DISASSEMBLING VISUAL BASIC APPLICATIONS LAST UPDATED : 26 JULY 2006 INDEX I. DISCLAIMER AND NOTICE

II. READING THIS TUTORIAL III. INTRODUCTION

IV. ASSUMPTIONS V. REQUIRED TOOLS

VI. STRUCTURE OF A VB PROGRAM VII. OUR FIRST PROGRAM

VIII. STRING COMPARISON IX. CONCLUSION X. IN THE NEXT UPDATE XI. REFERENCES DISCLAIMER AND NOTICE The information provided in this tutorial must not be used for Reverse Engineering any application.

THE TEXT HAS BEEN WRITTEN IN SUCH A WAY THAT THE READER CAN LEARN, AND NOT JUST GAIN INFORMATION WITHOUT KNOWING HOW STUFF WORKS.

If the Reader still chooses to break Protection Mechamisms after reading this tutorial, he/she shall alone be responsible for the damages cause and not the

Author. If you wish to post certain Sections of the Tutorial on a Website, you are free to do so provided you inform the author and publish the Selected Text from the Tutorial as it is without modification. The Author has not copied text or any other information directly from a Source. However, some information from some sources has been used to write this tutorial. These Sources have been mentioned in the References Section. You are permitted to continue reading the tutorial only if you agree to the text

given above. II. READING THIS TUTORIAL [RTUT]

Each Section in this Tutorial has a specific Topic Code enclosed in square brackets. This arrangement has been made so that you can jump to a specific

topic simply by searching for the topic code from your Browser. At many places in the tutorial, I've explained a few things which are almost

unnecessary to know when dealing with Visual BASIC programs but I've written them for those who are interested in Hacking (And I don't mean that 'hack-an-email-address' sort of a kid. The original meaning of Hacking has

been ruined by pathetic people like them. Hacking in literal terms stands for 'curiousity'.

The original meaning of a Hacker is: "A person who enjoys exploring the details of programmable systems, as opposed

to most users, who prefer to learn only the minimum necessary." The extra details I've given in this tutorial are for those who want to be such ethical hackers. The Topic Code [XTRA] and [/XTRA] has been given for marking "extra-information" sections and you are free to skip such sections. Text within the [XTRA]..[/XTRA] blocks is given for extra information. You can search for the Extra Information using the Topic Code. III. INTRODUCTION [ITRO] As long as you know Assembly Language, it is easy to read disassembled listings of executable files written in C/C++ or PASCAL, especially if you are using

IDA Pro as your disassembler. This is so because C and C++ Compilers generate (or at least try to) efficient code. Some Compilers like Borland C++ use simple

instructions for complex operations(also remember that this is not always the case) which make it easier to study them. Implementation of Code Constructs such

as loops, IF statements, Ternary IF statements, switch constructs etc. can be found very easily as each one is unique and distinct. However the same is not true for Applications written in Visual BASIC. VB Programs are said to be very slow and hence deliver poor performance. There is a reason for this. Visual BASIC programs unlike those written in other languages don't use Windows API Directly. Local functions present in VB Runtime Files are

called which call functions from the Windows API.Most of the Visual BASIC

functions are present in MSVBM60.DLL (if you've got Runtime Files ver. 6.0). So to study VB programs, we must disassemble and analyze the MSVBM60.DLL file as well. Since VB programs use such a complex API Function call procedure, programs tend to run slower.(There are other reasons as to why VB programs run slower but I won't be covering it as it's off-topic.) It becomes difficult to analyze VB Programs as it uses functions which are not part of the Windows API and hence we are not acquainted with them.

My primary aim in this Tutorial is to teach the reader how to understand disassembled listings of programs written in Visual BASIC.

My secondary aim is to help you realise why Visual BASIC is not suitable for writing small,fast and efficient programs.

Almost all authors of Visual BASIC books mention that Visual BASIC does not give you applications with good performance.

This tutorial tells you why. The Tutorial will talk about executable files compiled in Visual BASIC in Native Code ONLY and not p-code. After reading this tutorial, you should be able to disassemble,debug and understand Visual Basic Applications. You may also be able to reverse engineer Protection Mechanisms written in Visual BASIC and that's where the next section comes in. IV. ASSUMPTIONS [ASPT]

You are required to have a basic understanding of Visual BASIC,C,the Windows API and 80x86 Microprocessor Assembly Language.

It would be advisable to have a copy of Intel's 80x86 Instruction Set Manual. Intel provides this manual free of charge. If you need this manual, contact me.

This Instruction Set Reference is Volume 2 of Intel Architecture Software Developer's Manual.

The Software Developer's Manual consists of 3 volumes: : Basic Architecture - Order Number 243190 : Instruction Set Reference - Order Number 243191 : System Programming Guide - Order Number 243192 You can provide these Order Numbers to get a copy of these manuals. For this tutorial only Volume 2 is required. V. REQUIRED TOOLS [RQRT] You will need the following tools to proceed with the Tutorial.

* COMPILER : Visual Basic 6.0 * DISASSEMBLER : IDA Pro 4.x or higher

* DEBUGGER : OllyDebug Ver. 1.09d or higher * WINDOWS API DOCUMENTATION

* NuMeGa SmartCheck 6.x * VBDE version 0.85 by iorior

* VBReformer VBDE is not required but it's always better to have it as it gives addresses of entry-points of most VB procedures.

VBReformer is used to see the Property values of all objects in a Visual Basic Form. It even allows you to change the value of Object Properties such as Forms,

Command Buttons etc. Import Libraries can also been seen. This Application is not required for this Tutorial but it's better to have it.

NuMeGa SmartCheck is again not required but it is useful when we have no idea what a particular procedure of VB does. You can run a program from it like a Debugger and view its log files and find out which procedure is called and what operations are carried out etc. You can have API Documentation from MSDN or you can use the API Text Viewer Tool supplied with Visual Studio or browse MSDN Online (msdn.microsoft.com). Certain Applications like APIViewer will also do. I have given the names of the Tools that I have used. But you are free to use any disassembler and debugger as long as you are comfortable using it but I advice you to use the tools that I have used above. SoftIce is better than

OllyDebug but the latter is good enough for VB Programs so it doesn't matter which one you use.

Once you have the necessary knowledge and tools, you can proceed further. Let's begin.

VI. STRUCTURE OF A VB PROGRAM [SVBP] When you open a VB Program from IDA, you'll end up with the following code. start: push offset dword_4012B4 call ThunRTMain ; ---------------------------------------------------------------------------

dd 0, 300000h, 400000h, 0, 0E9960000h, 82E6FCDFh, 939C4C23h

dd 0EB969B2Fh, 73D5h, 0, 1, 34303230h, 72503033h, 63656A6Fh ; etc etc etc This doesn't make any sense does it? If you keep scrolling further you will see sections of code and data. Each Section has a meaning in VB Programs and you can see a general idea of a Visual BASIC program's Section Map below. 00401000: ... IAT (First Thunk ok apis) Next Section(NS):

... some data NS:

... transfer area (Jumps to imported functions) NS:

... lots of data NS:

... local transfer area (for internal event handlers) NS: ... other data NS: ... code NS: ... lots of data NS:

... .data Section Let us now start analysis from the entry point of the program.

push offset RT_Struct call ThunRTMain

It's C equivalent would have been: ThunRTMain(&RT_Struct);

A function ThunRTMain is called which accepts one parameter. We'll soon find out that the parameter is a structure. Simply putting a step over command on the CALL statement results in the execution of the Application. Wierd Isn't it? For Pascal,C and C++ Programs there is always a start() function that takes all CommandLine Parameters,Gets ProcessThreads,Module Handles etc. We didn't see anything of the sort in a Visual BASIC Program.

But actually, VB does have a start function. The start function code is placed in the ThunRTMain Function. Let's verify that by disassembling the MSVBM60.DLL

and viewing the ThunRTMain Function. I've mentioned only a part of the ThunRTMain Function Code.

ThunRTMain proc near arg_0 = dword ptr 8

mov esi, [ebp+arg_0] mov dword_7352F7DC, esi and [ebp+var_4], 0

lea eax, [ebp+StartupInfo] push eax ; lpStartupInfo

lea eax, [ebp+StartupInfo] push eax ; lpStartupInfo

call ds:GetStartupInfoA movzx eax, [ebp+StartupInfo.wShowWindow] mov dword_7352F7D8, eax push hModule push esi mov esi, offset dword_7352F470 mov ecx, esi call sub_7342DECD mov [ebp+var_1C], eax test eax, eax

jl short loc_7342DEC5 push 0 ; lParam

push 0 ; wParam push 1069h ; Msg

call ds:GetCurrentThreadId push eax ; idThread call ds:PostThreadMessageA ; Other Code or [ebp+var_4], 0FFFFFFFFh push 0 ; uExitCode

call ds:ExitProcess

jmp loc_734619B3 loc_7342DEC5: ; CODE XREF: ThunRTMain+60Rj push eax call sub_Free_Memory jmp short loc_7342DEB4 endp As you can see, it does call all the Functions that the start() function does in C and PASCAL programs. But what about CommandLine() Function from KERNEL32.DLL?

MSVBM60.DLL does call that function as well but that function call is placed in deeply nested function calls. You can open the Imports Window to see the

Imported Function and see the cross-reference to a procedure in MSVBM60.DLL The sub_Free_Memory procedure calls various API Functions but if you keep

reading the procedure, you'll soon come across the HeapFree() Function which is imported from kernel32.dll.

Now I guess you now know the purpose of the ThunRTMain Function. Let us now see what structure is passed to it. If we double-click on the RT_Struct offset, we reach an address containing certain values. It is a huge structure and each part needs to be seen one at a time. Explaining the Structure will take up a lot of time and since I want to focus on the Code Constructs of Visual BASIC, I won't explain the Structure Passed to ThunRTMain.

All I can tell you is that the structure contains the PE (Portable Executable) Header Details. It is this header that is read by Resource Editors.

I found a good source for understanding the structure that is passed to ThunRTMain and I suggest you read it if you are interested in knowing PE Header

Details. The link to the Article is given in the References [REFR] Section. The article is titled "VISUAL BASIC REVERSED - A decompiling approach" and is

written by Andrea Geddon. If the link is dead by the time you are reading this, you can contact me on my email address to get the article. VII. OUR FIRST PROGRAM [OFPR] Create a Form with a CommandButton. Click the CommandButton and add a simple Msgbox Code as shown below: Private Sub Command1_Click() Msgbox "Ssup"

End Sub Open the Compiled EXE File with IDA Pro.

Click the Strings Tab to find the "Ssup" String. Double-Click the String to find its cross-reference.

