Introduction
Thick client (Desktop) application penetration testing is the process of evaluating the security of a thick client application by simulating attacks to identify vulnerabilities. A thick client application, also known as a fat client, is a software program that runs on a local machine and communicates with a server or a database to perform its functions. Unlike thin clients, which rely on server-based processing, thick clients handle most of the processing on the client side, often providing rich user interfaces and high performance.
The comprehensive guidelines for thick client (desktop) application penetration testing are divided into three blog posts:
- This introductory blog post, which you are currently reading.
- The second post, titled “Thick Client (Desktop) Application Penetration Testing – Part 1“
- The third post, titled “Thick Client (Desktop) Application Penetration Testing – Part 2“
Thick client applications can be developed using various programming languages such as:
- .Net
- Java
- C/C++
- Microsoft Silverlight
The main focus areas have been derived from the OWASP Windows Binary Executable Files Security Checks Project
Architecture
Thick client applications can follow various architectural models, with the most common being:
2-Tier Architecture
- Client (Presentation Layer): The client application runs on the user’s machine, handling the user interface and some processing.
- Database Server (Data Layer): The server manages the database, responding to client queries and storing data.
- Client (Thick Client) / Presentation Layer:
- The client application runs on the user’s machine. Handles the user interface, user inputs, and some business logic.
- Directly communicates with the database server for data operations.
- Database Server / Data Layer:
- Manages data storage and retrieval.
- Processes queries sent by the client application.
- The database server is responsible for data management, security, and integrity.
3-Tier Architecture
- Client (Presentation Layer): Manages the user interface.
- Application Server (Business Logic Layer): Handles business logic and processing, interacting with the client and the database server.
- Database Server (Data Layer): Manages data storage and retrieval.
- Client (Thick Client) / Presentation Layer:
- The client application runs on the user’s machine. Manages the user interface and user interactions.
- Sends requests to the application server for processing.
- Application Server / Business Logic Layer:
- Acts as an intermediary between the client and the database server.
- Handles business logic, processing client requests, and enforcing business rules.
- Communicates with the database server to retrieve or store data.
- Database Server / Data Layer:
- Manages data storage and retrieval.
- Processes queries sent by the application server.
- Ensures data security, integrity, and availability.
Difference between 2-Tier Architecture and 3-Tier Architecture
OWASP Top 10 Thick Client or Desktop Application Security Risks (2021)
- The OWASP Desktop Application Security Top 10 outlines the most critical security risks for desktop applications, agreed upon by experts.
- Recognized globally, it serves as the essential first step for developers aiming to improve their secure coding practices.
- Adopting this document helps organizations begin mitigating these risks and fostering a culture focused on developing secure code.
| OWASP Top 10 Desktop App | Example |
| DA1 – Injections | SQLi, LDAP, XML, OS Command, etc. |
| DA2 – Broken Authentication & Session Management | OS / DesktopApp account Authentication & Session Management, Auth. for Import / Export with external Drive, Auth. for Network Shared Drives or other Peripheral devices |
| DA3 – Sensitive Data Exposure | Data in Memory post App Logout, Logs with Sensitive Info., Hardcoded Secrets in files, etc. |
| DA4 – Improper Cryptography Usage | Weak Keys or Usage of Outdated Cryptographic Algorithms, Inappropriate usage of Cryptographic Functions, reuse of Cryptographic Parameters across all Installations, Improper usage of Cryptography for Integrity check |
| DA5 – Improper Authorization | Weak File/Folder Permission per User Role, Missing Principle of Least Privilege approach, Improper User Roles |
| DA6 – Security Misconfiguration | Missing Code-Signing and Verification for File Integrity, Missing Code Obfuscation, Dll-Preloading or Injection, Race Conditions, lack of binary protection (Overflows, Null pointers, memory corruption), etc. |
| DA7 – Insecure Communication | Usage of weak TLS or DTLS Cipher-suites or Protocols, Unencrypted DB Queries in Transit, Absent Encrypted standard/custom protocol communication like HTTP, MQTT, COAP, etc. |
| DA8 – Poor Code Quality | Missing Code-Signing and Verification for File Integrity, Missing Code Obfuscation, Dll-Preloading or Injection, Race Conditions, lack of binary protection (Overflows, Null pointers, memory corruption) etc. |
| DA9 – Using Components with Known Vulnerabilities | Usage of Outdated Softwares, or Usage of Obsolete Components/Services of Windows/3rd Party vendors |
| DA10 – Insufficient Logging & Monitoring | Missing or Improper Logging of Activities, Missing Regular Monitoring to Detect Abuse |
Security
- 2-Tier: More vulnerable as the client has direct access to the database.
- 3-Tier: Improved security as the application server enforces access control and business rules.
Complexity
- 2-Tier: Simpler architecture with fewer components, but direct exposure of the database to the client.
- 3-Tier: More complex architecture with an additional layer, providing better separation of concerns and security.
Conclusion
Thick client (desktop) application penetration testing is essential for identifying and mitigating security vulnerabilities in applications that perform significant processing on the client-side and interact with servers or databases. Common vulnerabilities include insecure communication, SQL injection, and hardcoded credentials, which can be addressed through secure coding practices and robust encryption. The architectural models (2-tier and 3-tier) influence security considerations, with 3-tier offering better separation of concerns and security. Despite challenges like application complexity and encrypted communication, thorough penetration testing ensures a secure application environment, compliance with standards, and protection against data breaches.