
      <input type="file" onChange={(e) => uploadFile(e.target.files[0])} />
      <button onClick={() => generatePresignedURL('file1.txt', 'upload')}>
        Generate Upload URL
      </button>
      <button onClick={() => generatePresignedURL('file1.txt', 'download')}>
        Generate Download URL
      </button>
      {downloadURL && (
        <a href={downloadURL} download>
          Download file
        </a>
      )}
    </div>
  )
}

Use Cases

Upload files to the S3 bucket from your front end without exposing your AWS credentials.
Download files from the S3 bucket to your front end without exposing your AWS credentials.
Directly upload files to the S3 bucket from your front end without creating API to handle file uploads.

Bonus Code Snippet

// Code to get uploadUrl and put the file to the S3 bucket using fetch API.
const putFileToS3Api = async ({ uploadURL, file }) => {
  try {
    if (!file) throw new Error('No file provided')
    const res = await fetch(signedUrl, {
      method: 'PUT',
      headers: {
        'Content-Type': file.type ?? 'multipart/form-data',
      },
      body: file,
    })
    return res
  } catch (error) {
    console.error(error)
  }
}

const getUploadUrlApi = async ({ filename }) => {
  // Update the URL to your Graphql Endpoint.
  const res = await axios.get('http://localhost:3000/generate-presigned-url', {
    path: filename,
  })
  return res
  try {
  } catch (error) {
    console.error(error)
  }
}

export const uploadFileToS3Api = async ({ file }) => {
  try {
    if (!file) throw new Error('No file provided')
    const generateUploadRes = await getUploadUrlApi({ filename: file.name })
    if (!generateUploadRes.data.uploadURL)
      throw new Error('Error generating pre signed URL')
    const uploadRes = await putFileToS3Api({
      uploadURL: generateUploadRes.data.uploadURL,
      file,
    })
    if (!uploadRes.ok) throw new Error('Error uploading file to S3')
    return {
      message: 'File uploaded successfully',
      uploadURL: generateUploadRes.data.uploadURL,
      path: generateUploadRes.data.path,
    }
  } catch (error) {
    console.error(error)
  }
}

Call the uploadFileToS3Api function with the file you want to upload to the S3 bucket in one go. Additionally, you can use await Promise.all to upload multiple files at once.

const uploadFiles = async (files) => {
  const uploadPromises = files.map((file) => uploadFileToS3Api({ file }))
  const uploadResults = await Promise.all(uploadPromises)
  console.log(uploadResults)
}

Additional Resource

More information about AWS S3 presigned URLs can be found here

Conclusion

That's it. You have successfully learned how to generate pre-signed URLs for uploading and downloading files from the S3 bucket while keeping the bucket private.
I hope you find this blog helpful. If you have any questions, feel free to ask in the comments below or contact me on Twitter @thesohailjafri

Top comments (12)

Mike Stemle • Mar 18

There are a number of security problems here.

Never user plaintext AWS credentials

You shouldn't have plaintext AWS credentials in memory for your running web server. If you do, and someone is able to successfully inject code into your running server process, or otherwise dump heap, they could exfiltrate your credentials
Using plaintext credentials like this is often a recipe for never changing the credentials, which increases the harm an attacker could cause if those credentials are ever leaked.
If you're running your server in AWS, an execution role or instance profile should attach to an IAM policy document which gives you access. It is significantly safer to use IAM roles and STS than it is to use IAM users.
AWS IAM best practices discourage the use of IAM users: docs.aws.amazon.com/IAM/latest/Use...

Don't have public buckets

AWS is usually pretty clear about the risks of not having buckets configured to block all public access. It is far more secure to have your users upload the file to a server which then performs an s3:PutObject call.

If someone is able to get your service to give them signed URLs for uploading contents, you may very quickly find harmful files uploaded to your bucket.

Your CORS settings invite SSRF and CSRF

owasp.org/www-community/attacks/csrf

Your `axios` error handling is insecure

The axios module includes all authorization headers in the error object it returns, so your console.error() will log sensitive information.

Finally, you're using the old version of the AWS SDK

AWS-SDK v2 is being deprecated soon: aws.amazon.com/blogs/developer/ann...

The V3 of the SDK is pretty easy to use, and the nice thing about the change is that it's going to be similarly functional across all of the various libraries (e.g. Rust, Java, JavaScript), using similar patterns. Gone will be the days of language-specific AWS SDK patterns.

Sohail Jafri • Mar 18

Damn, thank you, I really mean it. Thats a lot to take but I will study one by one on the points you mentioned and try to improve my practice in existing and upcoming projects 🤝🙌😬

Sohail Jafri • Mar 18

During the production setup mostly the API routes stay under the auth middleware but I guess I can move the entire logic on the server which will take a single/multiple files and return the uploaded paths to keep it modular this I don't compose my bucket in any way

Mike Stemle • Mar 18

I very much appreciate you receiving that well. Security is hard, and security in the cloud is harder. There are a lot of tools but it's really hard to keep up with all of them.

If it helps, I never use IAM users. For humans, we should use federated authentication using something like Okta, or Auth0, and for infrastructure running code in AWS we should use execution roles or instance profiles.

Nobody can steal credentials which do not exist, or (in the case of AWS STS) are ephemeral and expire quickly.