Scroll up to the top of the procedure. You should see something like this:

[Explanation is partly given by comments after an instruction.] Command1_Click proc near var_64 = dword ptr -64h

var_5C = dword ptr -5Ch var_54 = dword ptr -54h

var_4C = dword ptr -4Ch var_44 = dword ptr -44h

var_3C = dword ptr -3Ch var_34 = dword ptr -34h var_2C = dword ptr -2Ch var_24 = dword ptr -24h var_14 = dword ptr -14h var_C = dword ptr -0Ch var_8 = dword ptr -8 ; Destructor Object var_4 = dword ptr -4 form_object = dword ptr 8 push ebp ; These two instructions

mov ebp, esp ; open the Stack Frame. sub esp, 0Ch ; Allocates 12 bytes on stack

push (offset exception_handler+1); Starts Exception Handler mov eax, large fs:0

push eax mov large fs:0, esp sub esp, 88h ; Allocates 136 bytes on stack push ebx push esi ; Saves Values of Registers push edi

; Loads the Destructor

mov [ebp+var_C], esp mov [ebp+var_8], offset destructor ; Allocating Dynamic Resources mov eax, [ebp+form_object] mov ecx, eax and ecx, 1 mov [ebp+var_4], ecx and al, 0FEh

push eax mov [ebp+form_object], eax

mov edx, [eax] call dword ptr [edx+4] ; Calls MSVBM60.Zombie_AddRef

mov ecx, 80020004h xor esi, esi

mov [ebp+var_4C], ecx mov eax, 0Ah mov [ebp+var_3C], ecx mov [ebp+var_2C], ecx mov [ebp+var_34], esi mov [ebp+var_44], esi mov [ebp+var_54], esi mov [ebp+var_64], esi

lea edx, [ebp+var_64] lea ecx, [ebp+var_24]

mov [ebp+var_24], esi mov [ebp+var_54], eax

mov [ebp+var_44], eax mov [ebp+var_34], eax

mov [ebp+var_5C], offset aSsup ; "Ssup" mov [ebp+var_64], 8 call ds:__vbaVarDup lea eax, [ebp+var_54] lea ecx, [ebp+var_44] push eax lea edx, [ebp+var_34] push ecx

push edx lea eax, [ebp+var_24]

push esi push eax

call ds:rtcMsgBox ; Calls the MsgBox Function lea ecx, [ebp+var_54]

lea edx, [ebp+var_44] push ecx lea eax, [ebp+var_34]

push edx lea ecx, [ebp+var_24]

push eax push ecx

push 4 call ds:__vbaFreeVarList add esp, 14h mov [ebp+var_4], esi push offset continue_after_jump jmp short fake_a_call_instr lea edx, [ebp+var_54] lea eax, [ebp+var_44] push edx lea ecx, [ebp+var_34]

push eax lea edx, [ebp+var_24]

push ecx push edx

push 4 call ds:__vbaFreeVarList add esp, 14h retn ; --------------------------------------------------------------------------- fake_a_call_instr:

retn

; --------------------------------------------------------------------------- continue_after_jump: mov eax, [ebp+arg_0] push eax mov ecx, [eax] call dword ptr [ecx+8] ; Calls MSVBM60.Zombie_Release mov eax, [ebp+var_4] mov ecx, [ebp+var_14]

pop edi pop esi

mov large fs:0, ecx pop ebx

mov esp, ebp pop ebp ; Closes Stack Frame

retn 4 Command1_Click endp Simply by looking at the entire procedure you can't exactly figure out what the hell happens when the whole subroutine is executed. If you know Assembly well and have had the patience to read through the code, you should notice a few neat things in the code. [XTRA] Before I begin explaining the procedure, I want to teach you how to recognise a

procedure in Visual BASIC. They can be called Procedure Signatures. 1) A Procedure has the open and close Stack Frame instructions.

2) The First Procedure in a VB Program is always preceded by 12 0xCC Bytes (which corresponds to the INT 3 Instruction) followed by

4 'T' bytes (0xE9) followed by 12 0xCC bytes. 3) Procedures other than the first are preceded by 10 NOP(0x90) Instructions.

: 1) STACK FRAME: The Open/Close Stack Frame Instructions are even found in C/C++ and Pascal programs and hence can be termed as a universal method of determining procedures. However that is not always the case. --> Many compilers just JMP instructions to fake a Call Instruction. This Jump is at times a CALL to a procedure. IDA Pro does not detect such CALL 'emulating' instructions but OllyDebug does recognise such code patterns.

--> Visual C++ allows the programmer to write naked functions. Naked functions mean that the compiler does not allocate space for its arguments nor does it

include the stack open and close frame instructions. But since we are dealing with Visual BASIC, we can ignore the second case. You

will see an example of the first case shortly. : 2) THE 0xCC BYTE

The 0xCC Byte is used to Generate the INT 3 Exception, which is known as the "CALL TO INTERRUPT" Procedure. It is used by Debuggers such as OllyDebug and SoftIce to set software Breakpoints. Debuggers insert the 0xCC byte before the

instruction which it wants to set a breakpoint on. As soon as the INT 3 Instruction is executed, Control is passed onto the Debuggers Exception Handler.

Here is the description taken directly from Intel's Software Developers Manual Volume 2 : Instruction Set Reference.

"The INT 3 instruction generates a special one byte opcode (CC) that is intended for calling the debug exception handler. (This one byte form is valuable because it can be used to replace the first byte of any instruction with a breakpoint, including other one byte instructions, without over-writing other code). To further support its function as a debug breakpoint, the interrupt generated with the CC opcode also differs from the regular software interrupts as follows: � Interrupt redirection does not happen when in VME mode; the interrupt is handled by a protected-mode handler. � The virtual-8086 mode IOPL checks do not occur. The interrupt is taken without faulting at any IOPL level."

That's how debuggers work. That's also the concept of certain anti-debugging techniques. Since the 0xCC code is injected by Debuggers before an instruction,

the CRC (Cyclic Redundancy Check) Value of the code also changes. Some Antidebugging techniques encrypt the program with a key which is the CRC value

of the program. When a program is being debugged, its CRC value changes and with the result the program doesn't get decrypted. Such methods are effective in stopping amateur wannabe hackers from understanding their code but its not foolproof and an expert hacker can get past this technique with ease. So much for what '0xCC' is. But why is it placed before the First Procedure in

VB Programs?

I've found no answer to that so far. This wastes a lot of space in a program. If you try to disassemble a Console Program written in Visual C++, you'll find many instructions which set parts of the stack to the '0xCC' value. You will also find 0xCC bytes scattered across the disassembled listing. If only Visual Studio was Open Source, we could have seen the code generation code and come up with an answer and improve the code generation code too. I hope you also realise why Open Source is slowly gaining momentum. 3) The 0x90 Byte

Here is the description taken directly from Intel's Software Developers Manual Volume 2 : Instruction Set Reference.

"Performs no operation. This instruction is a one-byte instruction that takes up space in the instruction stream but does not affect the machine context,

except the EIP register. The NOP instruction is an alias mnemonic for the XCHG EAX, EAX instruction."

This byte is injected into serial generation/checking procedures by amateur hackers where the protection mechanism is weak. This is known as bit-hacking. Sadly enough, bit-hacking STILL works for defeating plenty of today's Commercial Applications. Guess they never realised the importance for code-security. While writing programs in Assembly Language, if you use Forward Referencing in a few situations or use a wrong Jump Instruction to jump to certain addresses, chances are quite bright that the Assembler will fill in some bytes with the NOP instruction.

As a result, having the presence of the 0x90 Instruction in your code is considered bad programming.

But again, I see no reason why the 0x90 Byte is present in Visual BASIC. Removing such entries will reduce the executable size drastically.

Programs like VBDE rely on such Procedure Signatures to identify where a procedure is present.

[/XTRA] Let us start by analyzing the procedure in portions. First the Procedure opens the Stack Frame. Then it allocates 12 Bytes on the stack for the Destructor and other variables. ( We shall see the Destructor in detail after a short while. ) Then it allocates Dynamic Resources and calls the Zombie_AddRef Function. What does the Zombie_AddRef Function do? It Takes the Object Reference. In this function the parent object (in this case Form) is passed as a parameter

and uses AddRef to increment reference count of the object (instantiation). Since COM objects are responsible for their lifetime, the resources they use are

allocated until the reference count is 0, when it reaches 0 the objects enter zombie state & can be deallocated to free resources.

Refer COM object management documentation for more detailed information. Right after the call of the Zombie_AddRef Function there are MOV instructions

which assigns values to many variables. That follows a reference to the "Ssup" string followed by a call to the rtcMsgbox procedure. Why does it seem so wierd? Shouldn't it simply call the rtcMsgbox Function?

Let us find out why in a little more interesting manner. Intuition tells us that no matter what the function does, it will end up calling

the MessageBoxA or the MessageBoxW Function. So let's set a breakpoint on the MessageBoxA and MessageBoxW Functions.

To do that, start OllyDebug and load the Executable file by pressing F3. After the program is loaded, press Alt+E to open the Executable Modules Window. Double click USER32.DLL to open the disassembled listing of the User32.dll file. From there press Ctrl+N to open the Imports/Exports Window. Then Scroll over till you see the MessageBoxA and MessageBoxW Functions. Click them one at a time and press F2 to set a breakpoint. Now press F9 to run the program. The Application should open. Click the CommandButton. Now instead of the Debugger halting at a breakpoint of MessageBox, the MessageBox comes up without any halt to the Debugger. Why does this happen? Does this mean that rtcMsgBox has a seperate copy of the

MessageBox code within itself? Though it seems like a possible reason, it is unlikely to happen as Microsoft Developers built the Windows API so that they

could be reused for performance. So that means that some API Function is called which displays the MessageBox.

So let us try another experiment. In the same Imports/Exports Section of User32.dll we see 2 more MessageBox functions which are MessageBoxIndirectA and MessageBoxIndirectW. Let's try setting a breakpoint on both these Messages. After the breakpoint is set, press F9, and click the Command Button. This time, the Debugger halts at the MessageBoxIndirectA function. Interesting isn't it? All Visual BASIC Applications which use the Msgbox()

Function are actually calls to MessageBoxIndirectA and not MessageBox as thought.

This is an important characteristic. So the Next time you set a breakpoint on the MessageBox function and the debugger halts at a breakpoint, you can be pretty sure that someone has used the MessageBox() API Directly by consulting the API Text Viewer for the VB Declaration. Let us now see the prototype of the MessageBoxIndirect() API Function. Private Declare Function MessageBoxIndirect Lib "user32" Alias "MessageBoxIndirectA" (lpMsgBoxParams As MSGBOXPARAMS) As Long Only One Parameter? So then how is the Message Body and Title passed to the

Function? For that we'll need to see the declaration of the MSGBOXPARAMS Structure.

Private Type MSGBOXPARAMS cbSize As Long

hwndOwner As Long hInstance As Long

lpszText As String lpszCaption As String dwStyle As Long lpszIcon As String dwContextHelpId As Long lpfnMsgBoxCallback As Long dwLanguageId As Long End Type

This suggests that the required parameters are assigned to variables and the reference to that object is passed to that function.

