Content Type not enforced in s3 pre-signed POST

3 min read 06-10-2024
Content Type not enforced in s3 pre-signed POST


S3 Pre-Signed POST: Why Content Type Enforcement is Crucial

Uploading files directly to Amazon S3 using pre-signed POST requests offers a convenient and efficient way to handle file uploads in your applications. However, a critical security vulnerability can arise if you don't enforce the content type of uploaded files. This article will delve into the issue, explore its implications, and provide practical solutions for securing your S3 uploads.

The Problem: Unchecked File Types Can Open Doors for Malicious Activity

Imagine a scenario where your application allows users to upload profile pictures. You've implemented pre-signed POST requests to simplify the upload process. However, your code fails to enforce the expected "image/jpeg" or "image/png" content type. This seemingly innocent oversight can have serious consequences.

Original Code (Without Content Type Enforcement):

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

const params = {
    Bucket: 'your-bucket-name',
    Key: 'user-profile-picture.jpg',
    Fields: {
        'acl': 'public-read',
        'Content-Type': 'image/jpeg' // This is only a suggestion, not enforced
    },
    Conditions: [
        ['content-length-range', 0, 5000000]
    ],
    Expires: 3600 // 1 hour expiration time 
};

s3.createPresignedPost(params, (err, data) => {
    if (err) {
        console.error(err);
    } else {
        console.log(data); 
    }
});

In this code, we set the expected Content-Type to 'image/jpeg'. However, because it's not enforced, a malicious user could easily bypass this and upload a different type of file, such as an executable script, disguised as an image.

The Impact: Data Integrity and Security Risks

Without content type enforcement, you open yourself to several vulnerabilities:

  • Data Integrity: Uploaded files might not be what you expect, corrupting your data and potentially causing application errors.
  • Security Risks: Malicious files could be uploaded and executed on your server, compromising your infrastructure and sensitive data.
  • Increased Bandwidth Costs: Unwanted file uploads can consume unnecessary bandwidth, resulting in higher costs.

The Solution: Enforcing Content Type Through Validation and Policy

Preventing these vulnerabilities is crucial. Here's how you can ensure secure content type enforcement with your S3 pre-signed POST requests:

  1. Client-Side Validation:

    • Implement JavaScript validation on the client-side to restrict the allowed file types before upload.
    • Use libraries like FilePond or Dropzone.js to provide user-friendly file selection and validation.
  2. Server-Side Validation:

    • Pre-Signed POST Conditions: Use Conditions in your createPresignedPost request to enforce the expected content type. This forces clients to submit requests with the correct Content-Type header.
    • Lambda Trigger: You can use AWS Lambda triggered by an S3 event to perform additional validation upon file upload. This can check the content type against a whitelist and remove files that don't comply.

Updated Code (with Content Type Enforcement):

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

const params = {
    Bucket: 'your-bucket-name',
    Key: 'user-profile-picture.jpg',
    Fields: {
        'acl': 'public-read'
    },
    Conditions: [
        ['content-length-range', 0, 5000000],
        ['starts-with', '$Content-Type', 'image/'], // Enforces content type starting with 'image/'
        ['eq', '$Content-Type', 'image/jpeg']  // Enforces specific content type for image uploads
    ],
    Expires: 3600 // 1 hour expiration time 
};

s3.createPresignedPost(params, (err, data) => {
    if (err) {
        console.error(err);
    } else {
        console.log(data); 
    }
});
  1. S3 Bucket Policies:
    • Implement a bucket policy that denies writes if the Content-Type header doesn't match the allowed values. This adds an extra layer of security by blocking unauthorized uploads at the S3 level.

Additional Considerations:

  • Content Type Checking Libraries: Consider using libraries like mime-types or file-type to perform content type validation on the server-side.
  • Content Type Whitelists: Maintain a strict whitelist of allowed content types to minimize potential vulnerabilities.

Conclusion:

By implementing content type enforcement using a combination of client-side validation, server-side checks, and S3 bucket policies, you can significantly enhance the security of your S3 pre-signed POST uploads. Remember, protecting your data and application integrity requires a multi-layered approach that includes robust validation and a proactive mindset towards potential security threats.