performing-thick-client-application-penetration-test

name	performing-thick-client-application-penetration-test
description	Conduct a thick client application penetration test to identify insecure local storage, hardcoded credentials, DLL hijacking, memory manipulation, and insecure API communication in desktop applications using dnSpy, Procmon, and Burp Suite.
domain	cybersecurity
subdomain	penetration-testing
tags	["thick-client","desktop-application","dnSpy","Procmon","DLL-hijacking","binary-analysis","API-interception"]
version	1.0
author	mahipal
license	Apache-2.0
nist_ai_rmf	["MEASURE-2.7","MAP-5.1","MANAGE-2.4"]
atlas_techniques	["AML.T0070","AML.T0066","AML.T0082"]
nist_csf	["ID.RA-01","ID.RA-06","GV.OV-02","DE.AE-07"]

Performing Thick Client Application Penetration Test

Overview

Thick client (fat client) penetration testing assesses the security of desktop applications that run locally on user machines and communicate with backend servers. Unlike web applications, thick clients present a broader attack surface including local file storage, binary analysis, memory manipulation, DLL injection, process interception, and client-server communication. Common targets include banking applications, ERP clients (SAP GUI), trading platforms, healthcare systems, and legacy enterprise software.

When to Use

• When conducting security assessments that involve performing thick client application penetration test
• When following incident response procedures for related security events
• When performing scheduled security testing or auditing activities
• When validating security controls through hands-on testing

Prerequisites

• Application installer and valid credentials
• Windows/Linux test machine (isolated)
• Tools: dnSpy, Procmon, Process Hacker, Wireshark, Burp Suite, Echo Mirage, Fiddler, IDA Pro/Ghidra
• Administrative access to test machine

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Phase 1 — Information Gathering

Static Analysis

# Identify application technology
# Check file properties, signatures, framework (.NET, Java, C++, Electron)
file application.exe
# .NET -> dnSpy, JetBrains dotPeek
# Java -> JD-GUI, JADX
# C/C++ -> Ghidra, IDA Pro
# Electron -> extract asar archive

# Check for .NET framework
Get-ChildItem -Path "C:\Program Files\TargetApp" -Recurse -Filter "*.dll" |
  ForEach-Object { [System.Reflection.AssemblyName]::GetAssemblyName($_.FullName).FullName }

# Strings analysis
strings application.exe | findstr -i "password\|secret\|api\|key\|token\|jdbc\|connection"

# Check for hardcoded credentials
strings application.exe | findstr -i "username\|user=\|pass=\|pwd=\|admin"

# Review configuration files
type "C:\Program Files\TargetApp\app.config"
type "C:\Program Files\TargetApp\settings.xml"
type "%APPDATA%\TargetApp\config.json"

# Check for certificate pinning
strings application.exe | findstr -i "cert\|pin\|ssl\|tls"

.NET Decompilation with dnSpy

# Open application in dnSpy
1. Launch dnSpy
2. File > Open > Select application.exe and DLLs
3. Search for:
   - "password", "secret", "connectionString"
   - Authentication methods
   - Encryption/decryption functions
   - API endpoints and keys
   - License validation logic

# Look for:
- Hardcoded credentials in source
- Insecure encryption (DES, MD5, base64 "encryption")
- SQL queries (potential injection)
- Disabled certificate validation
- Debug/verbose logging with sensitive data