So That suggests that the many MOV instructions found before the rtcMsgbox call are used to initialise the MSGBOXPARAMS Structure.

To confirm our doubt, let's compare the MOV instructions with the code found before the MessageBoxIndirect function is called.

mov edx, [eax] mov [ebp+hWnd.lpszText], ecx mov ecx, [eax+8] mov eax, [eax+0Ch] push esi push ebx test ah, 40h mov [ebp+hWnd.hInstance], edi

mov [ebp+hWnd.lpszIcon], edi mov [ebp+hWnd.lpfnMsgBoxCallback], edi

mov [ebp+hWnd.cbSize], 28h mov [ebp+hWnd.hwndOwner], edx

mov [ebp+hWnd.lpszCaption], ecx mov [ebp+hWnd.dwStyle], eax

mov [ebp+hWnd.dwLanguageId], edi jz short loc_734A6133 mov [ebp+hWnd.lpfnMsgBoxCallback], offset sub_734A6098

loc_734A6133: mov esi, ds:MessageBoxIndirectA

lea eax, [ebp+hWnd] push eax ; LPMSGBOXPARAMSA

call esi ; MessageBoxIndirectA Interesting to see that....Isn't it? Next comes the __vbaFreeVarList Function. From its name we can see that it deallocates the address of a certain number of variables. This function actually does no work except call the __vbaFreeVar Function multiple number of times. Let us see how both functions work. __vbaFreeVar : Frees a Temporary Variable. __vbaFreeVar accepts only 1 Argument, which is the address of the variable to be deleted. This argument is ALWAYS passed through ECX. Uses the API Function __imp_SysFreeString()[Ordinal Number 6] from OLEAUT32.DLL

that carries out the actual deallocation of a variable. __vbaFreeVarList : Frees Temporary Variables.

Have a look at this Snippet: lea ecx, [ebp+var_54] ; Variable 1 stored in ecx

lea edx, [ebp+var_44] ; Variable 2 => edx push ecx ; Variable 1 pushed lea eax, [ebp+var_34] ; Variable 3 => eax push edx ; Variable 2 pushed lea ecx, [ebp+var_24] ; Variable 4 => ecx push eax ; Variable 3 pushed

push ecx ; Variable 4 pushed

push 4 ; Free 4 Temp. Variables call ds:__vbaFreeVarList The code is pretty easy to understand. This function frees temporary variables that are passed as arguments to it.Interestingly each memory location is 16 bytes wide. This is an interesting function as it can accept variable arguments. It's equivalent function call in C would be: __vbaFreeVarList(4,&var_24,&var_34,&var_44,&var_54);

where its declaration would be: int __vbaFreeVarList(int NUMBER_OF_VARIABLES_TO_FREE,<addresses of the vars>...)

{ // CODE

} If we analyse the code of __vbaFreeVarList, we actually find multiple calls of

__vbaFreeVar. Have a look at the source code of __vbaFreeVarList. public __vbaFreeVarList __vbaFreeVarList proc near arg_0 = dword ptr 4 arg_4 = dword ptr 8 arg_8 = dword ptr 0Ch mov ecx, [esp+arg_4] ; Address of Variable to delete push esi ; Saves value of esi

lea esi, [esp+4+arg_8] ; esi = Address of Next Variable call __vbaFreeVar ; function call

mov eax, [esp+arg_4] ; eax gets value of num of ; variables to be freed

cmp eax, 1 ; If eax<=1 then jbe short freed_all_vars ; jump to end of function.

push edi ; Value of edi is saved on stack lea edi, [eax-1] ; edi=eax-1 ; THIS IS A WHILE LOOP : while(edi){ /*CODE*/ } loop_start: mov ecx, [esi] ; ecx=Address of Variable to Delete add esi, 4 ; esi = Address of Next Variable call __vbaFreeVar ; function call dec edi ; edi--;

jnz short loop_start ; Jump to beginning of the loop if ; edi is not ZERO.

; Well Written Code.No need for a ; CMP Instruction as DEC

; Instruction affects the ZERO Flag pop edi ; Value of edi is restored.

freed_all_vars: pop esi ; Value of esi is restored. retn ; Return to the calling function

ENGINE:7352009D __vbaFreeVarList endp As you can see, __vbaFreeVarList uses a while loop to free each variable one by

one using the __vbaFreeVar Function. Notice that the address of the variable to be freed is stored in ECX always.

You can disassemble the __vbaFreeVar Function to confirm that. Now let us see what happens when after the MessageBox is shown. This is the most interesting part. After the MessageBox is displayed, a clean up code is executed that deallocates all the variables used in the entire procedure. Have a look at these statements in the Command1_Click() Code. push offset continue_after_jump jmp short fake_a_call_instr ; --------------------------------------------------------------------------- fake_a_call_instr:

retn ; ---------------------------------------------------------------------------

continue_after_jump: ; code

This is the 'CALL-Simulation' instruction. If you recall, before a call function is executed, the processor pushes the location of the instructions which are supposed to receive control after execution of a function is over. VB instead of issuing a call instruction simulates it using the push, jmp and retn instructions. It is small sections of code like this that reduce Visual BASIC's efficiency and performance.

Let us still see why this is done.

The CALL-Simulation Instruction calls the MSVBM60.Zombie_Release function. This is the destructor code. Doesn't that remind you of something? The instruction mov [ebp+var_8], offset destructor contains the offset of the destructor code. But is this so? Double-click on the 'offset destructor' text and you'll land up here. destructor dd 80005h, offset loc_401A7E, 0, offset loc_401A85, 102825FFh Hmm...this contains more offsets? By simply double-clicking the offsets you land

up at the destructor code again. That's why the CALL simulation code is used so that the destructor code looks like its an inline function.

If you're more curious, you can also double-click the 'exception_handler' text to see where that leads to.

Well, after a long journey into the Command1_Click() Procedure, we're finally done analyzing it.

From this point onwards, I shall explain only the important section of code rather than explain such intricate details once again. Let us proceed further. This Time let us create a Visual BASIC Application using only a module. We shall use the Main Subroutine. Use this code: Sub Main() MsgBox "Ssup"

End Sub What you will realise that the Procedure code is an exact copy of the code we

dealt with earlier. This means that Form Procedures and Module Procedures are treated alike. This also means that the Command Button code procedure had no

chance of using any information of the Form Object. Let's take another example.

VIII. STRING COMPARISON [STR1] Create a form without any controls. The code in the form module is as follows: Private Sub Form_Load() If "Sanchit" <> InputBox("ssup") Then MsgBox "wrong" Else MsgBox "Right" End If

End Sub The resultant code is shown below (after stripping unimportant instructions)

Form_Load proc near ; Variables and Arguments shown

push ebp mov ebp, esp

sub esp, 0Ch push (offset vba_exception_handler+1) mov eax, large fs:0

push eax mov large fs:0, esp

sub esp, 0F0h push ebx

push esi push edi mov [ebp+var_C], esp mov [ebp+var_8], offset destructor mov eax, [ebp+arg_0] mov ecx, eax and ecx, 1 mov [ebp+var_4], ecx and al, 0FEh push eax

mov [ebp+arg_0], eax mov edx, [eax]

call dword ptr [edx+4] ; Zombie_AddRef() xor eax, eax

mov ebx, 80020004h mov edi, 0Ah ; code... lea edx, [ebp+var_98] lea ecx, [ebp+var_28] ; MOV [var],register Instructions

mov [ebp+var_90], offset aSsup ; "ssup"

mov [ebp+var_98], 8 call ds:__vbaVarDup lea eax, [ebp+var_88] push offset aSanchit ; "Sanchit" lea ecx, [ebp+var_78] push eax lea edx, [ebp+var_68] push ecx

lea eax, [ebp+var_58] push edx

lea ecx, [ebp+var_48] push eax

lea edx, [ebp+var_38] push ecx

lea eax, [ebp+var_28] push edx push eax call ds:rtcInputBox mov edx, eax lea ecx, [ebp+var_18] call ds:__vbaStrMove push eax

call ds:__vbaStrCmp mov esi, eax

lea ecx, [ebp+var_18] neg esi

sbb esi, esi neg esi

neg esi call ds:__vbaFreeStr lea ecx, [ebp+var_88] lea edx, [ebp+var_78] push ecx lea eax, [ebp+var_68] push edx lea ecx, [ebp+var_58]

push eax lea edx, [ebp+var_48]

push ecx push edx

lea eax, [ebp+var_38] lea ecx, [ebp+var_28]

push eax push ecx push 7

call ds:__vbaFreeVarList add esp, 20h

mov [ebp+var_50], ebx test si, si

mov [ebp+var_58], edi mov [ebp+var_40], ebx mov [ebp+var_48], edi mov [ebp+var_30], ebx mov [ebp+var_38], edi jz short jump_if_right lea edx, [ebp+var_98] lea ecx, [ebp+var_28] mov [ebp+var_90], offset aWrong ; "wrong" mov [ebp+var_98], 8

call ds:__vbaVarDup lea edx, [ebp+var_58]

lea eax, [ebp+var_48] push edx

lea ecx, [ebp+var_38] push eax push ecx lea edx, [ebp+var_28] push 0 push edx

call ds:rtcMsgBox

lea eax, [ebp+var_58] lea ecx, [ebp+var_48] push eax lea edx, [ebp+var_38] push ecx lea eax, [ebp+var_28] push edx push eax

jmp short free_up_resources ; ---------------------------------------------------------------------------

jump_if_right: lea edx, [ebp+var_98]

lea ecx, [ebp+var_28] mov [ebp+var_90], offset aRight ; "Right"

mov [ebp+var_98], 8 call ds:__vbaVarDup lea ecx, [ebp+var_58] lea edx, [ebp+var_48] push ecx lea eax, [ebp+var_38] push edx push eax

lea ecx, [ebp+var_28] push 0

push ecx call ds:rtcMsgBox

lea edx, [ebp+var_58] lea eax, [ebp+var_48]

push edx lea ecx, [ebp+var_38] push eax lea edx, [ebp+var_28] push ecx push edx free_up_resources: push 4

call ds:__vbaFreeVarList add esp, 14h

mov [ebp+var_4], 0 push offset call_emulate

jmp short jump_as_a_call ; ---------------------------------------------------------------------------

lea ecx, [ebp+var_18] call ds:__vbaFreeStr lea eax, [ebp+var_88]

lea ecx, [ebp+var_78] push eax

lea edx, [ebp+var_68] push ecx

lea eax, [ebp+var_58] push edx lea ecx, [ebp+var_48] push eax lea edx, [ebp+var_38] push ecx lea eax, [ebp+var_28] push edx push eax push 7

call ds:__vbaFreeVarList add esp, 20h

retn ; ---------------------------------------------------------------------------

jump_as_a_call: retn ; --------------------------------------------------------------------------- call_emulate: mov eax, [ebp+arg_0] push eax

mov ecx, [eax]

call dword ptr [ecx+8] ; Zombie_Release() mov eax, [ebp+var_4] mov ecx, [ebp+var_14] pop edi pop esi mov large fs:0, ecx pop ebx mov esp, ebp

pop ebp retn 4

Form_Load endp Let us first see the Prototype of the InputBox Function

Function InputBox(Prompt As String, Title As String, Default as String , _ xpos As Long , ypos As Long, helpfile As String ,context As Long ) _

