Thick Client Pentesting Guide

Hello Hackers,

My name is Madhurendra, and today in this blog, I’m going to write about thick application pentesting. By the end of this article, you’ll understand what thick applications are and why it’s important to test their security.

Madhurendra Kumar

You’ll learn about the thick client, common security risks, great resources for thick client pentesting, popular tools and techniques, andand finally vulnerable labs for practice.

Here is what this article covers:

1. Introduction to Thick Applications.
2. Why Pentesting Thick Applications is Important.
3. Common Security Risks and Vulnerabilities.
4. Tools and Techniques for Pentesting.
5. Vulnerable Labs of thick client application.

What is a Thick Client?

A thick client, also known as a fat client or rich client, is a software application that performs most of its operations locally on the user’s device. This includes data processing, storage, and execution. Thick clients are capable of functioning with or without network connectivity, providing a rich user experience with more features and faster performance.

What is a Thin Client?

A thin client is a lightweight computer that relies heavily on a central server for processing activities. Thin clients are primarily used for display and input tasks, while most of the data processing and storage happens on the server. This makes them more dependent on network connectivity and central server resources.

Architecture

Thick Client Architecture

1. Two-Tier Architecture

• Client Tier: The thick client application running on the user’s device handles presentation, business logic, and data processing.
• Server Tier: The database server stores and manages data.
• Example: Early versions of Microsoft Access.

2. Three-Tier Architecture

• Client Tier: The thick client manages the user interface.
• Application Server Tier: This middle layer processes client requests and business logic.
• Database Server Tier: The backend server stores and manages data.
  Example: Modern enterprise applications like SAP, Postman, Outlook.

Thin Client Architecture

• Client Tier: The thin client acts primarily as an interface for user input and display.
• Server Tier: The server handles all processing, business logic, and data storage.
• Example: Virtual Desktop Infrastructure (VDI) environments where the desktop environment is hosted on a central server.

Thick Client vs Thin Client

Let’s explore few more concepts

Standalone Thick Clients vs Networked Thick Clients

1. Standalone Thick Clients: Operate independently of a network.
Example: Microsoft Word, Adobe Photoshop.

2. Networked Thick Clients: Operate locally but also connect to a server for additional functionalities.
Example: Microsoft Outlook, Microsoft Teams.

Stateless Thin Clients vs Stateful Thin Clients

1. Stateless Thin Clients: Do not retain any user data between sessions.
Example: Devices used for accessing web-based applications.

2.Stateful Thin Clients: Retain some user data and settings between sessions to improve user experience.
Example: Thin clients in a VDI environment.

Basics of Thick Clients

Proxy Aware vs Proxy Unaware

1. Proxy Aware: Can route network traffic through a proxy server, allowing for monitoring and control.
   Example: Web browsers configured to use a proxy.
2. Proxy Unaware: Communicate directly with servers, making network traffic monitoring difficult.
   Example: Custom-built applications without proxy configuration options.

Photo by Arget on Unsplash

Why pentesting of Thick Client is required?

Thick client pentesting is crucial because without it, vulnerabilities could remain undiscovered, potentially leading to serious consequences such as unauthorized access to sensitive data, manipulation of application functionality, and even complete system compromise. By not conducting pentesting, organizations risk exposing their users to data breaches, financial losses, damage to reputation, and regulatory non-compliance. Regular pentesting helps preemptively address these risks, ensuring robust security measures are in place to protect both the organization and its stakeholders.

Madhurendra Kumar

Amazing Thick client pentestin resouces:

• https://www.cyberark.com/resources/threat-research-blog/thick-client-penetration-testing-methodology
• https://www.darkrelay.com/post/thick-client-penetration-testing
  https://payatu.com/blog/thick-client-penetration-testing/
• https://qualysec.com/thick-client-pen-testing-a-comprehensive-guide/
  https://medium.com/@abhishek.offsec/thick-client-security-testing-the-essential-guide-78f0015ed94f
• https://infosecwriteups.com/thick-client-pentest-modern-approaches-and-techniques-part-1-7bb0f5f28e8e
• https://qualysec.com/thick-client-pen-testing-a-comprehensive-guide/
• https://www.threatintelligence.com/blog/thick-client-application-penetration-test

Most Common Tools used for Thick Client Pentesting:

The is list is not complete, however this is most common tools used during conducting thick client pentesting.

• Network Traffic Analysis Tools — Wireshark, Fiddler, Burp Suite, OWASP ZAP
• Proxy Tools — Burp Suite, OWASP ZAP, Charles Proxy, Proxyman
  Static Tools for Identifying Development Language — CFF Explorer, PEid, Detect It Easy (DIE), Strings
• .NET Decompilers and Deobfuscators — dnSpy, ILSpy, JetBrains DotPeek, de4dot, NeonFuscatorDeobfuscator
• Network Sniffers — Wireshark, TCPView, SmartSniff, tcpdump, Microsoft Network Monitor 3.4
• Proxy Tools for Intercepting Traffic — Burp Suite, Fiddler, Echo Mirage, Charles Web Debugging Proxy
• File Analysis Tools — Process Monitor, Regshot, Process Explorer, Process Hacker
• DLL Hijacking Testing Tools — DLLSpy, Robber
• Tools for Locating Sensitive Data Storage on Files and Registry — Process Monitor, Regshot, Process Explorer, Process Hacker

Thick Client Pentesting Check List:

Here arelist few nice check list dedicated for thick client.

• https://github.com/Hari-prasaanth/Thick-Client-Pentest-Checklist
• https://github.com/m14r41/PentestingEverything

Github Repository dedicate thick client:

• https://github.com/RakeshKengale/RaKKeN/blob/master/Index/Thick_Client.md

Vulnerable Thick Client Application:

Explore a list of vulnerable thick client applications to practice and improve your skills safely.

• https://github.com/srini0x00/dvta
• https://github.com/kartikdurg/Terrible-Thick-Client
• https://github.com/Warxim/vucsa
• https://github.com/DarkRelayLabs/VWA
• https://github.com/NetSPI/BetaFast

Thank you for reading the blog. Stay tuned for more content in the future. Looking forward to seeing you in the next one. Happy hacking!