razzia's tutorial for crippled programs
(The beautiful creation of the "RazziaPad")
by razzia
(19 August 1997)
Courtesy of Fravia's page
of reverse engineering
Well, once more I'm amazed: Razzia's
essays are always outstanding, but this one will make history in the scene:
he's showing us with a very easy to follow example the road to the most
sublime advanced cracking that you can dream of: ADDING functionality to
ANY target you fancy... that's exactly what our aim should always be: they
speak (falsely) of "object" oriented programming, as if the
toy languages they
use could really be used to integrate smoothly completely different compiled
code (try to integrate visualbasic with visual c++ and you'll see what
I mean) and Razzia throw them all possible eggs in the face DEMONSTRATING
here, under your privileged eyes, dear readers of my page, how (relatively)
easy it is to do such "novelties" and "modernities" using good old "obsolete"
assembly.
I have no words: Razzia is a great cracker (as if we did not know it already)
he may join the +HCU anytime he wishes, I'm sure +ORC would love this essay
(hope you are reading this from wherever you are now, Master!).
So, once more, READ this essay, by all means, HEAD what Razzia teaches and
try this out on other targets until you master this. Here you'll go, right
now, from
"dilettante" to "advanced" crackers, here and now!
razzia's tutorial for crippled programs
Introduction
I think every cracker knows the feeling when you download a program
and it turns out to be crippleware. We feel disappointed because we didn't
even get a fair chance in the fight. But in this tutorial i want to
show you that the fight doesnt necessarily have to end there. There is
a good chance we can add the missing code to the program. It will not
be easy, and a good deal of knowledge about windows is needed, but the
satisfaction of a crack will in such cases be greater.
Tools and References
Before you start an operation like this you will need to know
how windows programs work, and how windows exe files are construc-
ted. You will need to study the following documents :
- http://www.microsoft.com/win32dev/base/pefile.htm
A document about the structure of the PE header (Win32 files are
called PE files).
- Vlad magazine #6
This virus magazine explains in this issue how they made the
first win95 virus called Bizatch, souce code is included. This
issue also has another document about the PE header. In the past
this document was included in the microsoft win32 SDK package.
But somehow (!) microsoft stopped doing that.
- Fravia's tutorials about Taskman and Filemon
This will give you a good understanding about how windows programs
look when they have been compiled to assembly.
- Wap32 example that comes with Tasm 5.
This example will show you how to make win32 programs in assembly.
This program is also used as the host for Bizatch, so you will
find its source also in Vlad #6.
- A windows programming book
The tools i have used in this case are Softice 3.0, Tasm 5, Hiew 5.65,
IDA PRO 3.6 and pedump (19585 bytes).
A few words about Win32 programs
Every win95 executable file consists of 2 parts : the PE header and
the sections. The PE header contains all kind of information for the os
about how to threat this file. The sections are grouped by their
functionality. For example there is one section for the programs code,
one for its data, one with its resources, one with the table of
imported functions and a few more.
Now, what happens when win95 loads a program is this:
First an environment is created for the program where it gets its own
virtual address-space. Then win95 has to decide where in this virtual
address-space it should place the program. That information is avail-
able in the PE header. The PE header contains the desired imagebase
of the program, this is the adress the program wants to be loaded at.
Then windows takes all the sections and places them in memory
beginning at the imagebase (default imagebase is 400000h). Where
exactly it places the sections is also stated in the PE header. Every
section has its own so called RVA (Relative Virtual Address). This is
just an offset relative to the imagebase.
Once the sections are in memory, windows has to know how to threat
those sections. It has to know which section contains a stucture with
the resources, which one has the scructure of the import table etc.
That is also stated in the PE header with RVA's to the beginning of the
various so called data directories.
Then finally windows has to jump to the programs code. This entrypoint
is in the PE header as the entrypoint RVA.
