Call to Interrupt Procedure

Created the Friday 10 March 2023. Updated 6 months, 2 weeks ago.

This anti-debugging technique involves using the INT n instruction to generate a call to the interrupt or exception handler specified with the destination operand.

To implement this technique, the int 0x03 instruction is executed, followed by a ret (0xCD03, 0xC3) nested in a __try, __except block. If a debugger is present, the except block will not be executed, and the function will return TRUE, indicating that a debugger is running.

This technique can be used to prevent analysts from analyzing and manipulating the malware's code during runtime.

Technique Identifier

U0124

Technique Tags

Anti-debugging X86 instruction RET

Code Snippets

Description

This code is implementing an anti-debugging technique that checks if a debugger is present in the environment. The TestDebugger() function generates a software interrupt (INT 3) using inline assembly code in x86 architecture.

If a debugger is present, it would intercept this interrupt, and the __except block will not execute, resulting in the TestDebugger() function returning false. On the other hand, if no debugger is present, the __except block will be executed, and the function will return true.

The main() function calls the TestDebugger() function and prints a message indicating whether a debugger was found or not. This code could be used as a security measure to protect against reverse engineering or malicious attacks by detecting if a debugger is present during runtime.

#include <stdio.h>
#include <windows.h>

bool TestDebugger() 
{
    __try
    {
        __asm  //x86 implementation
        {
            _emit 0xCD 
            _emit 0x03 //INT 03
            _emit 0xC3 //RET
        }
    }
    __except (EXCEPTION_EXECUTE_HANDLER)
    {
        return false;
    }

    return true;
}

int main()
{
    if(TestDebugger())
    {
        printf("Found debugger!\n");
    }
}

Additional Resources

External Links

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INT n/INTO/INT 3--Call to Interrupt Procedure