MalDoc In PDF : A Novel Technique For Evading Detection

The cybersecurity community has recently been alerted to a sophisticated attack method known as “MalDoc in PDF,” which involves embedding a malicious Microsoft Word file within a seemingly harmless PDF document.

This technique, identified by JPCERT/CC, allows attackers to bypass traditional security measures by exploiting the dual nature of these files, which can be opened in both PDF viewers and Microsoft Word.

How MalDoc In PDF Works

1. Embedding Malicious Content: Attackers append an MHT file created in Word, containing malicious macros, to the end of a PDF file. This results in a file that maintains a PDF signature but can also be opened in Word.
2. Execution of Malicious Macros: When opened in Word, the embedded macros execute, potentially leading to malicious activities such as downloading and deploying malware.
3. Evasion of Detection: Traditional PDF analysis tools like pdfid may fail to detect the malicious components, as the file appears as a legitimate PDF.

To combat this technique, cybersecurity experts recommend using tools specifically designed for analyzing malicious Word files:

• OLEVBA: This tool effectively identifies embedded macros in Word documents, allowing for the detection of malicious elements within MalDoc in PDF files.
• Yara Rules: These can be used to create detection rules that identify discrepancies in file extensions and structures, providing warnings when incompatible file types are detected within PDF documents.

The MalDoc in PDF technique highlights the evolving nature of cyber threats and the need for advanced detection methods.

While it does not bypass settings that disable auto-execution of Word macros, its ability to masquerade as a PDF poses significant challenges for automated malware analysis and traditional security software.