As String If we disassemble the MSVBM60.DLL File and scroll over to the rtcInputBox function, we can confirm this from the prototype that IDA provides us. Most of the code is irrelevant to us but the important chunk of code of the function (which took me quite some time to find) is given below: push edi ; Context Pushed push ebx ; HelpFile Pushed push [ebp+arg_8] ; ypos Pushed

push [ebp+arg_C] ; xpos Pushed push eax ; Default pushed

push ecx ; Title Pushed push edx ; Prompt pushed

call sub_7349DC68 ; Function that causes Inputbox ; At this point EAX contains the entered string

push [ebp+arg_4] ; BSTR mov edi, eax call esi ; __imp_SysFreeString push [ebp+arg_0] ; BSTR call esi ; __imp_SysFreeString push [ebp+var_4] ; BSTR call esi ; __imp_SysFreeString push [ebp+var_1C] ; BSTR

call esi ; __imp_SysFreeString mov eax, edi ; Sets EAX to the Return Value

pop edi pop esi

pop ebx leave ; Close Stack Frame

retn 1Ch ; Returns from here In the first portion of the code, the push instructions push all the seven parameters to the function that creates the InputBox Dialogbox. I know that with

the current example that I'm disassembling, it's not quite possible to believe that all the push instructions stand for what I've mentioned. So what you can do

is disassemble the following code given below and set a breakpoint on the PUSH instructions in the MSVBM60.DLL File using OllyDebug or SoftIce.

str1 = InputBox("This is prompt","This is Title","Default-Val",10,20, _ "c:\help.hlp",1) With this you can actually verify the contents of the push instruction to confirm what I've written. Now, after the call instruction you can see a PUSH instruction pushing the contents of a variable. This is a parameter for the SysFreeString Function. Next is a MOV instruction transferring the contents of the EAX register into EDI. EAX at this point of time contains the address of the String that we filled in the Text Box. This is done to save the value of EAX. Then the SysFreeString Function is called. This function takes 1 argument which

is the string that needs to be deallocated. This function does not return any value after execution.

Did you notice something stupid? If the SysFreeString Function does not return any value, then why were the

contents of EAX saved in EDI? And why is there a need to have a MOV EAX , EDI instruction? What a waste of precious bytes. This is deploring code generated by Visual C++ 6.0 Compiler (which was used to write the MSVBM60.DLL File) and since Visual BASIC programs use this routine, it results in slow, sluggish programs.

And this is just one function...imagine what would happen if we analyzed all of

them? Anyway, the original contents of the registers are restored, the stack frame is closed (with LEAVE) and the function returns after adding 0x1C bytes to ESP. Now let's have a look at this code portion. call ds:rtcInputBox mov edx, eax lea ecx, [ebp+var_18] call ds:__vbaStrMove

push eax call ds:__vbaStrCmp

mov esi, eax lea ecx, [ebp+var_18]

neg esi sbb esi, esi

neg esi neg esi call ds:__vbaFreeStr After the call of rtcInputBox, EAX contains the entered string in the Text Box of the InputBox dialog. The address of this string is moved to EDX. Is this done to save the contents of the String for the compare function? As obvious as it may seem, it is really not like that. Let us see. The __vbaStrMove function moves a String from one place in memory to another

place. On Analysis of the function code it is found that this function accepts two arguments and returns one value as shown below:

PARAMETERS: EDX : Source String

ECX : Destination RETURNS : Source String. Sets EAX to EDI (which holds value of EDX)

Now we see that by setting the value of EAX to EDX, we are setting the entered string as a SOURCE string to be copied into another location. This is neat. Now the Source String is pushed again and the __vbaStrCmp Function is called. This on first glance looks wierd again. It seems that the StrCmp Function accepts only one argument. Then what does it compare it with? If you scroll above you will find a push instruction that pushes the address of the string "Sanchit" ( push offset aSanchit ; "Sanchit" ) Such cases remind us that unlike code generated by Pascal and C/C++ compilers,

Visual BASIC functions can have it's arguments pushed anywhere and not right before the function call.

Keep this thing in mind when you set out to disassemble your own Visual BASIC programs.

Now after the Comparing function returns its value via the EAX register, it is copied to ESI.

Then the LEA instruction is used to load the address of a variable in the ECX register. Now since we are aware of VB's tricks we know that this is a parameter to the __vbaFreeStr function. You should notice that the same variable which

held the value of the entered string is now being passed to this function to deallocate it as its not required after the comparison has been done.

Now let's talk about this code fragment: neg esi

sbb esi, esi neg esi neg esi call ds:__vbaFreeStr The NEG statement's actual use is to change the sign of a number for example, from 3 to -3. But this one has an indirect use. This Instruction affects many flags. But the one it is meant for is the Carry Flag (CF) which is used in the next SBB Instruction.If ESI is equal to 0 then CF is reset to 0 and otherwise is set to 1. Now the SBB Instruction stands for Subtraction with Borrow. In this case it can

be translated to this: ESI = ESI - (ESI + CF);

This is where the previous NEG instruction comes into picture. The next two NEG instructions are of no use. You can take this as another

example to show why VB code is slow. Following this is the __vbaFreeStr Function which deallocates space for a string variable. Now you might wonder that if comparison has been performed, then why isn't there any JUMP instruction? If you keep reading the code you will find the TEST and JUMP Instructions after

the seven variables used have been cleared. These instructions are found

together in C/C++ and Pascal programs but in Visual BASIC this isn't always the case. That's why Visual BASIC programs take up more time for analysis compared to C and Pascal programs. I'm not discussing the rest of the code as it has been covered in the previous section. Now let's discuss a Hack-tip. This type of String Comparision protection is used in many popular Shareware

Applications. Removing such protection isn't much of a job and I'll discuss a cleaner way to

hack this protection. Remember the two useless NEG instructions before the __vbaFreeStr Function call?

Since these instructions are of no practical use, we can replace these two bytes with XOR ESI, ESI which consumes only 2 bytes. That way the TEST instruction

would always result in ZERO making control jump always to the Msgbox("Right") code. A lot of Hackers use such 'useless' code to implement such hacking techniques. IX. CONCLUSION [END1] This isn't exactly the end of the tutorial. I'd rather say that it is the end for now. This topic is vast and I'm trying to include explanation and analysis of all Visual BASIC functions. If I plan to release this tutorial with all functions inclusive, it's going to take a lot of time.

So i've decided to post this tutorial first and keep updating it every 15 to 20 days. You can check the LAST UPDATED Section in the beginning of the Tutorial to

see how recently this tutorial has been updated. Keep checking for updated versions every month.

I hope you've enjoyed my tutorial as I've put in a lot of hard work and time on writing this.

Since I haven't come across any Books or Articles on this subject, I don't quite know what exactly is expected from my tutorial. I would appreciate it if you could email me suggestions and comments on this tutorial. I may not be able to reply to every email, but I do read each one of them. Thanks. X. IN THE NEXT UPDATE [NXTU] In the next update of this tutorial, I plan to give an explanation of which function is called when a certain Visual BASIC function is called. I then

analyze the function, it's parameters and return value, methods to improve the function and much more.

XI. REFERENCES [REFR] -> MSDN (msdn.microsoft.com)

-> "VISUAL BASIC REVERSED - A decompiling approach" by Andrea Geddon.

VISUAL BASIC REVERSED - A decompiling approach

Author: AndreaGeddon

Abstract

Keywords: Reverse Code Engineering, Visual Basic, VB, Decompiling

Frameworks are getting more and more popular today, Visual Basic is one of them. Personally i hate frameworks,and also most reversers do. So, why this tutorial? We can consider both the light and the dark side of the problem:frameworks usually put a lot of code in the compiled programs, so it becomes hard to find the way among all thatjungle. But they also use sets of pre-built objects, so theese objects are always the same and can be recognized,helping the reverser to understand the code itself. In a VB PE you have a lot of information inside the exe, so youcan easily extract all the information you need about all components of the program. To analyze a VB application Iused this program that was written by a friend of mine (thank you _d31m0s_!). It’s a sort of name/serial crackme,but we are not interested in serial fishing, we are interested in how it works and how the vb knows how to build theapp itself. I asked my friend to write it adding some event handling (colors, on over, etc) and a simple algorithmto check serial. He also wrote the proggy using more source files and making various subs (some null sub too).We also have the source of all, but we will check them later.

Let’s make some introduction now!

————————————

1 INTRODUCTION

Before VB5 the VB programs were not truly traduced in assembler, they were coded in Pseudo Code (hehe youall remember those hating pcode exe!), and the VB virtual machine had the task of interpreting the pseudo codeand execute it. Those programs were linked to vbrun100, vbrun200, vbrun300, vbrun400 dlls (depending on theversion); well thing are a little different because there were variations between 16bit or 32bit modules (pcodewere mostrly 16bit apps), but this is not what we are looking for. Today we have version 5 and 6 of VB, theyuse MSVBVM50.dll and MSVBVM60.dll, and now VB exes are really compiled and traduced in asm. As you allknow you can’t use usual breakpoints like ”GetWindowTextA” when debugging VB programs, infact you shoulduse the apis exported from the VBVM dll (e.g. for a serial i would use __vbaStrCmp, or rtcMsgBox), if you wantto use theese apis in softice you just have to load the VB dll (in winice.dat or via symbol loader). This will helpyou debug VB applications.

2 ANALISYS

Let’s start disassembling the proggy. I am using Ida, i advice you to use it too, but you can use other disassemblersif you want. First of all lets have a general look, so we can have a general idea. You can easily see the following:

00401000

... IAT (First Thunk ok apis)

004010F0

... some data

004011A5

... transfer area (declspec(dllimport) style)

0040130E

... lots of data

004023EC

... local transfer area (for internal event handlers)

0040242C

... other data

00402E44

... code

00403D06

... other data

Ok we have a general idea of the mapping of the program. Notice that all read only data is in the .text section,the data before and after the code contains names of imported functions (IT original first thunks), however nowwe begin analisys.

We start from entry point. What we see is:

00401310 push offset RT_MainStruct00401315 call ThunRTMain

if you debug this, you execute the call and the program runs. The analisys here is very simple: the entry pointsimply consists of:

ThunRTMain(&RT_MainStruct);

2

this function is the main function of the VBVM (the Thunder Runtime Engine) and has one parameter, thepointer to a complex structure that describes all the application. It is clear that all the data, all the callbacksand so on are described by this structure, so we can extract a lot of information. We just have to check this bigstructure! Let’s go!

