VMProtect CRC Handling (CRC Checks) Part 1

Hello Friends,
Today I am gonna share a trick for bypassing the CRC checks of VMProtect. VMProtect is using a hash system to verify the checksum of file. Here I have a file protected by VMProtect. I patched a byte and now It is showing a Message Box.


This is the MessageBox that will appear if we modify the Original file. Our goal is bypass the check and make the file working.


For bypassing CRC checks. Firstly we gonna set a HWBP on execution on api CreateFileW and then HWBP on CreateFileMappingW and then MapViewOfFile. Our last bp is MapViewOfFile.


Once the last HWBP executed clear all the Breakpoints and press CTRL+F4 and we will reached at the end of the function. Now we can see that the EAX holds the VA of our file. In my case VA is 01AC0000

Now lets find the VA 01AC0000 in memory and set memory breakpoint on access and execute the target. 

After execution we reached at
 00630A5E   3202             XOR AL,BYTE PTR DS:[EDX]                 ; crc function break


Lets investigate this function. Press F7 and see whats happening. I analysed the function and found some values.

0062FB53   D0ED             SHR CH,1                                 ; loop start
0062FB55   89C1             MOV ECX,EAX
0062FB57   51               PUSH ECX
0062FB58   60               PUSHAD
0062FB59   E8 3D150000      CALL VMProtec.0063109B
0063109B   C1E0 07          SHL EAX,0x7
0063109E   66:0FA3DB        BT BX,BX
006310A2   9C               PUSHFD
006310A3   C1E9 19          SHR ECX,0x19
006310A6   F9               STC
006310A7  ^E9 AAF9FFFF      JMP VMProtec.00630A56
00630A56   F9               STC
00630A57   09C8             OR EAX,ECX
00630A59   F9               STC
00630A5A   0FBAE5 15        BT EBP,0x15
00630A5E   3202             XOR AL,BYTE PTR DS:[EDX]                 ; crc function break
00630A60  ^E9 ACE3FFFF      JMP VMProtec.0062EE11
0062EE11   66:C74424 08 030>MOV WORD PTR SS:[ESP+0x8],0x503
0062EE18   42               INC EDX
0062EE19   68 D7988109      PUSH 0x98198D7
0062EE1E   891C24           MOV DWORD PTR SS:[ESP],EBX
0062EE21   68 E8F9D11A      PUSH 0x1AD1F9E8
0062EE26   E9 3F1F0000      JMP VMProtec.00630D6A
00630D6A   FF4D 00          DEC DWORD PTR SS:[EBP]                   ; length of bytes
00630D6D   53               PUSH EBX
00630D6E   E8 52DDFFFF      CALL VMProtec.0062EAC5
0062EAC5   8D6424 3C        LEA ESP,DWORD PTR SS:[ESP+0x3C]
0062EAC9   0F85 84100000    JNZ VMProtec.0062FB53                    ; loop jump
0062EACF   68 6D5A4433      PUSH 0x33445A6D
0062EAD4   8945 00          MOV DWORD PTR SS:[EBP],EAX               ; final hash
0062EAD7   51               PUSH ECX
0062EAD8   9C               PUSHFD
0062EAD9   60               PUSHAD
0062EADA   8D6424 2C        LEA ESP,DWORD PTR SS:[ESP+0x2C]
After some analysis I found that there is a loop function which is creating a hash for a fixed size of bytes. I posted the whole function above. I removed the mem bp and put a soft bp at VA 0062EAD4 because this is the first write after loop. Now run the target. EAX holds a hash in my case hash is 99FE991A

Now I decided to compare the hash with original file. So I fire up olly and load the Original target and do the same procedure above after stop at VA 0062EAD4 I saw that EAX holds different hash. Now copy the eax and put it at our target and try to run. Bingo! the file run without any error. It means we need to write an inline patch for bypassing it purely. We will do it in next part.

Hopefully this will helps for bypassing VMP CRC.

PS : My English is not so good but I think people can understand my feelings. :D. If you have any questions regarding this please let me know.
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Author

Hi, Its me GautamGreat. A Reverser, Programmer and a developer. Developed a lot of programs. Cracking is my hobby I' am student of Class 12th, Who has a mobile phones repairing office. A blogger from heart and a student from mind.

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