The above words about the PE header is not meant as a replacement for
the PE document i mentioned at the beginning of this tutorial. I would
recommend to every (win32) cracker to study the PE header thoroughly.
It will give you a much better understanding of win95 programs.
Our target : Notepad
I dont program much, but when i do they are small programs most of
the time, ie keygens, patches. To edit my code i like to use Note-
pad. But one disadventage of Notepad is that it doesn't show linenumbers.
So when the compiler gives an error with linenumber, i have to load
another editor to find the line with the error. Therefore it would be
nice if i could add some code that shows me the current line of the
cursor.
The reason i chose Notepad as an example in this tutorial is that it
is small and simple. So its perfect for learning purposes.
I dont know if Notepad.exe is the same on every version of win95,
the one i have is 35.328 bytes long.
Strategy
Well, its time now to think about the 'crack'. We want to add some
code to a program and want that code to interact with the existing
code. We can do it in two steps. First we have to find out what code
we are gonna add. Then we will have to append this code to the file
and patch the file at some places so it will jump to the new code
at the right times.
Part 1 : find out what code we need
In order to find out what code we have to add we will need to do
some investigation on the target program.
The 'heart' of every windows program is its WndProc procedure. This
procedure is called by windows everytime the user has interacted
with the programs window. Windows passes to this procedure a few
variables, like a windows message, so that WndProc knows what happened
and can perform what needs to be done to keep the window updated.
In the case of notepad the main program generates a childwindow
of the 'edit' class (to get more information about this class check
your windows programming books).The edit childwindow gets the size of
the mainwindow all the time. In order to print our own text to the
mainwindow we will have to reduce the size of the 'edit' childwindow
so it wont cover the complete mainwindow area.
Everytime the user changes the size of the mainwindow, windows
calls the WndProc functions with the WM_SIZE message. The WndProc
procedure then can react to this event, ie adjust the size of the
'edit' childwindow to the new size of the mainwindow.
Lets locate this WndProc procedure and see how it handles the
WM_SIZE message. You can do that either with Softice or IDA. Lets
chose the more relaxed way, and do it with IDA.
Run IDA and load Notepad.exe. There are a lot of ways to get to
WndProc procedure, but the easiest way is via the RegisterClass
function. You see, at the beginning of every windows-program the main
window is 'registered' to windows. The parameter to RegisterClass
is a variable of WNDCLASS, which contains a pointer to the WndProc
function (check your winAPI reference).
So, press control-l in IDA and press return on the RegisterClassExA
function (notepad uses this variation of RegisterClass). IDA will show
you:
00402B16 lea eax, [ebp-30h]
00402B19 mov dword ptr [ebp-08h], offset aNotepad
00402B20 mov dword ptr [ebp-28h], offset loc_401AAD ;<- WndProc !
00402B27 mov dword ptr [ebp-10h], 6
00402B2E mov dword ptr [ebp-2Ch], 1000h
00402B35 push eax
00402B36 mov [ebp-24h], edi
00402B39 mov [ebp-20h], edi
00402B3C call ds:RegisterClassExA
Easily we see that loc_401AAD is the location of the WndProc proce-
dure. At loc_401AAD IDA shows :
00401AAD loc_401ADD: ;WndProc
00401AAD push ebp
00401AAE mov ebp, esp
00401AB0 push esi
00401AB1 push edi
00401AB2 mov esi, [ebp+0Ch]
00401AB5 cmp esi, 5 ;beware that WM_SIZE = 5
00401AB8 ja short loc_401ACE
00401ABA jz loc_401BC6 ;jump here for WM_SIZE
00401AC0 cmp esi, 2
00401AC3 jz loc_401BB9
00401AC9 jmp loc_401B51
We see that here the program checks to see which message it is dealing
with and branches accordingly. To find out which value corresponds
to WM_SIZE you can look it up in the header files that come with
a win32 c++ compiler (there is a free one at www.cygnus.com)'or you
can do '? wm_size' in softice,
or you can download right now winuser.h
from the Watcom C/C++ (version 11) package... the compiler you
should buy, btw... courtesy of Fravia :-)
We also see that for a WM_SIZE msg the program branches to
loc_401BC6. If you examine the code at that location you will see a
call to this procedure:
0040113F sub_40113F proc near
0040113F push 1
00401141 mov eax, ds:dword_406004
00401146 push 0
00401148 push eax
00401149 call ds:InvalidateRect
0040114F push 1
00401151 mov eax, ds:dword_406004
00401156 push dword ptr [esp+0Ch] ;Height
0040115A push dword ptr [esp+0Ch] ;Width
0040115E push 0
00401160 push 0
00401162 push eax ;handle of 'edit' win
00401163 call ds:MoveWindow ;Update size of 'edit' win
00401169 retn 8
00401169 sub_40113F endp
Note that the handle of the 'edit' child window is apparently stored
in (virtual) address 406004.