00401970 RT_MainStruct db ’VB5!’ ;signature

00401974 db 1Ch ;00401975 aVb6it_dll db ’#VB6IT.DLL’,0 ;italian language00401980 dd 000401984 dd 2Ah00401988 dd 00040198C dd 000401990 dd 0A0000h00401994 dd 410h00401998 dd 409h0040199C dd 0004019A0 dd offset ProjectStruct004019A4 dd 30F016h004019A8 dd 0FFFFFF00h004019AC dd 8004019B0 dd 1004019B4 dd 2004019B8 dd 0E9h004019BC dd offset DialogsStruct004019C0 dd offset ExternStruct004019C4 dd offset ProjectInfo004019C8 dd 78h004019CC dd 7Eh004019D0 dd 84h004019D4 dd 85h004019D8 dd 0004019DC dd 0004019E0 dd 0004019E4 dd 0

the first field is the signature of the struct itself. Note that the program has been written with VB 6.0, butthe signature is ”VB5!”, probably they didnt change it for cross compatibility? Or just forgot it? Who knows!However we can see various infos, the ”VB6IT.dll” that should be the module to load the language (the apphas been written with italian version of vb). What should really abtract your attention are the four pointers(ProjectStruct, DialogsStruct, ExternStruct, ProjectInfo). I gave them a name because i know their function,you should see just four addresse in the dasm. However we will analyse theese structs to find infos we need.

3

0040131C ProjectInfo dd 0 ; DATA XREF: 004019C4o00401320 dd 30h00401324 dd 40h00401328 dd 00040132C dd 0EACF9A13h00401330 dd 4F93898Bh00401334 dd 0DF0C5493h00401338 dd 159AAEEDh0040133C dd 000401340 dd 10000h00401344 a020430progetto db 0,0,’020430Progetto1’,000401356 aC000 db ’-C000-’0040135C dd 0 ;

here we can see various numbers, in particular there is the project name as last field. If we go and search in thesource of the app, we can see in the .vbp the following line

Reference=*\G{00020430-0000-0000-C000-000000000046}#2.0#0#D:\WINNT\System32\STDOLE2.TLB#OLE Automation

so we can think all the data bytes represent the type of project, modules contained etc etc. This isn’t very useful,so we go on. We go back to main struct and see another sub structure:

004018F8 ExternStruct dd 6 ;flag

004018FC dd offset InsideImport ;import data

it is a bit harder to figure out what this structure is. This handles other imported functions, they can be insideor outside virtual machine module. The flag indicates the type of import (6 = inside, 7 = outside). If you lookat InsideImport you will find:

00402B64 InsideImport dd offset Descr00402B68 dd offset Thunk

Descr will point to four dwords that seem to be the same in all vb apps. Thunk will contain a pointer to an areawhere addresses (of some code of the virtual machine) are stored. I analysed an other vb app, its ExternStructis:

//snippet from another vb app

00401A5C ExternStruct dd 7 ;outside00401A60 dd offset ImportData00401A64 dd 6 ;inside00401A68 dd offset InsideImport//end snippet from another vb app

4

as you can see in that app there was an external function, infact the app used ShellExecute to open the internetbrowser and link to a site. The ImportData is as follows:

//snippet from another vb app

00402DA4 ImportData dd offset aShell32 ;modulename00402DA8 dd offset aShellexecutea ;apiname00402DAC align 800402DB0 dd offset IAT_Data//end snippet from another vb app

where you can see you have the pointers to the name of the module and the name of the api to import. TheIAT_Data fields points to thunking data (hInstance of module and api address). This data is also used from theprimitive DLL_Import of VB, used to thunk to outside apis. Back to our app, we check the DialogsStruct pointer:

00401A00 DialogsStruct[0] dd 50h ;sizeof struct

00401A04 dd 356022C6h00401A08 dd 400E3F28h00401A0C dd 495240B8h00401A10 dd 0C9491BB6h00401A14 dd 000401A18 dd 000401A1C dd 000401A20 dd 000401A24 dd 000401A28 dd 310h00401A2C dd 000401A30 dd 000401A34 dd 000401A38 dd 000401A3C dd 000401A40 dd 596h00401A44 dd 000401A48 dd offset MainDialog00401A4C dd 4Ch00401A50 DialogsStruct[1] dd 50h ;sizeofstruct00401A54 dd 78CBBB9Fh00401A58 dd 401EB563h00401A5C dd 0DB80B296h00401A60 dd 7EFFD31Ah00401A64 dd 000401A68 dd 000401A6C dd 000401A70 dd 000401A74 dd 300401A78 dd 100h00401A7C dd 000401A80 dd 000401A84 dd 000401A88 dd 0

5

00401A8C dd 000401A90 dd 1A3h00401A94 dd 000401A98 dd offset AboutDialog00401A9C dd 9Ch

we have an array of dialog descriptors. Each descriptor has various data values, in particular has in the 19th fielda pointer to a struct were the resource info are stored. Theese structure have variable size, because it dependson the data contained by the resources. Lets go and see MainDialog; i will not paste all the data but only theimportant things.

004013C2 aForm1 db 5,0,’Form1’,0

004013CC aLeimcrackme db 0Bh,0,’Leimcrackme’,0

004013E2 IconData db 6,3,0,0,6Ch,74h,0,0,0FEh...

004016FA dw 2Dh ; clientleft004016FE dw 14Ah ; clienttop00401702 dw 1248h ; clientwidth00401706 dw 0B7Ch ; clientheight

00401715 aText2 db 5,0,’Text2’,0

00401731 aText1 db 5,0,’Text1’,0

0040174D aCommand2 db 8,0,’Command2’,00040175A aCheck db 7,0,’&Check!’,0

00401777 aCommand1 db 8,0,’Command1’,000401784 aAbout db 6,0,’&About’,0004017A2 aLabel2 db 6,0,’Label2’,0004017AD aSerial db 7,0,’Serial:’,0004017D1 aLabel2_0 db 6,0,’Label2’,0004017DC aName db 5,0,’Name:’,0004017FC aLabel1 db 6,0,’Label1’,000401807 aWhoeverTriesTh db ’’,0,’Whoever tries...

you can see all the components of the crackme, their data, etc etc. IconData is the raw data of the icon of themain dialog.

6

If we look at the source we have:

Begin VB.Form Form1BorderStyle = 3 ’Fixed DialogCaption = "Leimcrackme"ClientHeight = 2940ClientLeft = 45ClientTop = 330ClientWidth = 4680Icon = "andre.frx":0000LinkTopic = "Form1"

you can see the icon data is encoded in the .frx file, which usually have big data. So the Icon field links toandre.frx file and 0000 is the offset of the starting data. Infact also the last label is linked as follows:

Caption = $"andre.frx":030A

infact in the frx file after the raw icon data there is at offset 30Ah the string of that label. Of course in thecompiled app all the infos (.frm and .frx) are built in the dialog sutrcture. In the same way you can see the infosabout the second dialog (AboutDialog). Now we go and check the most important structure:

00401AA0 ProjectStruct dd 1F4h ;signature?

00401AA4 dd offset Tree00401AA8 dd 000401AAC dd offset StartOfCode00401AB0 dd offset EdnOfCode00401AB4 dd 1238h00401AB8 dd offset DataVar100401ABC dd offset vbaExceptHandler00401AC0 dd offset StartOfData00401AC4 dd 84h dup(0)00401CD4 dd offset ExternStruct00401CD8 dd 1

Let’s see the fields: there is a pointer to the exetern imports, we already covered this structure. There areStartOfCode and EndOfCode vars, they indicate where the executable code starts and where it ends. The codeis delimited by two signatures (E9E9E9E9h starting, 9E9E9E9E ending) and some 0xCC padding. There is alsoa field that is the pointer to the base per-thread exception handler, that is __vbaExceptHandler (but of coursethe code will install others handerls). There are the pointers to StartOfData and another pointer that pointsto StartOfData + 8, it seems theese values are common for all applications. In the 84h dup(0) space you canprobably find infos about path of the project etc, usually you see here some unicode strings, nothing interesting.The remaining field, Tree, is a descriptor of code modules as they were in the source (and as they are oranized inthe compiled exe).

7

So we are going to check this struct; it is a bit complex, so pay attention:

00402434 Tree dd 0

00402438 dd offset VB_Func0040243C dd offset TreeData00402440 dd 0FFFFFFFFh00402444 dd 000402448 dd offset UnkVar10040244C dd 1F1CB8D4h00402450 dd 42793AE6h00402454 dd 51A97A97h00402458 dd 41E1033h0040245C dd 4000Ah00402460 dd 40004h00402464 dd offset ModulesList ;ptr to ModulesList[0]00402468 dd 00040246C dd 000402470 dd 000402474 dd offset aProgetto1_0 ; "Progetto1"00402478 dd 409h0040247C dd 410h00402480 dd 000402484 dd 2

VB_Func is a pointer to a location that is filled at runtime with some address of the vbvm, there are other fieldssuch as the project name and an unknown var, but the interesting things are TreeData and ModuleList. Let’ssee TreeData:

00402DE0 TreeData dd 000402DE4 dd offset Tree ;back pointer00402DE8 dd 0FFFFFFFFh00402DEC dd 000402DF0 dd offset FormList00402DF4 dd 000402DF8 dd 000402DFC dd 000402E00 dd 0FFFFFFFFh00402E04 dd 000402E08 dd offset ProjectInfo200402E0C dd offset RawData100402E10 dd offset R_UnkVar100402E14 dd offset ProjectInfo200402E18 dd offset RawData200402E1C dd offset R_UnkVar200402E20 dd offset ProjectInfo200402E24 dd offset RawData300402E28 dd offset R_UnkVar300402E2C dd offset ProjectInfo200402E30 dd offset RawData400402E34 dd offset R_UnkVar4

8

00402E38 dd 000402E3C dd offset RawData500402E40 dd offset R_UnkVar5

what we got here? A back pointer to tree struct, some raw data and ather infos about the project (you can see atprojectInfo2 some data and usually you will find the string ’C:\Programmi\Microsoft Visual Studio\VB98\VB6.OLB’),the important pointer is FormList. Let’s see:

00402D18 FormList dd offset Form[0] ;form100402D1C dd 0FFFFFFFFh ;module100402D20 dd 0FFFFFFFFh ;module200402D24 dd offset Form[1] ;form2infact if we see the .vbw file we find:

Form1 = 186, 224, 986, 702, , 207, 76, 652, 524, CModule1 = 236, 214, 1036, 662,Module2 = 166, 196, 966, 644,Form2 = 100, 283, 900, 731, , 173, 118, 973, 566, C

so the FormList simply points to a list of tied dialogs to the forms/modules of the project. Note that modulescorrespond to .bas files, forms to .frm files (which include .frx for raw resources). Let’s go on:

