PDF Executable Files: A Security Risk

TL;DR

Yes, a file named agreement.execds.pdf could contain an executable file hidden inside it. PDFs can embed other files, and attackers exploit this to deliver malware. You should scan any PDF from untrusted sources with antivirus software before opening it, and be cautious even if the scan is clean.

Understanding the Risk

PDF (Portable Document Format) files are designed to contain text, images, and fonts. However, they also allow for embedding other types of content, including JavaScript, Flash objects, and even entire executable files. This flexibility can be abused by attackers.

How it Works

1. Embedding Executables: Attackers embed malicious executables within the PDF structure. These aren’t visible in a normal file listing.
2. Exploiting PDF Viewers: Older or poorly updated PDF viewers might automatically execute embedded content when the PDF is opened, without warning.
3. Social Engineering: Attackers often disguise malicious PDFs as legitimate documents (like invoices, agreements, etc.) to trick users into opening them. The .execds extension is a red flag – it’s not standard and suggests something unusual.

Steps to Check for Hidden Executables

1. Scan with Antivirus: This is the first step! Use a reputable antivirus program to scan agreement.execds.pdf before opening it. Most modern antivirus software can detect embedded malware.

   # Example using ClamAV (command line) - install if needed
   clamscan agreement.execds.pdf

2. Examine File Headers: Use a hex editor to inspect the file’s header. A legitimate PDF will start with %PDF-. If you see other unexpected headers, it’s suspicious.

   Note: This requires technical knowledge and is not recommended for beginners.

3. Extract Embedded Files (Advanced): Tools like pdfid or peep can extract embedded files from a PDF. These tools show you what’s *inside* the PDF.
   • pdfid: A Python script to identify PDF features and potential threats.

     # Install pdfid (requires Python) - pip install pdfid
     pdfid agreement.execds.pdf

   • peep: Another tool for examining PDF structure.

     # Download from https://github.com/jesparza/peep
     ./peep agreement.execds.pdf

4. Sandbox Analysis (Advanced): If you’re highly suspicious, run the PDF in a sandbox environment (like Cuckoo Sandbox) to observe its behaviour without risking your system.

   Note: This requires significant technical expertise.

Protecting Yourself

• Keep Software Updated: Regularly update your PDF viewer (Adobe Acrobat Reader, Foxit Reader, etc.) to patch security vulnerabilities.
• Be Wary of Unknown Sources: Don’t open PDFs from untrusted senders or download them from suspicious websites.
• Enable Protected Mode: Adobe Acrobat Reader has a ‘Protected Mode’ feature that limits the PDF viewer’s access to your system, reducing the risk of exploitation.
• Disable JavaScript (If Possible): If you don’t need it, disable JavaScript execution in your PDF viewer settings. This prevents malicious scripts from running.

  In Adobe Acrobat Reader: Edit > Preferences > JavaScript and select ‘Do not allow JavaScript execution’.