Disk Forensics — Digital Evidence Analysis

Analyze disk images and file systems to recover evidence, reconstruct timelines, and identify artifacts.

Evidence Handling Principles

• Always work on copies, never originals
• Verify image integrity with hash comparison before analysis
• Mount everything read-only
• Document every command and finding
• Preserve timestamps — never modify source evidence

Methodology

Step 1: Image Identification and Integrity

Identify the image format and verify integrity:

file <image>                    # Identify format (E01, dd/raw, VMDK, VHD)
sha256sum <image>               # Compare to provided hash

For E01 images, use ewfinfo to extract metadata.

Step 2: Partition Layout

Examine the partition structure:

fdisk -l <image>                # Partition table
mmls <image>                    # Sleuth Kit partition layout

Calculate mount offsets: sectorstart × sectorsize

Step 3: Mount and Explore

Mount read-only and survey the file system:

mount -o ro,loop,offset=<bytes> <image> /mnt/evidence
ls -laR /mnt/evidence

For encrypted volumes, identify the encryption type and request the key/passphrase.

Step 4: File System Analysis (Sleuth Kit)

fsstat -o <offset> <image>              # File system details
fls -r -o <offset> <image>             # Full file listing (deleted files marked with *)
icat -o <offset> <image> <inode>       # Extract specific file by inode

Step 5: Artifact Recovery

Deleted files: Use fls to find (marked with *), icat to extract by inode.

File carving: Run foremost or scalpel on unallocated space to recover files by header signatures.

Hidden data:

• NTFS alternate data streams
• HFS+ resource forks
• Check image files for steganography: exiftool, binwalk, steghide

System artifacts:

• Browser history: ~/.mozilla, ~/Library/Safari, AppData\Local\Google
• System logs: /var/log/*, Windows Event Logs
• Registry hives (Windows): SAM, SYSTEM, SOFTWARE, NTUSER.DAT
• Recently accessed files, USB device history, prefetch files

Step 6: Metadata and Timestamps

exiftool <file>                 # EXIF, XMP, IPTC metadata
stat <file>                     # MAC times (Modified, Accessed, Changed)

For NTFS: examine $MFT timestamps and $UsnJrnl for change journal entries.

Use mactime (Sleuth Kit) to generate a unified timeline from body files.

Step 7: Keyword Search

strings <image> | grep -i <keyword>    # Raw string search across image

Use bulk_extractor for automated extraction of emails, URLs, credit card numbers, and other structured data.

Step 8: Timeline Construction

Collect all timestamps into a unified timeline. Cross-reference file events with log entries. Flag anomalies:

• Timestamps before the OS install date
• Future-dated files
• Gaps in otherwise continuous log sequences
• Timestamps inconsistent with timezone settings

Output Format

# Forensic Analysis Report
## Case: [identifier]
## Image: [filename] — SHA256: [hash]
## Date of Analysis: [date]

### Image Integrity
- Hash verified: [yes/no]
- Algorithm: [SHA256]

### Partition Layout
| # | Type | Start | Size | File System |
|---|------|-------|------|-------------|

### Key Findings
#### Finding 1: [Title]
- **Evidence:** [file path or artifact]
- **Content:** [description]
- **Timestamp:** [UTC]
- **Significance:** [why this matters]

### Recovered Files
| File | Source | Recovery Method | SHA256 | Significance |
|------|--------|-----------------|--------|-------------|

### Timeline
| Timestamp (UTC) | Event | Source | Notes |
|-----------------|-------|--------|-------|

### Conclusions
[Summary of findings and their implications]

Boundaries

• Work only on provided images and files
• Maintain read-only access at all times
• Document chain of custody for real investigations
• For CTF challenges, focus on finding flags and solving the challenge
• Never modify evidence or suggest evidence tampering
• Refuse requests involving unauthorized device access

References

• NIST SP 800-86: Guide to Integrating Forensic Techniques
• The Sleuth Kit documentation
• SANS Digital Forensics cheat sheets