PDF Upload Security Risks

TL;DR

Yes, an inaccessible (or seemingly harmless) uploaded PDF can harm a server. PDFs can contain malicious code that exploits vulnerabilities in PDF viewers or the server itself. Proper validation and sanitisation are crucial.

How a PDF Can Be Harmful

Even if you don’t display the PDF directly in a browser, simply storing it can be risky. Here’s how:

• Malicious JavaScript: PDFs support JavaScript. A malicious script could execute when someone opens the file (even locally) or during server-side processing.
• Exploits in PDF Viewers: Older versions of Adobe Reader and other viewers have known vulnerabilities that a crafted PDF can trigger.
• File System Attacks: A specially designed PDF could attempt to exploit bugs in the file system when opened, potentially leading to arbitrary code execution on the server if processed incorrectly.
• Denial-of-Service (DoS): Large or complex PDFs can consume excessive resources during processing, causing a DoS attack.

Steps to Protect Your Server

1. Input Validation: Always validate the file extension and MIME type before accepting an upload.
   • Don’t rely solely on the client-side check (it can be bypassed).
   • Use server-side checks. For example, in PHP:

2. File Size Limits: Restrict the maximum allowed PDF file size to prevent DoS attacks.
   • Configure this in your web server settings (e.g., Apache, Nginx) or application code.
3. Sanitisation/Scanning: This is the most important step.
   • Virus Scanning: Use a reputable antivirus scanner to scan uploaded PDFs before storing them. ClamAV is a popular open-source option.

     clamscan /path/to/uploaded/pdf_file.pdf

   • PDF Parsing and Validation Libraries: Use libraries specifically designed for PDF parsing to identify potentially malicious content.
     • PDFiD: A Python tool that identifies features within a PDF file, helping detect suspicious elements.
     • peepdf: Another Python library for analyzing PDFs; it can help find JavaScript and other embedded objects.
     • Ghostscript: While powerful, be cautious when using Ghostscript directly as it has had security vulnerabilities in the past. Use a wrapper or carefully control its input.
4. Sandboxing/Isolation: If you need to process PDFs server-side (e.g., for indexing), do so within a sandboxed environment.
   • Containers (Docker) or virtual machines can provide isolation.
5. Regular Updates: Keep your PDF viewers, libraries, and operating system up to date with the latest security patches.
6. Content Security Policy (CSP): If you display PDFs in a browser, use CSP headers to restrict JavaScript execution.
   • Example header:

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

7. Storage Location and Permissions: Store uploaded PDFs in a dedicated directory with limited permissions. Prevent direct execution of scripts from that directory.

Important Considerations

• Zero Trust: Assume all uploaded files are potentially malicious until proven otherwise.
• Ongoing Monitoring: Regularly review your security measures and logs for suspicious activity.