We also see that for every WM_SIZE message the 'edit' childwindow gets
updated with a call to MoveWindow with the new size of the mainwindow.
But we dont want the edit childwindow to be the full size of the
mainwindow.
In order to get a smaller heigth we have to place a jump to our
own code at the line with (virtual) adress 00401156. Such a jump takes
5 bytes, so our return adress will have to be 0040115E. For the code
that gets overwritten we will make up in our code.
Own our code will look like :
Own_Code_1:
sub esp,8
push ebx ;Save ebx temporarily
mov ebx,[esp+0Ch+4] ;Get heigth
sub ebx,20 ;Substract 20
mov [esp+4+4],ebx ;Push heigth
mov dwHeight,ebx ;Save Height
mov ebx,[esp+0Ch] ;Get width
mov [esp+4],ebx ;Push width
pop ebx ;Restore ebx
jmp 0040115E ;Return to where we left
dwHeigth dd 0
One note here is appropiate. We save the Height because we will need
this value in the printing part of our code, as you will see if you
continue reading my essay.
Ok, one down, one more to go (as you will see). We want to print
some text on the mainwindow. Everytime a window needs to get
'repainted', for example when another window overlapped it or when
the user changed the windows size, windows will call the WndProc
procedure with WM_PAINT. This means we have to 'trap' the WM_PAINT
msg in the WndProc function and make a jump to our own routine, the
one that prints text.
We will also need to print every time the vertical position
of the cursor has changed. The easiest way to do that is by ignoring
all the possible messages that inform about a change in the cursor
position, and instead compare the new cursor position with the old
one.
If you examine the WndProc code you will see it doesnt do anything
with the WM_PAINT message. So, at the beginning of the WndProc procedure
we have to insert a jump to our own code.
The beginning of WndProc procedure looked like this:
00401AAD loc_401ADD: ;WndProc
00401AAD push ebp
00401AAE mov ebp, esp
00401AB0 push esi ;<- good place for jump
00401AB1 push edi
00401AB2 mov esi, [ebp+0Ch]
00401AB5 cmp esi, 5 ;<- return here
00401AB8 ja short loc_401ACE
00401ABA jz loc_401BC6
00401AC0 cmp esi, 2
00401AC3 jz loc_401BB9
00401AC9 jmp loc_401B51
A nice place to put the jump to our code for processing WM_PAINT
will be at the line with (virtual) adress 00401AB0. And again, the
jumping code occupies 5 bytes, so it will return at adress 00401AB5.
