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CHFI (312-49) Digital Forensics Simulation

This simulation tests your ability to identify common anti-forensics techniques. You will learn to perform file signature analysis using hexadecimal headers to determine a file's true nature, regardless of its extension.

Investigation Scenario

A suspect is believed to be exfiltrating sensitive financial records from a corporate workstation. A quick triage of the suspect's USB drive reveals a folder full of family vacation photos. However, the investigator notices one specific file is unusually large for a standard image. To verify the file's integrity, the investigator opens the file in a Hex Editor to examine its internal structure.

Evidence Collected

Offset(h) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F Decoded Text 00000000 25 50 44 46 2D 31 2E 37 0A 25 E2 E3 CF D3 0A 31 %PDF-1.7.%âãÏÓ.1 00000010 20 30 20 6F 62 6A 0A 3C 3C 2F 54 79 70 65 2F 43 0 obj.<

Question

An image file has a .jpg extension, but when checked with a Hex Editor, the header starts with %PDF-1.7. This is known as:
Hint: The first few bytes of a file are called the "Magic Number" or file signature. They dictate the actual format of the data, regardless of what the file is named.

Expert Analysis

1. What Evidence Shows

The hex dump clearly shows the ASCII representation %PDF-1.7 at the very beginning of the file (offset 0), which corresponds to the hexadecimal values 25 50 44 46. However, the file is named with a .jpg extension. A valid JPEG file should start with the hex signature FF D8 FF E0 (or similar markers like FF D8 FF E1).

2. Forensic Stage

Examination: Performing file signature analysis to identify anomalies, defeat basic anti-forensics, and reveal the true nature of collected data.

3. Why the Correct Answer is Correct (A)

This is a classic example of a mismatching file extension. Threat actors frequently alter a file's extension to bypass basic Data Loss Prevention (DLP) systems, evade casual visual inspection, or prevent default applications from opening the file correctly. The file header (magic number) always indicates the true underlying file structure.

4. Why Others are Wrong

B. File corruption: Incorrect. Corruption typically results in randomized, missing, or null data at the header level. It does not perfectly restructure the header into a completely different, valid format (like transforming a JPG header flawlessly into a PDF header).

C. Data compression: Incorrect. Compressed archives (like ZIP or RAR) have their own specific magic numbers (e.g., 50 4B 03 04 for PKZIP). Compressing a file does not result in a plaintext PDF header.

D. Encryption: Incorrect. Encrypted files exhibit extremely high entropy (randomness) and lack recognizable plaintext ASCII headers like %PDF-1.7, unless wrapped in a specific recognizable encryption container.

5. Real-World Forensic Action

A digital forensic investigator rarely relies on the Windows operating system's file associations. Instead, they ingest the evidence into a forensic suite (like Autopsy, FTK, or EnCase) or use command-line tools like file or sigcheck. These tools automatically compare every file's extension against its internal magic number. When a mismatch is flagged, the investigator re-exports the file with its correct extension (in this case, .pdf) to safely examine the exfiltrated document.

MINI LESSON: Magic Numbers & Signature Analysis

The OS relies on extensions (like .docx, .exe) to know which program to use, but the application relies on the Magic Number (File Signature) to know how to parse the bits. Common signatures every investigator should know include:

  • JPEG: FF D8 FF
  • PNG: 89 50 4E 47
  • PDF: 25 50 44 46 (%PDF)
  • ZIP/DOCX/XLSX: 50 4B 03 04 (PK..)
  • Executable (PE): 4D 5A (MZ)

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