OutputDebugString

This Windows API is often used by developers for debugging purpose. It will display a text to the attached debugger. This API is also used by Malware to open a communication channel between one or multiple processes.

It is possible to use OutputDebugString in addition of GetLastError / SetLastError to detect debugger presence.

Technique Identifiers

U0117 B0001.016

Technique Tag

outputdebugstring

Evasion Categories

Anti-Debugging

Code Snippets

program OutputDebugString;

{$APPTYPE CONSOLE}

uses
  WinAPI.Windows,
  System.SysUtils;

var AErrorValue : Byte;

begin
  try
    randomize;

    AErrorValue := Random(High(Byte));

    SetLastError(AErrorValue);

    OutputDebugStringW('TEST');

    if (GetLastError() = AErrorValue) then
      WriteLn('Debugger detected using OutputDebugString() technique.')
    else
      WriteLn('No debugger detected using OutputDebugString() technique.');
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

About Author Open Snippet

Author: Jean-Pierre LESUEUR (DarkCoderSc) / Target Platform: Windows

#include <stdio.h>
#include <Windows.h>

int main()
{
	SetLastError(0);
	
        // Send string to the debugger
	OutputDebugStringA("Hello friend");

	if (GetLastError() != 0)
	{
		printf("Debugger detected!!\n");
	}
        system("pause");
	return 0;
}

About Author Open Snippet

Author: Thomas Roccia (fr0gger) / Target Platform: Windows

Detection Rules

import "pe"

rule Detect_OutputDebugStringA_iat: AntiDebug
{
	meta:
		Author = "http://twitter.com/j0sm1"
		Description = "Detect in IAT OutputDebugstringA"
		Date = "20/04/2015"

	condition:
		pe.imports("kernel32.dll","OutputDebugStringA")
}

rule:
  meta:
    name: check for OutputDebugString error
    namespace: anti-analysis/anti-debugging/debugger-detection
    author: michael.hunhoff@fireeye.com
    scope: function
    mbc:
      - Anti-Behavioral Analysis::Debugger Detection::OutputDebugString [B0001.016]
    examples:
      - Practical Malware Analysis Lab 16-02.exe_:0x401020
  features:
    - and:
      - api: kernel32.SetLastError
      - api: kernel32.GetLastError
      - api: kernel32.OutputDebugString

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.

OutputDebugStringA function (debugapi.h) - Win32 apps | Microsoft Docs

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Created

March 18, 2019

Last Revised

March 24, 2026