Interrupts

Created the Monday 18 March 2019. Updated 5 months, 4 weeks ago.

Adversaries may use exception-based anti-debugging techniques to detect whether their code is being executed in a debugger. These techniques rely on the fact that most debuggers will trap exceptions and not immediately pass them to the process being debugged for handling.

By triggering an exception and checking whether it is handled properly, the adversary's code can determine whether it is being executed in a debugger and take appropriate action, such as exiting or altering its behavior. This can be achieved using interrupt instructions such as INT 3 or UD2 to trigger the exception. This technique can be used to evade detection and make reverse engineering more difficult.

Technique Identifier

U0106

Code Snippets

Description

This code defines a function named IsDebuggerPresent that uses a try-except block to trigger an interrupt instruction and generate an exception. If the exception is handled, it means the code is being executed in a debugger and the function returns true. If the exception is not handled, it means the code is not being executed in a debugger and the function returns false. The main function calls IsDebuggerPresent and takes appropriate action based on the result. This is just an example and more advanced implementations may be needed for more complex scenarios.

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

// Function to check if the code is being executed in a debugger
bool IsDebuggerPresent()
{
    __try
    {
        // Trigger an interrupt instruction to generate an exception
        __debugbreak();
    }
    __except (EXCEPTION_EXECUTE_HANDLER)
    {
        // If the exception is handled, it means the code is being executed in a debugger
        return true;
    }

    // If the exception is not handled, it means the code is not being executed in a debugger
    return false;
}

int main()
{
    if (IsDebuggerPresent())
    {
        std::cout << "Debugger detected" << std::endl;
        // Take appropriate action, such as exiting or altering behavior
    }
    else
    {
        std::cout << "No debugger detected" << std::endl;
        // Continue with normal execution
    }

    return 0;
}

Detection Rules

                                    rule AntiDebugging_Interrupt {
  condition:
    // Check for presence of __try and __except blocks
    uint32(0) == 0x00646120 and uint32(4) == 0x00646120 and
    // Check for presence of __debugbreak or interrupt instructions such as INT 3 or UD2
    (uint8(8) == 0xCC or uint8(8) == 0xF1 or uint8(8) == 0xCC)
}

Contributors

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.

http://www.autosectools.com/anti-debugging-with-exceptions.pdf