By Het Mehta | Published: 2025-06-03 | Last Updated: 6/11/2025
Disclaimer
Performing penetration testing without explicit, written permission from the application owner is illegal and unethical. This content is provided for educational purposes only.
This checklist provides a structured approach to thick client application penetration testing. Thick client applications, unlike web or mobile apps, often run directly on the user's machine, interacting with local resources, the operating system, and potentially remote backend services. This introduces unique attack surfaces and testing methodologies.
It covers key phases from reconnaissance to detailed analysis, drawing inspiration from general application security principles and specific thick client attack vectors.
Ensure you have the necessary tools and environment for thick client VAPT:
# On Linux/macOS to identify file type
file /path/to/thickclient.exe
# On Windows (using PowerShell to check .NET assembly)
[System.Reflection.AssemblyName]::GetAssemblyName("C:\path\to\thickclient.exe")
# On Windows, use Process Explorer to see loaded DLLs
# On Linux, use lsof to list open files by a process
lsof -p <PID> | grep .so
# On Windows, list active connections and associated executables
netstat -ano | findstr ESTABLISHED
# On Linux, list listening ports and associated processes
sudo netstat -tulpn
# On Windows, use Procmon to filter for File System and Registry activity
# Filter: Process Name is <app_name>.exe, Operation is WriteFile/RegSetValue
# On Windows, use OLE/COM Object Viewer to enumerate COM objects
# For Named Pipes, look for \\.\pipe\ entries in Procmon or use PipeViewer.
# Unpack UPX compressed executable
upx -d /path/to/packed_executable.exe
# Extract printable strings from a binary
strings /path/to/executable | grep -i "password\|api_key\|secret"
# On Windows using findstr
findstr /s /i "password api_key secret" C:\path\to\app\*.*
<!-- Example: Insecure configuration in a .NET App.config -->
<configuration>
<appSettings>
<add key="DatabaseConnectionString" value="Data Source=server;Initial Catalog=db;User ID=sa;Password=admin"/>
<add key="DebugMode" value="true"/>
</appSettings>
</configuration>
// Example: Insecure file write in C
FILE *fp = fopen("sensitive_data.txt", "w");
fprintf(fp, "Username: %s\nPassword: %s\n", username, password);
fclose(fp);
/* Example: Insecure hostname verifier in Java */
import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.SSLSession;
HostnameVerifier allHostsValid = new HostnameVerifier() {
public boolean verify(String hostname, SSLSession session) {
return true; /* ALWAYS returns true - INSECURE! */
}
};
// Example: Buffer overflow vulnerability
char buffer[10];
strcpy(buffer, user_input); /* No size check */
// Example: SQL Injection in local SQLite DB
sqlite3_snprintf(1024, sql, "SELECT * FROM users WHERE username='%q'", user_input);
# Procmon filter example for sensitive file access
# Process Name: <app_name>.exe
# Path: C:\Users\<username>\AppData\Local\<app_data_folder>\*
# Operation: WriteFile, RegSetValue
# Result: SUCCESS
# On Windows, check permissions
icacls "C:\Program Files\YourApp"
# On Linux, check permissions
ls -la /opt/YourApp
# Wireshark filter for specific custom protocol port
tcp.port == 12345
# Using ProxyCap/Proxifier to force thick client traffic through Burp/Fiddler
# Configure application to use SOCKS proxy (Burp's default is 8080)
# Using Volatility to dump process memory (example for Windows)
vol.py -f <memory_dump.raw> --profile=Win7SP1x64 procdump -p <PID> -D .
# Searching for strings in a memory dump (Linux)
strings <process_dump.dmp> | grep -i "password\|token"
// Example: Frida script to hook and log parameters of a Windows API (e.g., CryptEncrypt)
// This is conceptual; actual implementation depends on the specific API and target
// Load the specific module if not already loaded
const advapi32 = Module.findExportByName("advapi32.dll", "CryptEncrypt");
if (advapi32) {
Interceptor.attach(advapi32, {
onEnter: function (args) {
console.log("CryptEncrypt called!");
console.log("hKey:", args[0]);
console.log("hHash:", args[1]);
console.log("fFinal:", args[2].toInt32());
console.log("dwFlags:", args[3]);
console.log("pbBuffer (input data):", args[4].readByteArray(args[5].readUInt()));
this.pbBufferLen = args[5]; // Store pointer to length
},
onLeave: function (retval) {
console.log("CryptEncrypt returned:", retval);
console.log("pbBuffer (output data):", this.pbBufferLen.readByteArray(this.pbBufferLen.readUInt()));
}
});
}
# Example: Python script to connect to a Named Pipe (Windows)
import win32file
import win32pipe
pipe_name = r"\\.\pipe\MyApplicationPipe"
try:
handle = win32file.CreateFile(
pipe_name,
win32file.GENERIC_READ | win32file.GENERIC_WRITE,
0, None,
win32file.OPEN_EXISTING,
0, None
)
# Write/Read data to/from the pipe
win32pipe.SetNamedPipeHandleState(handle, win32pipe.PIPE_READMODE_MESSAGE, None, None)
win32file.WriteFile(handle, b"ATTACK_COMMAND")
resp = win32file.ReadFile(handle, 4096)
print(f"Received: {resp[1].decode()}")
win32file.CloseHandle(handle)
except Exception as e:
print(f"Error: {e}")
# Example: Fuzzing command line arguments with a long string
./thickclient.exe --config "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA..."
# Example: Fuzzing a configuration file with malformed XML/JSON
# Create a malformed config.xml and launch the app with it
# On Windows, check service permissions (replace "YourService" with actual service name)
sc qc "YourService"
icacls "C:\Program Files\YourApp\YourService.exe"
# Check for unquoted service paths using wmic
wmic service get name,displayname,pathname,startmode | findstr /i "auto" | findstr /i /v "c:\windows\\" | findstr /i /v """
# Example Wireshark filter for unencrypted HTTP basic auth
http.authbasic
# Check for custom protocols in Burp/Fiddler and use their extensibility to parse
# For binary protocols, manual analysis or custom scripts might be needed.
# Example: Testing for SQL Injection in an API parameter
POST /api/v1/search HTTP/1.1
Host: backend.thickclientapp.com
Content-Type: application/json
{"query": "product' OR 1=1--"}
# Example: Testing for Command Injection in a backend endpoint
POST /api/v1/execute_command HTTP/1.1
Host: backend.thickclientapp.com
Content-Type: application/json
{"command": "ping 127.0.0.1 & whoami"}
# Example: Testing for Insecure Direct Object Reference (IDOR) on a backend API
# This would be part of the traffic intercepted from the thick client
GET /api/v1/user_data?id=123 HTTP/1.1 --> Change '123' to another user's ID
Host: backend.thickclientapp.com
Authorization: Bearer <valid_token_for_user_123>
...
# Example: Testing for XXE via XML payload
POST /api/v1/xml_parser HTTP/1.1
Host: backend.thickclientapp.com
Content-Type: application/xml
<?xml version="1.0"?>
<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
<data>&xxe;</data>
This checklist provides a robust framework for conducting thick client application penetration tests. Remember that thick client applications often present unique challenges due to their interaction with the local operating system and diverse underlying technologies. Adapt your testing based on the specific application's architecture and always stay updated with the latest security research and tools.