00402D60 Form[0] dd 0

00402D64 dd offset FormDescriptor[0]00402D68 dd 0FFFFFFFFh00402D6C dd 000402D70 dd 000402D74 dd 000402D78 dd offset FlagList00402D7C dd 000402D80 dd offset UnkData00402D84 dd offset UnkData00402D88 dd offset UnkData00402D8C dd 000402D90 dd 000402D94 dd 000402D98 dd 58h00402D9C dd 4

9

i will not paste Form[1] because its identical to Form[0]. We see some pointers to some data (mostly they pointto null values), and then a pointer to FormDescriptor. Here we stop for now, we have to go back to checkTree.ModulesList structure (from there we will reach again FormDescriptor structs):

00402488 ModulesList[0] dd offset FormDescriptor[0]0040248C dd 0FFFFFFFFh00402490 dd offset Flags_000402494 dd 000402498 dd 00040249C dd 0004024A0 dd offset aForm1_0 ; "Form1"004024A4 dd 5004024A8 dd offset OptionalData1004024AC dd 0FFFFh004024B0 dd 18083h ;Flags_1004024B4 dd 0004024B8 ModulesList[1] dd offset ModuleDescriptor[0]004024BC dd 0FFFFFFFFh004024C0 dd offset Flags_1004024C4 dd 0004024C8 dd offset unk_modvar1004024CC dd 0004024D0 dd offset aModule1 ; "Module1"004024D4 dd 7004024D8 dd 0004024DC dd 0FFFFh004024E0 dd 18001h004024E4 dd 0004024E8 ModulesList[2] dd offset ModuleDescriptor[1]004024EC dd 0FFFFFFFFh004024F0 dd offset Flags_1004024F4 dd 0004024F8 dd offset unk_modvar2004024FC dd 000402500 dd offset aModule2 ; "Module2"00402504 dd 300402508 dd 00040250C dd 0FFFFh00402510 dd 18001h00402514 dd 000402518 ModulesList[3] dd offset FormDescriptor[1]0040251C dd 0FFFFFFFFh00402520 dd offset Flags_200402524 dd 000402528 dd 00040252C dd 000402530 dd offset aForm2 ; "Form2"00402534 dd 200402538 dd offset OptionalData20040253C dd 0FFFFh00402540 dd 18083h00402544 dd 0

10

voil. We can easily see names and order of the forms/modules of the project. Note that the first field is adescriptor for the Module/Form, they are different descriptors, so they have a different structure. We can go andsee ModuleDescriptors:

00401938 ModuleDescriptor[0] dd 10001h0040193C dd offset Tree ;back pointer00401940 dd 000401944 dd 0FFFFFFFFh00401948 dd 0FFFFFFFFh0040194C dd 000401950 dd offset ModulesList[1] ;back pointer00401954 dd offset MD0_UnkVar00401958 dd 00040195C dd 7D63150h00401960 dd 000401964 dd 000401968 dd 00040196C dd offset RT_MainStruct ; ptr to following address

nothing of interest here. The other module descriptor is like this so i won’t paste it. Let’s see form descriptors:

00401F48 FormDescriptor[0] dd 100401F4C dd offset Tree ;back pointer00401F50 dd 000401F54 dd offset Form[0] ;back pointer00401F58 dd 0FFFFFFFFh00401F5C dd 000401F60 dd offset ModulesList ;back pointer00401F64 dd offset DataVar100401F68 dd 000401F6C dd 7D98E18h00401F70 dd 000401F74 dd 000401F78 dd 000401F7C dd offset FD0_Raw100401F80 dd 100401F84 dd offset FD0_Raw200401F88 dd 000401F8C dd offset FD0_Raw100401F90 dd 100401F94 dd offset FD0_ControlsList00401F98 dd 000401F9C dd offset FD_Raw300401FA0 dd 7 ;number of controls in list00401FA4 dd offset FD0_ControlsList00401FA8 dd 1B70005h00401FAC dd 6C0068h00401FB0 dd offset FD0_Dispatcher00401FB4 dd offset FD0_UnkVar100401FB8 dd 000401FBC dd 1324FCh

11

we see some unknown vars and some back pointers, what we care of is FD0_COntrolsList and FD0_Dispatcher.We see there are 7 controls in the list, so let’s see the list:

00401FC8 FD0_ControlsList[0] dd 180040h ;control type00401FCC dd 34h ;ID100401FD0 dd offset RawData100401FD4 dd 30005h ;ID200401FD8 dd 000401FDC dd 000401FE0 dd offset LocalDispatcher[0]00401FE4 dd 7DC1BF0h00401FE8 dd offset aText2_0 ; "Text2"00401FEC dd 30005h00401FF0 FD0_ControlsList[1] dd 180040h ;control type00401FF4 dd 38h ;ID100401FF8 dd offset RawData100401FFC dd 30004h ;ID200402000 dd 000402004 dd 000402008 dd offset LocalDispatcher[1]0040200C dd 7DC1BF0h00402010 dd offset aText1_0 ; "Text1"00402014 dd 30004hetc...

we have all the components of the dialog, which is fundamental in event tracking. You can see names for thecontrols, some strange values (ID1 and ID2, semms like they are internally used as resource id) and LocalDis-patcher. This is the point of all! LocalDispatcher infact points to a structure that contains all event handlers ofthe controls! Let’s see LocalDispatcher[0]:

004020F4 LocalDispatcher[0] dd 0004020F8 dd offset FD0_ControlsList[0]004020FC dd offset FormDescriptor[0]00402100 dd offset i_EVENT_SINK_QueryInterface00402104 dd offset i_EVENT_SINK_AddRef00402108 dd offset i_EVENT_SINK_Release0040210C dd 18h dup(0)

the first field is always zero. The second is a backpointer to the parent structure in the controls list, the thirdis a backpointer to parent FormDescriptor structure. After we find some basic handlers which are present in allcontrols, then we have no other handlers. This is the dispatcher of a ”label” control, so it has no other handlers!

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So we can choose another control in the list:

00402040 ControlsList[3] dd 110040h00402044 dd 40h00402048 dd offset RawData30040204C dd 30002h00402050 dd 000402054 dd 000402058 dd offset LocalDispatcher[3]0040205C dd 7DC1C10h00402060 dd offset aCommand1_0 ; "Command1"00402064 dd 30002h

as before we go and see the LocalDispatcher[3]:

00402278 LocalDispatcher[3] dd 00040227C dd offset ControlsList_3_00402280 dd offset FormDescriptor_0_00402284 dd offset i_EVENT_SINK_QueryInterface00402288 dd offset i_EVENT_SINK_AddRef0040228C dd offset i_EVENT_SINK_Release00402290 dd offset onClickAbout

00402294 dd 10h dup(0)

voil, what we were searching for! We have the onClickAbout pointer, that points to:

004023FD onClickAbout:004023FD sub dword ptr [esp+4], 3Fh00402405 onClickAbout1:00402405 jmp onClickAboutRoutine

that is a transfer area (internal function dispatcher) for local event handlers. So now we know what routine isexecuted when ”About” button is pressed. The problem is, how did I know that it is a onClick handler? It couldbe a onOver, onMove etc etc? To answer this question let’s see the Label1 dispatcher, we know that it has aonOver handler, we can see it at runtime (and we know it is label1 from line 004017FC remember?)

13

00402330 LocalDispatcher[5] dd 000402334 dd offset ControlsList[5]00402338 dd offset FormDescriptor[0]0040233C dd offset i_EVENT_SINK_QueryInterface00402340 dd offset i_EVENT_SINK_AddRef00402344 dd offset i_EVENT_SINK_Release00402348 dd 00040234C dd 000402350 dd 000402354 dd 000402358 dd 00040235C dd 000402360 dd 000402364 dd 000402368 dd 00040236C dd offset onOverLabel100402370 dd 8 dup(0)

we see the common handler and backpointers, then far away the onOver handler pointer. So my idea is thatevery field of the LocalDispatch structure is a pointer to a given event handler (00402348 would be onClick etc),in addition the LocalDIspatch structure seems to be not exactly the same for all control types, so if you want tomap all controls handlers you should write an app which uses all possible handlers and see where they are placedin this struct. Now we can go back to FormDescriptor[0] and check the last pointer, that was FD0Dispatcher, itpoints to

004020E0 FD0_Dispatcher dd offset onClickAboutPre1004020E4 dd offset onClickAbout1004020E8 dd offset onClickCheck1004020EC dd offset OnOverForm1004020F0 dd offset onOverLabel11

a simple list to all import transfers addresses (they point directly to the jumps, the pointers in local dispatchersinstead point to previous line!). As for this form, we can go in ModulesList[3] and check FormDescriptor[1], wewill find the controls list and associated event handlers.

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In particular, in this form there is a moving button, if we check the LocalDispatcher of this button we see:

00401E70 FD1_LocalDispatcher[1] dd 000401E74 dd offset FD1_ControlsList[1]00401E78 dd offset FormDescriptor_1_00401E7C dd offset i_EVENT_SINK_QueryInterface00401E80 dd offset i_EVENT_SINK_AddRef00401E84 dd offset i_EVENT_SINK_Release00401E88 dd offset onClickOk00401E8C dd 000401E90 dd 000401E94 dd 000401E98 dd 000401E9C dd 000401EA0 dd 000401EA4 dd 000401EA8 dd 000401EAC dd offset onOverOk00401EB0 dd 7 dup(0)

the other controls in the list just have default handlers.

We have mapped all the resource and relative event handlers just by examining data structures (and using a bitof zen!).

Now we can work on the code. Let’s say we want to find a correct serial for a given name. We know that theroutine called when the Check button is pressed is at address 00402FD0, so we start from there:

-standard function initialization

00402FD0 push ebp ;allocate private stackframe00402FD1 mov ebp, esp00402FD3 sub esp, 0Ch

00402FD6 push offset vbaExceptHandler ;installing default seh00402FDB mov eax, large fs:000402FE1 push eax00402FE2 mov large fs:0, esp00402FE9 sub esp, 74h00402FEC push ebx ;save registers area00402FED push esi00402FEE push edi

-loading destructors00402FEF mov [ebp-0Ch], esp00402FF2 mov dword ptr [pDestruct], offset Destructors_2

-allocating dynamic resource00402FF9 mov esi, [ebp+8]00402FFC mov eax, esi00402FFE and eax, 100403001 mov [ebp-4], eax

15

00403004 and esi, 0FFFFFFFEh00403007 push esi00403008 mov [ebp+8], esi0040300B mov ecx, [esi]0040300D call dword ptr [ecx+4] ; Zombie_AddRef

the parent object (the form) is passed as parameter (ebp+8, sorry i forgot to resolve ebp based frame function inida!) and the COM tecnology uses AddRef to increment reference count of the object (instantiation). For thoseof you that don’t know this, COM objects are responsible for their lifetime, the resources they use are allocateduntil the reference count is ¿0, when it reaches 0 the objects enter zombie state and can be deallocated to freeresources (well, things are a little more complex, see COM object management documentation to know more onthis topic).

