Windows Reverse Engineering - Reverse Engineering Examples
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Before beginning your practical journey, there is one final issue to note. Similar to software debugging, reverse engineering by definition goes in reverse. In other words, you must be able to think backward. Zen meditation skills will serve you better than many years of formal programming education. If you are good at solving verbal brain-teaser riddles on long trips with friends, you will probably be good at RCE. In fact, master reversers like +Fravia recommend cracking while intoxicated with a mixture of strong alcoholic beverages. While for health reasons we cannot recommend this method, you may find that a relaxing cup of hot tea unwinds your mind and allows you to think in reverse. The following segments walk you through live examples of Windows reverse engineering.
Since it is illegal to defeat protections on copyrighted works, reverse engineers now program their own protection schemes for teaching purposes. Thus, crackmes are small programs that contain the heart of the protection scheme and little else.
Example 1: A Sample Crackme
Example 1 is Muad’Dib’s Crackme #1.
The sample binaries (crackmes) used in this chapter may be downloaded from our web site at http://www.securitywarrior.com.
This is a simple program, with a twist. The program’s only function is to keep you from closing it. For example, when you run the program you will see an Exit button. However, pressing the Exit button does not work (on purpose). Instead, it presents you with a nag screen that says, “Your job is to make me work as an exit button” (Figure 2-12).
Thus, the crackme emulates shareware or software that has features removed or restricted to the user (i.e., crippleware). Your job is to enable the program in order to make it fully functional. Fortunately, the program itself gives you a great clue. By searching the disassembled program for the following string:
"Your job is to make me work as an exit button"
you will probably be able to trace back to find the jump in the program that leads to functionality—i.e., a working Exit button.
Once you have installed IDA Pro, open your target (in our case, Muad’Dib’s Crackme #1) and wait for it to disassemble. You will be looking at the bare, naked ASM. Go straight for the protection by searching the convenient list of strings that IDA Pro has extracted (Figure 2-13).
Double-clicking on our target string takes us directly to the target code in the disassembly (Figure 2-14).
We arrive at this code:
*Reference To: KERNEL32.ExitProcess, Ord:0075h |
:00401024 E843000000 Call 0040106C
;( ThisCalls ExitProcess when we click on theWindows Exit Cross)
:00401029 55 push ebp
:0040102A 8BEC mov ebp, esp
:0040102C 817D0C11010000 cmp dword ptr [ebp+0C], 00000111
:00401033 751F jne 00401054
:00401035 8B4510 mov eax, dword ptr [ebp+10]
:00401038 6683F864 cmp ax, 0064
:0040103C 752A jne 00401068
:0040103E 6A00 push 00000000
*Possible StringData Ref from Data Obj ->"GOAL:"
|
:00401040 682F304000 push 0040302F
; This references the text in the MessageBox *Possible StringData Ref from Data Obj ->"Your job is to make me work as an exit button!" |
:00401045 6800304000 push 00403000
:0040104A FF7508 push [ebp+08]
;These lines push the Caption and Handle of the MessageBox *Reference To: USER32.MessageBoxA, Ord:01BBh
:0040104D E832000000 Call 00401080
:00401053 EB2A jmp 00401068
This is the call to the annoying message box that we want to bypass! We need to patch this address to jump to the Exit Process API. This is the heart of the protection.
Looking back at line 401024, we see it calls the exit process 0040106C, as follows:
*Referenced by a CALL at Address:
|:00401024 ;This made the call to 0040106C
*Reference To: KERNEL32.ExitProcess, Ord:0075h
This is the Exit Process API call that we need.
|:0040106C FF2504204000 jmp dword ptr [00402004]
Thus, we will patch with this jump instead. We replace the bytes at offsets 40104D and 401053 with those at offset 40106C, and when we click on the Exit button, the program will exit and the nagging message box will not appear.
The best way to patch it is to replace these lines:
:0040104D E832000000 Call 00401080
:00401053 EB2A jmp 00401068
with the following:
:0040104D FF2504204000 jmp dword ptr [00402004]
:00401053 90 nop
Thus, 0040104D now jumps to the ExitProcess address. The program exits appropriately when we click on either the X or the Exit button. 00401053 is extraneous, so we can just NOP it; this involves changing the JMP to a NOP (no operation).
In order to do the actual opcode patching, you need to open the program in a hex editor. After you have installed the hex editor, simply right-click the binary program in Windows and select “open with Ultra Edit.” You will see the raw hex code (Figure 2-15) ready to be patched.
How do we find the bytes that we need to patch? Search the hex dump for a unique string of hex bytes that represents the target code. For example, to find:
:0040104D E832000000 Call 00401080
:00401053 EB2A jmp 00401068
we search for its unique hex string (Figure 2-16):
E832000000EB2A
The key is to search for a hex string that is long enough that it will be unique in the application.
Make sure to search using hex, rather than ASCII.
Once you have found the target bytes, carefully replace them to bypass the jump. Then, simply save the binary application again and run it. In our example, the program exits properly when you click the Exit button.
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Next: Example 2: Reversing Malicious Code >>
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