Our own code for the WM_PAINT msg will look like this :
Our_Code_2:
push esi
push edi
mov esi, [ebp+0Ch] ; Esi contains current message now
push 0
push -1
push EM_LINEFROMCHAR
push [406004] ; (Handle of 'edit' child win)
call SendMessageA ; After this call EAX will have
; the y position of cursor
cmp eax,dwLine ; Has cursor pos changed ?
jnz update_line_num ; If yes, print the new line num
cmp esi,0Fh ; Is the current msg a WM_PAINT ?
jnz no_update_needed ; If no , return to WndProc
update_line_num:
mov dwLine,eax ; Save new y position of cursor
inc eax
push eax
push offset szFormat
push offset sLineNumber
call wsprintfA ; Convert number to ascii
push [406000h] ; (Handle of main window)
call GetDC ; Get the device context
mov theDC,eax ; Save dc
push 0
push 0
push 0
push 14
push sLineNumber
push 0
push [dwHeigth]
push [theDC]
call TabbedTextOut ; Print the text
push [theDC]
push [406000h] ; (Handle of main window)
call ReleaseDC ; Release the DC
no_update_needed:
mov esi, [ebp+0Ch]
jmp 0401AB5h ; Return to WndProc
dwLine dd 0
szFormat db '%#05d',0
sLineNumText db ' line : '
sLineNumber db 6 dup (?)
theDC dd 0
A few notes about the above code. First of all i used only api functions
that were already imported by Notepad.exe (check its import table).
What if the API's functions we need were not in the import table?
I will explain at the end of this tutorial how to solve this problem.
Secondly, some functions need the handle of the main window as a
parameter. You can find its (virtual) address with IDA if you look
where notepad stores it after the first call to CreateWindowExA.
Finally, this code is not final. Consider it as a sketch, or a blueprint.
Some minor, but very important, changes will be necessary.
Part 2 : Appending our code
Now we are ready to start thinking about adding our code to
notepad.exe. Our main concern will be to make code that is not
dependant on the memory location it get placed. This is because our
code gets compiled independently of the notepad code. Therefore our
instuctions with addresses will not get relocated by the win32 loader.
Lets start from the beginning and deal with the problems as they
occur on our path.
The first question to deal with is where should we place our
code inside the target. The easiest way is to append our code to the
last section of notepad.exe.
Let's examine the header of the last section, which happens to be
the .reloc section.
At the dos prompt type 'pedump notepad.exe > out.txt'. If you have
downloaded pedump, this will make a file with the header information
of notepad.
The last section header shows :
06 .reloc VirtSize: 0000091E VirtAddr: 0000B000
raw data offs: 00008000 raw data size: 00000A00
relocation offs: 00000000 relocations: 00000000
line # offs: 00000000 line #'s: 00000000
characteristics: 42000040
INITIALIZED_DATA MEM_DISCARDABLE MEM_READ
We see that there is a space of A00h-91Eh = E2h bytes left in the
section. For me this was not enough, mainly because i am a bad coder
and produce too big code. So, first thing that should be done is to
increase the size of this section. I increased both Virtual Size and
raw data size to B00h.
Our code needs also to write to its variables, so another thing we
have to do with this section is to add a MEM_WRITE property to its
flags.
An easy way to find the file-offsets of the places we want to change
is to load notepad.exe with Hiew. You will find the header easily
because the first 8 bytes contain its name, ".reloc" in this case.
This section header tells us also what the RVA of our code is going
to be. The RVA of the section itself is B000h , it has 91eh bytes, so
that means the RVA of our code will be B91Eh. (As a reminder, a RVA
is just an offset/distance to the image base)
This knowledge is needed for the jumps from notepad to our code and
vice versa. Knowing that in 32 bit programs all jumps get compiled to
relative jumps (opcode is E9), we can compile those jumps (total of 4)
in tasm with :
jmp $+ (RVA_of_destination_code - RVA_of_current_instruction)
For example, we needed a jump from 00401156 to Our_code_1.
We know the RVA of Our_code_1 is B91Eh. So we can produce the opcode
for this jump with : jmp $+(B91Eh - 1156h) .