-background color changing

00403010 xor eax, eax00403012 lea edx, [ebp-24h]00403015 mov ebx, 80020004h0040301A mov edi, 0Ah0040301F mov [ebp-24h], eax ;zero vars00403022 push edx00403023 mov [ebp-34h], eax00403026 mov [ebp-44h], eax00403029 mov [ebp-1Ch], ebx0040302C mov [ebp-24h], edi0040302F call ds:rtcRandomNext ;get random fp values00403035 fstp dword ptr [COlor_R]00403038 lea eax, [ebp-34h]0040303B mov [ebp-2Ch], ebx0040303E push eax0040303F mov [ebp-34h], edi00403042 call ds:rtcRandomNext00403048 fstp dword ptr [Color_G]0040304B lea ecx, [ebp-44h]0040304E mov [ebp-3Ch], ebx00403051 mov ebx, [esi]00403053 push ecx00403054 mov [ebp-44h], edi00403057 call ds:rtcRandomNext ;color_B not stored, used directly eax0040305D fmul ds:_0255 ;multiply each rand fpu * 25500403063 mov edi, ds:__vbaR8IntI200403069 fnstsw ax0040306B test al, 0Dh0040306D jnz loc_40312F ;fpexception00403073 call edi ; __vbaR8IntI2 ;cast from _fpu real 8 bytes_ to _integer 2 bytes_00403075 fld dword ptr [ebp-7Ch]00403078 fmul ds:_02550040307E push eax0040307F fnstsw ax00403081 test al, 0Dh00403083 jnz loc_40312F ;fpexception00403089 call edi ; __vbaR8IntI2

16

0040308B fld dword ptr [ebp-78h]0040308E fmul ds:_025500403094 push eax00403095 fnstsw ax00403097 test al, 0Dh00403099 jnz loc_40312F ;fpexception0040309F call edi ; __vbaR8IntI2004030A1 push eax004030A2 call ds:rtcRgb004030A8 push eax ;rgb resulting from previous calculus004030A9 push esi ;form object instance004030AA call dword ptr [ebx+64h] ; MSVBVM_UnkFunc2 (Set back form color)004030AD test eax, eax004030AF fnclex004030B1 jge short loc_4030C2 ;taken

004030B3 push 64h004030B5 push offset dword_402590004030BA push esi004030BB push eax004030BC call ds:__vbaHresultCheckObj

004030C2 lea edx, [ebp-44h]004030C5 lea eax, [ebp-34h]004030C8 push edx004030C9 lea ecx, [ebp-24h]004030CC push eax004030CD push ecx004030CE push 3 ;number of objects004030D0 call ds:__vbaFreeVarList ;free vars used in previous rtcRandomNext

-name/serial check004030D6 add esp, 10h004030D9 call VoidCheck004030DE test ax, ax004030E1 jnz short loc_4030E8004030E3 call SerialValidation004030E8 mov dword ptr [ebp-4], 0004030EF Destr_2_0:004030EF wait004030F0 push offset loc_403110004030F5 jmp short EndRoutine

17

SerialValidation is the routine that checks the serial, so we will check it (see later). VoidCheck is the routine thatchecks if text fields are void, if so display the error message then exit.

-destructors and ending stuff

004030F7 Destr_2_1: ;called in case of error004030F7 lea edx, [ebp-44h]004030FA lea eax, [ebp-34h]004030FD push edx004030FE lea ecx, [ebp-24h]00403101 push eax00403102 push ecx00403103 push 3 ;number of vars to free00403105 call ds:__vbaFreeVarList0040310B add esp, 10h0040310E retn0040310F EndRoutine0040310F retn ;goes to 0040311000403110 mov eax, [ebp+8] ;ptr to form object00403113 push eax00403114 mov edx, [eax]00403116 call dword ptr [edx+8] ;Zombie_Release (decrease reference count00403119 mov eax, [ebp-4] ;for form object)0040311C mov ecx, [ebp-14h]0040311F pop edi ;save registers area00403120 pop esi00403121 mov large fs:0, ecx ;restore exception handler00403128 pop ebx00403129 mov esp, ebp ;delete private stackframe0040312B pop ebp0040312C retn 4

0040312F jmp loc_4011AC ; __vbaFPException

are you beginning to feel the VB framework? It’s really easy as you can see, the structure of the code is alwaysthe same.

18

Again let’s see all the code, so you can understand how easy is code analisys:

SerialValidation

004032C0 push ebp ;allocate private stackframe004032C1 mov ebp, esp004032C3 sub esp, 8004032C6 push offset vbaExceptHandler ;allocate exception handler004032CB mov eax, large fs:0004032D1 push eax004032D2 mov large fs:0, esp004032D9 sub esp, 158h004032DF push ebx ;registers save area004032E0 push esi004032E1 push edi

-destructors allocation and initialization004032E2 mov [ebp+var_8], esp004032E5 mov [ebp+var_4], offset Destructors_5004032EC mov eax, Form1Instance004032F1 xor edi, edi004032F3 cmp eax, edi ;is form1 instanced?004032F5 mov [ebp+var_20], edi ;zero vars004032F8 mov [ebp+var_30], edi004032FB mov [ebp+var_40], edi004032FE mov [ebp+var_50], edi00403301 mov [ebp+var_54], edi00403304 mov [ebp+var_58], edi00403307 mov [ebp+var_5C], edi0040330A mov [ebp+var_6C], edi0040330D mov [ebp+var_7C], edi00403310 mov [ebp+var_8C], edi00403316 mov [ebp+var_9C], edi0040331C mov [ebp+var_AC], edi00403322 mov [ebp+var_BC], edi00403328 mov [ebp+var_CC], edi0040332E mov [ebp+var_DC], edi00403334 mov [ebp+var_EC], edi0040333A mov [ebp+var_FC], edi00403340 mov [ebp+var_10C], edi00403346 mov [ebp+var_11C], edi0040334C mov [ebp+var_13C], edi00403352 mov [ebp+var_14C], edi00403358 mov [ebp+var_15C], edi0040335E jnz short loc_403375 ;jump if form1 istanced00403360 push offset Form1Instance00403365 push offset FormDescriptor_0_0040336A call ds:__vbaNew2 ;if not instanced the form wuold00403370 mov eax, Form1Instance ;have been created here-copy vars from form instance to local vars00403375 mov ecx, [eax]00403377 push eax ;form object instance

19

00403378 call dword ptr [ecx+308h] ;MSVBVM_UnkFunc (some sort of addref)0040337E mov esi, ds:__vbaVarMove00403384 mov ebx, 900403389 lea edx, [ebp+var_6C]0040338C lea ecx, [ebp+Text1] ;note, text1 object, not string!0040338F mov [ebp+var_64], eax ;(parameter in ecx, fastcall style)00403392 mov [ebp+var_6C], ebx00403395 call esi ; __vbaVarMove ;copy text1 in local var00403397 mov eax, Form1Instance0040339C cmp eax, edi0040339E jnz short loc_4033B5 ;avoid allocation of form if it exists004033A0 push offset Form1Instance004033A5 push offset FormDescriptor_0_004033AA call ds:__vbaNew2004033B0 mov eax, Form1Instance004033B5 mov edx, [eax]004033B7 push eax004033B8 call dword ptr [edx+30Ch] ; MSVBVM_UnkFunc (as above)004033BE lea edx, [ebp+var_6C]004033C1 lea ecx, [ebp+Text2] ;note, text2 object, not string!004033C4 mov [ebp+var_64], eax ;(parameter in ecx, fastcall style)004033C7 mov [ebp+var_6C], ebx004033CA call esi ; __vbaVarMove ;copy text2 in local var

as you can see COM tecnology strikes again: in the form object there is the list of controls, each one with his owndata (text controls have a simple string). So what does the code do? It simply copies the data from those objectin local variables (Text1 and Text2), it uses __vbaVarMove to copy data. Attention: it does not copy only thestring, but the whole text object! The parameter is passed via ecx register (fastcall convention), and is a pointerto the memory space that receives object data. So if you want to know the data of the object (the text in thiscase), after __vbaVarMove go to data pointed by Text1 (ebp-50h), and see the third dword: it is a pointer to theunicode string for name inserted. So the structure should be as follows:

RT_Text_Object:

+00 SizeOf(RT_Text_Object)

+04 Method1

+08 TextPointer

+0C etc (other values or methods)

ok, we now know that the proggy has copied locally the data of text objects, we also know the addresses of thisobjects, so we can easily track all movements on theese strings.

20

-check length of name

004033CC mov eax, 1004033D1 lea ecx, [ebp+Text1]004033D4 mov [ebp+var_104], eax004033DA mov [ebp+var_F4], eax004033E0 lea eax, [ebp+var_10C]004033E6 lea edx, [ebp+var_6C]004033E9 push eax004033EA mov esi, 2004033EF push ecx ;object (Text1)004033F0 push edx ;var object that receives length004033F1 mov [ebp+var_10C], esi004033F7 mov [ebp+var_FC], esi004033FD mov [ebp+var_114], edi00403403 mov [ebp+var_11C], esi00403409 call ds:__vbaLenVar ;get length of text1 (name)0040340F push eax ;var object for target value of subtraction00403410 lea eax, [ebp+var_FC]00403416 lea ecx, [ebp+var_7C]00403419 push eax ;var object for value to subtract (1)0040341A push ecx ;var object for result of subtraction0040341B call ds:__vbaVarSub

some easy function here, the first function gets the Text1 and gets its length. As before keep in mind that thepointers points to OBJECTS, so vbaLenVar does no return the length of the string, but the object that containsthe length. The object is as before:

RT_Var_Object:

+00 SizeOf(RT_Text_Object)

+04 Method1

+08 Data (length of string)

+0C etc (other values or methods)

so you must look the third dword at the memory pointed by the result pointer of vbaLenVar. Same thing for allother __vba***. So here it gets length of string and decrements it by one.

