Thick Client Penetration Testing

What Is Thick Client Testing?

Unlike web apps, thick clients run directly on user devices—often with elevated privileges and embedded secrets. Our testing simulates reverse engineering, API tampering, DLL injection, and more.

• Analyze application logic and workflows
• Test interactions with local storage and databases
• Intercept API traffic between client and server
• Decompile .NET assemblies and inspect for secrets

Thick client assessments combine static and dynamic analysis with API-level penetration testing for complete coverage.

What Do We Look For?

• Hardcoded credentials or connection strings
• DLL hijacking or insecure library loading
• Insecure use of the Windows registry or file system
• Insufficient input validation within local forms
• Local privilege escalation via misused permissions
• API calls that bypass server-side validation

Why It’s Critical

Thick clients often connect directly to internal databases or critical infrastructure. If their security model lacks proper validation or enforcement, attackers can manipulate the client to perform unauthorized actions, extract sensitive data, or serve as a bridge into the internal network.

• Prevent attackers from abusing your client binaries
• Protect sensitive configurations and encryption keys
• Prevent internal privilege escalation risks
• Support compliance with security frameworks such as GDPR, ISO 27001, SOC 2, NIST, PCI DSS, HIPAA, and CIS Controls.
• Test the app as a user would—with elevated control

Our Testing Approach

We use tools like dnSpy, ILSpy, Fiddler, Burp Suite, and custom scripts to inspect .NET assemblies and application behavior. Initially, we interact with the application like a regular user to understand its intended workflows, functionality, and user roles.

This helps us identify surface-level features and potential areas of interest. Next, we decompile the application to analyze its internal logic, look for hardcoded secrets, insecure configurations, and authentication mechanisms. In parallel, we intercept and monitor traffic using proxies like Burp Suite or Fiddler to understand how the application communicates with backend services, APIs, or other systems.

By combining static and dynamic analysis, we’re able to identify trust boundaries, attack surfaces, and insecure implementations that may not be visible through black-box testing alone. Our goal is to approach testing like a motivated attacker but with the discipline and structure of a professional audit.

Real World Examples

One client had developed an internal thick client application to manage and annotate case related files stored on a central file server. The application was intended to control access and maintain an audit trail of user actions. However, during testing, we discovered that end users could bypass the application entirely by directly mounting the backend file share.

This allowed them to access, modify, or delete sensitive files without any oversight, logging, or restrictions exposing the organization to significant fraud and data integrity risks.

Engagement Process

1. Scoping & Quotation: Drop us an email, and we’ll either arrange a scoping call or send over our scoping forms. Based on the information you provide, we’ll prepare a quote for the work. Once everything is agreed upon, we’ll send through the necessary paperwork and agreements, and then schedule the assessment.
2. Assessment Phase: On the scheduled day, our team begins testing. If we detect any critical or high risk vulnerabilities during the assessment, we alert you immediately to help you respond quickly.
3. Results & Reporting: You’ll receive a professionally written report detailing each vulnerability, its business impact, technical detail, and precise guidance for remediation.
4. Free Retesting: Once you’ve resolved the major findings, we offer complimentary retesting for high and critical issues—so you can close the loop with confidence.