Almost every malware exists out there has a functionally to make the post-detection analysis more difficult. Threat actors use various anti-debugging techniques, one of the more common ones used to check whether a debugger is on via IsDebuggerPresent() Windows API call. In this blog post, we will discuss how to bypass this technique by Frida.

why Frida?

Frida is a dynamic instrumentation toolkit. It gives a simple interface where you can develop complex hooking logic rapidly and make changes to it as your requirements. Frida supports Windows, macOS, GNU/Linux, iOS, Android, and QNX.

Prepare a sample PE

To allow us to dynamically test our function hooks I wrote a small Windows test application harness in C++. You can see the main functionality below, it’s short and easy to understand.

// target.cpp

#include <Windows.h>
#include <iostream>

int main()
        bool check = IsDebuggerPresent();
        if(check == 0)
            std::cout << "cool!" << std::endl;
            std::cout << "common .." << std::endl;

        return 0;

Lets try to look at IsDebuggerPresent() C++ function prototype.

BOOL WINAPI IsDebuggerPresent(void);

According to Microsoft docs, if the current process is not running in the context of a debugger, the return value will be zero.

Bypassing IsDebuggerPresent()

First of all, you need to install Frida, start from here: documentation. You can test your installation by running frida-ps.exe in Powershell or CMD.

How The Frida works? Frida will inject Google’s V8 engine to a specific process. After that it will run JavaScript code and generate a dynamic hook. Lets make our hand dirty and write some JavaScript codes. :)

For hooking purpose we will use Interceptor API.

Interceptor.attach(target, callbacks): intercept calls to target function. it’s a NativePointer specifying the address of the function you would like to intercept calls to.

To begin, we need to find the address of IsDebuggerPresent() function by using DebugSymbol API.

// poc.js

isDebuggerPresentAddr = DebugSymbol.getFunctionByName("IsDebuggerPresent")
console.log("function address : " + isDebuggerPresentAddr)

Using the Interceptor, we can quickly hook the application and write some basic JS to make IsDebuggerPresnt to always return 0. Notice the use of isDebuggerPresentAddr in the code.

// poc.js

Interceptor.attach(isDebuggerPresentAddr, {
        onEnter: function (args) {
			console.log("IsDebuggerPresent() get called ...");
        onLeave: function (retval) {

retval is a NativePointer-derived object containing the raw return value. We can use replace() to change the return value.

Now it’s time to test the bypass method. First we spawn process without injecting our code:

frida.exe .\target.exe

Frida will stop on Entrypoint.Now try to attach your debugger to the target.exe process and then type %resume command to continue execution. debugger will be detected by the process. debugger got detect Run the following command to inject our code and bypass the check:

frida.exe .\target.exe -l .\poc.js

You can see the result below. debugger not detect

The IsDebuggerPresent() has been bypassed successfully. :)


Read more