GIF Image DOS Attack Prevention

TL;DR

Large or maliciously crafted GIF images can cause a denial-of-service (DoS) attack by consuming excessive client-side resources. This guide explains how to mitigate this risk.

Solution Guide

1. Understand the Problem: GIFs, unlike JPEGs and PNGs, are decoded entirely on the client’s browser. A very large GIF or one with many frames can take a long time to process, freezing the browser or causing it to crash.
2. Image Size Limits (Server-Side): Implement limits on the maximum allowed file size for uploaded images. This is your first line of defence.
   • For PHP: You can check $_FILES['image']['size'] before saving the image.
   • Example PHP code:

      2048000) {
       echo 'Image too large!';
       exit;
     }
     ?>

3. Image Dimension Limits (Server-Side): Limit the maximum width and height of uploaded images. Large dimensions contribute to larger file sizes.
   • Use image processing libraries like GD or ImageMagick on your server to check these values before saving.

      1000 || $height > 800) {
         echo 'Image dimensions too large!';
         exit;
     }
     ?>

4. GIF Optimisation (Server-Side): Automatically optimise uploaded GIFs to reduce their file size and number of frames.
   • Tools like ImageMagick can be used for this. Consider reducing the colour palette or removing unnecessary frames.

5. Content Security Policy (CSP): Implement a CSP to restrict where images can be loaded from, reducing the risk of malicious GIFs being injected.
   • Add a header like this to your web server configuration:

     Content-Security-Policy: img-src 'self'

     This only allows images from the same domain as your website.

6. Client-Side Validation (JavaScript): Add client-side validation to check image size and dimensions before uploading, providing a better user experience.
   • Example JavaScript:

     const fileInput = document.getElementById('imageUpload');
     fileInput.addEventListener('change', function() {
       if (this.files[0].size > 2048000) {
         alert('Image too large!');
         this.value = ''; // Clear the selected file
       }
     });

7. Lazy Loading: Implement lazy loading for images, so they are only loaded when they come into view.
   • This can help prevent a large number of GIFs from being processed at once.
8. Regular Security Audits: Regularly scan your website and server for vulnerabilities, including potential image-related attacks.