21

-main for() cycle

00403421 push eax ;(strlen(Text1)-1) object00403422 lea edx, [ebp+var_11C]00403428 lea eax, [ebp+var_15C]0040342E push edx0040342F lea ecx, [ebp+var_14C]00403435 push eax00403436 lea edx, [counter]00403439 push ecx0040343A push edx ;counter

0040343B call ds:__vbaVarForInit ;prepare for() cycle00403441 mov edi, ds:__vbaVarMul00403447 mov ebx, ds:__vbaVarAdd

-loop here0040344D test eax, eax ;is cycle finished? (true = loop, false = exit)0040344F jz EndCheck ;then check ends00403455 mov eax, Form1Instance0040345A test eax, eax0040345C jnz short loc_403473 ;is form1 instanced?0040345E push offset Form1Instance ;create instance if not00403463 push offset FormDescriptor[0]00403468 call ds:__vbaNew20040346E mov eax, Form1Instance00403473 mov ecx, [eax] ;use current instance00403475 push eax ;form1 instance00403476 call dword ptr [ecx+308h] ;MSVBVM_UnkFunc0040347C lea edx, [ebp+var_5C]0040347F push eax00403480 push edx00403481 call ds:__vbaObjSet ;set object var to [ebp+var_5C] ptr00403487 mov eax, 10040348C lea ecx, [counter]0040348F mov [ebp+var_F4], eax ;set vars to true00403495 mov [ebp+var_A4], eax0040349B mov [ebp+var_114], eax004034A1 mov eax, [ebp+var_5C]004034A4 mov [ebp+var_94], eax004034AA lea eax, [ebp+var_AC]004034B0 push eax ;value to add (1)004034B1 lea edx, [ebp+var_11C]004034B7 push ecx ;target value of addiction (counter)004034B8 lea eax, [ebp+var_8C]004034BE push edx ;value to add004034BF push eax ;target004034C0 mov [ebp+var_104], 64h004034CA mov [ebp+var_10C], esi004034D0 mov [ebp+var_FC], esi004034D6 mov [ebp+var_AC], esi

22

004034DC mov [ebp+var_11C], esi004034E2 mov [ebp+var_5C], 0004034E9 mov [ebp+var_9C], 9004034F3 call ebx ; __vbaVarAdd004034F5 push eax ;result object004034F6 call ds:__vbaI4Var ;convert result object to int_4_bytes004034FC lea ecx, [ebp+var_9C]00403502 push eax ;position00403503 lea edx, [ebp+var_BC]00403509 push ecx ;length0040350A push edx ;target string (name)0040350B call ds:rtcMidCharVar ;extract one char from name at counter+1 position00403511 lea eax, [ebp+var_BC]00403517 lea ecx, [ebp+var_54]0040351A push eax ;extracted char object0040351B push ecx ;name string0040351C call ds:__vbaStrVarVal00403522 push eax ;extracted char value00403523 call ds:rtcAnsiValueBstr ;get ansi value from extracted value00403529 push eax ;ansi string of value of mid0040352A call ds:__vbaStrI2 ;convert it to int_2_bytes

this code increments the for() counter and gets the char at counter+1 position with mid function. Then it obtainsthe numeric value of the extracted char, then at the end it converts it into a unicode string representing that charin decimal number. Example: at iteration x it extracts Name[x+1] char, let’s assume it is an A (0x41). Then itgets the numeric value (0x41) and then the unicode string 65 (dec for 0x41), unicode means the string will be(0x36 0x00 0x35 0x00).

00403530 mov edx, eax00403532 lea ecx, [ebp+var_58] ;ptr to unicode int2bytes00403535 call ds:__vbaStrMove ;move unicode decimal number in ebp+var480040353B push eax ;unicode decimal number0040353C call ds:rtcR8ValFromBstr ;convert unicode decimal number string

;in floating point value00403542 call ds:__vbaFpI4 ;convert previous fp number in int4bytes

23

here we have the numeric value of the extracted char in eax

00403548 cdq00403549 mov ecx, 0Ah ;divisor0040354E mov [ebp+var_13C], 300403558 idiv ecx ;int4bytes % 100040355A lea eax, [ebp+var_10C] ;trash result00403560 lea ecx, [counter] ;trash divisor00403563 mov [rest], edx ;save rest of division00403569 lea edx, [ebp+var_40]0040356C push edx ;result string0040356D push eax ;value 100 (for multiplication)0040356E lea edx, [ebp+var_FC]00403574 push ecx ;target (counter)00403575 lea eax, [total]00403578 push edx ;value to add (1)00403579 push eax ;target (result)0040357A call ebx ; __vbaVarAdd ;increment counter0040357C lea ecx, [ebp+var_7C]0040357F push eax00403580 push ecx00403581 call edi ; __vbaVarMul ;100 * (counter+1)00403583 push eax00403584 lea edx, [ebp+var_13C]0040358A lea eax, [ebp+var_CC]00403590 push edx00403591 push eax00403592 call edi ; __vbaVarMul ;(100*(counter+1)) * previous calculus

;on extracted char00403594 lea ecx, [ebp+var_DC]0040359A push eax0040359B push ecx0040359C call ds:__vbaVarInt ;cast to int the result of all calculus004035A2 lea edx, [ebp+var_EC]004035A8 push eax ;int numeric value004035A9 push edx ;string that gets total004035AA call ds:__vbaVarCat ;cat unicode string of int numeric value

;to unicode string of total004035B0 mov edx, eax004035B2 lea ecx, [ebp+var_40]004035B5 call ds:__vbaVarMove ;copy string of total to [ebp+var_40]004035BB lea eax, [ebp+var_58]004035BE lea ecx, [ebp+var_54]004035C1 push eax004035C2 push ecx004035C3 push esi ;esi = 2 (number of vars)004035C4 call ds:__vbaFreeStrList ;free temp vars004035CA add esp, 0Ch004035CD lea ecx, [ebp+var_5C]004035D0 call ds:__vbaFreeObj ;free temp object

24

004035D6 lea edx, [ebp+var_BC]004035DC lea eax, [ebp+var_AC]004035E2 push edx004035E3 lea ecx, [ebp+var_8C]004035E9 push eax004035EA lea edx, [ebp+var_9C]004035F0 push ecx004035F1 lea eax, [total] ;temp total004035F4 push edx004035F5 push eax004035F6 push 5004035F8 call ds:__vbaFreeVarList ;free 5 temp vars004035FE add esp, 18h00403601 lea ecx, [ebp+var_15C]00403607 lea edx, [ebp+var_14C]0040360D lea eax, [counter]00403610 push ecx ;maximum value of counter00403611 push edx ;incremental step00403612 push eax ;actual counter00403613 call ds:__vbaVarForNext ;if actual counter < maximum then return true00403619 jmp loc_40344D ;else return false

this is the loop for serial calculus

0040361E EndCheck: ; CODE XREF: Check+18Fj0040361E call nullsub_1 ;null calls00403623 call nullsub_1 ;I asked _d31m0s_ to add them00403628 call nullsub_1 ;just to check some things0040362D call nullsub_100403632 call nullsub_100403637 call nullsub_10040363C lea ecx, [ebp+var_40]0040363F lea edx, [ebp+var_30]00403642 push ecx ;calculated serial (text object)00403643 push edx ;inserted serial (text object)00403644 call ds:__vbaVarTstEq ;compare the two string objects0040364A test ax, ax ;true = equal, false = different0040364D jz short Error ;if strings are different then error message0040364F call OkMessage ;else ok message00403654 wait ;useless waste...00403655 push offset loc_4036FE0040365A jmp short Ending0040365C0040365C Error:0040365C call ErrorMessage00403661 wait00403662 push offset loc_4036FE00403667 jmp short Ending0040366900403669 Destructor: ;called in case of error00403669 lea eax, [ebp-58h]0040366C lea ecx, [ebp-54h]

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0040366F push eax00403670 push ecx00403671 push 200403673 call ds:__vbaFreeStrList00403679 add esp, 0Ch0040367C lea ecx, [ebp-5Ch]0040367F call ds:__vbaFreeObj00403685 lea edx, [ebp-0ECh]0040368B lea eax, [ebp-0DCh]00403691 push edx00403692 lea ecx, [ebp-0CCh]00403698 push eax00403699 lea edx, [ebp-0BCh]0040369F push ecx004036A0 lea eax, [ebp-0ACh]004036A6 push edx004036A7 lea ecx, [ebp-9Ch]004036AD push eax004036AE lea edx, [ebp-8Ch]004036B4 push ecx004036B5 lea eax, [ebp-7Ch]004036B8 push edx004036B9 lea ecx, [ebp-6Ch]004036BC push eax004036BD push ecx004036BE push 9004036C0 call ds:__vbaFreeVarList004036C6 add esp, 28h004036C9 retn004036CA Ending:004036CA lea edx, [ebp+var_15C] ;free vars004036D0 lea eax, [ebp+var_14C]004036D6 push edx004036D7 push eax004036D8 push 2004036DA call ds:__vbaFreeVarList004036E0 mov esi, ds:__vbaFreeVar004036E6 add esp, 0Ch004036E9 lea ecx, [counter]004036EC call esi ; __vbaFreeVar004036EE lea ecx, [ebp+var_30]004036F1 call esi ; __vbaFreeVar004036F3 lea ecx, [ebp+var_40]004036F6 call esi ; __vbaFreeVar004036F8 lea ecx, [ebp+var_50]004036FB call esi ; __vbaFreeVar004036FD retn ;return to 004036FE004036FE mov ecx, [ebp-10h]00403701 pop edi ;save register area00403702 pop esi00403703 mov large fs:0, ecx ;restore seh handler0040370A pop ebx

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0040370B mov esp, ebp ;delete private stackframe0040370D pop ebp0040370E retn

ok, now we discovered how the serial is built, in particular if you see the __vbaVarTestEq function, the twoparameters passed are two string objects, one for serial inserted and one for the correct serial, so you can do serialfishing here without calculating. The algorithm we reversed is:

nser = nser & Int(100 * (i + 1) * (Val(Asc(Mid(Form1.Text1, i + 1, 1))) Mod 10))

so debugging vb applications is really easy, just keep in mind that functions use Objects instead of direct values.Once you know this, COM jungle will not be a problem!

Serial for "AndreaGeddon" is 50000160050042007008000011000, find your own!

3 SUMMARY

I hope that at the end of this tutorial you will have learned how to debug visual basic applications. As you cansee it is really easy, you do not even need SmartCheck. The exe itself is full of precious infos, you can easily findall event handlers. Then you just have to analyse the code to understand what the program does. Rememberthat COM tecnology is object based, so when you see functions and you analyse their parameters and returnvalues, you know that you are considering OBJECTS, not the values directly. The values you search (strings,numbers, etc) will be encoded in the object. We encountered a lot of rtc* and __vba* apis here, they always useobjects, infact when there is an addition you see __vbaVarAdd instead of a simple ”add” asm instruction, thisis because the function adds two number objects! Naturally the code uses also asm instructions for direct valuearithmetics, this happens when there is some casting such as __vbaI4Var function etc etc.

I wrote this tute because my intent is vb full decompiling, now that you know the structure of the compiled exesyou can understand that decompiling is possible, and is relatively easy. Hope I will write a proggy about this oneday!

GREETS AND THANKS

Thanks to _d31m0s_ who wrote the vb app for this tutorial (i will kill him for lame messages in it!), greets to allRET friends and great reversers! Greets to all UIC members an to all #crack-it people, see you all guys!

GoodBye

[AndreaGeddon] andreageddon@hotmail.com my mail

www.andreageddon.8m.com my lame italian site

[RET] www.reteam.org RET’s great site

[UIC] www.quequero.org italian university of cracking

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