Another point of concern is that our code makes use of 'local'
variables, like "mov dwHeigth,ebx". In order to be able to
read/write to variables without needing relocation we can use the
same old good trick that all exe-protectors and virii have been
using until today :
call next_instruction
next_instruction:
pop ebp
sub ebp, offset next_instruction ;ebp has now 'delta' offset
mov [ebp + offset dwHeigth], ebx
This code is independent from the memory location it gets placed at.
There is just one more thing left to take cafe of : the calls to
the api functions. If you examine with Hiew the calls to api's from
the original code of notepad, you will see that in reality they have
the form :
call [address_of_a_dword_var]
So, they are infact indirect calls trough a dword variable which
contains the address of the api function in question.
Win95 uses this method of calling api's because in this way it
needs to put the address of the relevant api function at only one
place.
So, for us to make use of an api function we need to find out the
RVA of the dword variable that contains the address of the api
function we want to use.
This can be done by examining the operand of the inscruction in Hiew.
For example for a SendMessageA call Hiew shows :
FF1554744000 call SendMessageA ;USER32.dll
Which in reality is
FF1554744000 call [00407454]
So, the RVA of this variable is 7454. To calculate the address of
this variable in memory you still have to calculate the image base,
which is : EIP_At_start_of_own_code - B91Eh. Then the address of the
variable is ofcourse : image base + RVA.
In practice :
Our_code_1: ; <-- RVA = B91Eh
...
...
call next_instruction
next_instruction:
pop ebp
sub ebp, offset next_instruction ; ebp has now 'delta' offset
...
...
mov eax,ebp
add eax, offset Our_code_1 ; <-- EAX has now the
; the EIP at Our_code_1
sub eax, B91eH ; <-- EAX has image base
add eax, 7454h ; <-- [EAX] has now the
; address of SendMessageA
call [eax] ; call SendMessageA
The same method can be used to address variables of the original
code of notepad, like the variable with (virtual) address 406000h that
contained the handle of the main window.
The complete source to patch notepad.exe and add the extra code
is included with this tutorial. Also is included is the notepad.exe
that i have.
DOWNLOAD EVERYTHING razzia.zip HERE!
Final notes
We have seen that we can add any code we want to any win32
program.
The question now is how we can know what code we have add to a
crippled program ?
Sometimes it is obvious. For example when we are dealing with a
save disabled program and the file it should generate is a text file.
If the output file is not a text file, it can be helpful to check
the Swag file format encyclopedia.
(available at http://www.gdsoft.com/swag/swag.html).
Sometimes examining old versions of a program can help. Or examining
'lite' versions of programs which are not crippled.
Also examining the load function can be helpful.
Another question is what should we do if we need to use api functions
which are not imported by our target.
The easiest solution to that is just to replace 2 entries in the
import table with the kernel32 functions GetModuleHandle and
GetProcAdress. With those two functions you can get the address of
every function you fancy.
If you also change the entry point RVA to let it point at some code
that you have added, which will put the addresses of the two original
functions at the places they were supposed to get, then there will be no
problem.
The Greetings
I would like to take advantage of this oppurtunity and thank the
following persons for everything they thought me in the past :
THE_OWL, rANDOM, aCP, madmax, lost_soul and xygorf.
Also i would like to greet the wonderfull ppl in #cracking,
#cracking4newbies and #Pc97 :
Psychotrn, lordbyte, j0b, ThePharao, musashi, sharp, mailman, dwolf,
niabi, josephco, StarDogg Champion (Brain is fried dude ;--), tHATDUDE,
cyberlatin, blorgth, sice_boy, jacky_x, teraphy and everybody
else i forgot..
Finally my special thanks go to +ORC, Fravia+ and +gthorne.
august 1997 - razzia
(c) razzia 1997. All rights reserved
You are deep inside Fravia's page of reverse engineering,
choose your way out:
project 6
homepage
links
anonymity
+ORC
students' essays
academy database
tools
cocktails
antismut CGI-scripts
search_forms
mail_Fravia
Is reverse engineering illegal?