Attackers Embedding Malicious Word Files into PDFs to Evade Detection

Introduction

Cybercriminals are deploying a new evasion technique called “MalDoc in PDF”, where they embed malicious Word documents inside PDF files to bypass traditional security tools. This sophisticated attack allows macros to execute once the file is opened in Microsoft Word, compromising systems while evading detection from PDF security scanners, sandboxes, and antivirus solutions.

How the MalDoc in PDF Attack Works

Dual-Nature Composition

These malicious files maintain PDF signatures and file structure, appearing harmless to standard PDF scanners. However, the embedded Word document with macros is appended after the legitimate PDF object, allowing the file to be opened by Microsoft Word.

Exploiting File Associations

The attackers typically use a .doc extension. Due to Windows default file association, the file is automatically routed to open in Word, triggering embedded macros that establish command and control (C2) connections and execute malicious code.

Why It's Dangerous

• Bypasses PDF Scanners: Tools like pdfid fail to detect the malicious components.
• Escapes Sandbox Detection: Hybrid files evade sandbox and antivirus classification.
• Macro Execution Risk: If macros are enabled, embedded code runs instantly.

How to Stay Safe

• Disable Macros by Default: Ensure macro execution is disabled unless explicitly required.
• Use Tools like OLEVBA: This tool detects malicious Office macros, including hidden ones in hybrid files.
• Custom Yara Rules: Deploy rules that identify both PDF and Office document structures.
• Educate Users: Train employees to recognize suspicious file behaviours and avoid enabling macros.

How Indian Cyber Security Solutions (ICSS) Helps

Indian Cyber Security Solutions (ICSS) provides comprehensive protection against sophisticated attacks like MalDoc in PDF through:

• Web Application Penetration Testing (WAPT): Identifying and mitigating document-based vulnerabilities.
• Advanced Threat Detection: Spotting hidden malware that traditional tools miss.
• Security Awareness Training: Empowering users to detect and respond to phishing and document-based threats.

With a proven track record and strong client portfolio, ICSS ensures secure transactions and data protection against evolving cyber threats.

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Conclusion

As cyber threats grow more sophisticated, techniques like MalDoc in PDF highlight the need for layered defences. Organizations must combine technical tools with user awareness to stay ahead. Partner with ICSS to fortify your defences and protect your business from emerging document-based attacks.