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  1. #1
    Dwar
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    [Asm] Codecaving tutorial

    Codecaving tutorial


    Codecaving is, basically, to put a jump before a target offset that needs to be patched to an empty or unimportant zone in memory, put your modified code there, and then jump back under the target address.

    What's the point of doing this?
    Codecaving allows you to get the effects of modifying an offset that could be blacklisted by poor anti-cheat software (ex: Warden in Warcraft 3), since you never modify it directly.
    You could therefore take the offsets of some hack and make them almost "undetectable" (considering you keep that hack private or simply for yourself).


    Pseudo example:

    Before the code cave and patching:
    • 1 Address 1 and its content
      2 Target
      3 Address 3 and its content

      4 Unimportant stuff
      5 Unimportant stuff
      6 Unimportant stuff


    (Unimportant stuff is mostly 00's and INT3's)

    After the Code cave and patching:
    • 1 JUMP to address 4
      2 Target
      3 Address 3 and its content

      4 Address 1's content
      5 Patched target
      6 JUMP back to address 3


    As you can see, the target code is still being patched, but not at Address 5 instead of Address 2.

    Let's do a codecave now!

    Things you will need:
    • An offset to patch
    • A real-time debugger (OllyDBG)


    For this example, we are going to use some stupid offset I found while offset-hunting in Warcraft 3:

    6F39B991 BA 08000000 MOV EDX,8

    to

    6F39B991 BA 00000000 MOV EDX,0

    Effect: Removes ground textures (ground becomes all black).

    Let's analyze the bunch of code over that address. Open up Warcraft 3 and get in a custom game. Then start OllyDBG and attach war3.exe, then right click -> go to -> 6F39B991 or CTRL+G -> 6F39B991 (you may need to do this twice to get to the address).

    We must first find some empty memory zone in which we will input our code.

    By scrolling down to the end of Game.dll, I found out this neat little place:
     6F85BE24   0000             ADD BYTE PTR DS:[EAX],AL
    6F85BE26 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE28 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE2A 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE2C 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE2E 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE30 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE32 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE34 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE36 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE38 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE3A 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE3C 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE3E 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE40 0000 ADD BYTE PTR DS:[EAX],AL
    6F85BE42 0000 ADD BYTE PTR DS:[EAX],AL

    etc.

    Then let'a go back to our target address, 6F39B991, and analyse what's around it.
     6F39B974   8D4C24 24        LEA ECX,DWORD PTR SS:[ESP+24]
    6F39B978 E8 E3A8C7FF CALL Game.6F016260
    => 6F39B97D E8 1E50C7FF CALL Game.6F0109A0
    6F39B982 39AE 30030000 CMP DWORD PTR DS],EBP
    6F39B988 74 07 JE SHORT Game.6F39B991
    6F39B98A 8BCE MOV ECX,ESI
    6F39B98C E8 4FEDFFFF CALL Game.6F39A6E0
    6F39B991 BA 08000000 MOV EDX,8 // Target offset
    6F39B996 8D4C24 5C LEA ECX,DWORD PTR SS:[ESP+5C] // Return address

    We will use a normal jump to get there.

    6F39B97D seems like a good place to start our jump since it is 5 bytes long. Take that address's info down in notepad or w/e.

    6F39B97D E8 1E50C7FF CALL Game.6F0109A0

    PAUSE OllyDBG and change this line:

    6F39B97D E8 1E50C7FF CALL Game.6F0109A0

    to

    6F39B97D E9 A2044C00 JMP Game.6F85BE24

    To do this, Select then right click the address -> Assemble or simply press spacebar then type: jmp 6F85BE24 and press enter (don't fill with NOP's)

    Now press enter on that address to go the empty zone.

    On 6F85BE24, we will place the info of the address we took down in note. It was

    CALL Game.6F0109A0

    Like we did just a minute ago, we will right click -> assemble.

    Type call 6F0109A0 and press enter.

    Now that we have added this line, we need to also add everything else that was between it and our target address:
     6F39B982   39AE 30030000    CMP DWORD PTR DS:[ESI+330],EBP
    6F39B988 74 07 JE SHORT Game.6F39B991
    6F39B98A 8BCE MOV ECX,ESI
    6F39B98C E8 4FEDFFFF CALL Game.6F39A6E0
    6F39B991 BA 08000000 MOV EDX,8 // target address

    So, just like we did before, click on the line below the one we modified and assemble this:

    Then, on the next line, assemble this twice:

    The reason we are putting 2 NOP's here instead of JE SHORT 6F39B991 is because 6F39B991 is way out of range for the short jump.

    Then, on the next line:

    Then, on the next line:

    Now let's modify MOV EDX,8 to MOV EDX,0 to get the black floor effect in-game.

    Assemble at 6F85BE38:

    Alright, we are almost done! What we need to do now is jump back to 6F39B996, the address that was just below our target.

    Assemble this on the next line:

    Your memory should now look like this:

    Your jump:

    Your code cave:

    Alright, now we need to jump to the memory zone we just made.

    Assemble this at:


    You are now ready to press on PLAY in OllyDBG. Your floor should be black.
    © Tyrano
    Please, post your questions on forum, not by PM or mail

    I spend my time, so please pay a little bit of your time to keep world in equilibrium

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  3. #2
    pohkak
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    Спасибо за туто Двар . Немог бы ты описать как заменить в клиенте оригинальную строку на желаемую ?
    я не имею ввиду оллидбг . Я имею ввиду если я хочю методом иньекта переписать оригинальный код своим кодом как это будет выглядеть ?
    Are you hackers ?...No, wee are russians!

  4. #3
    SteelAthena
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    Question [Thank you] and Need an Suggestion

    First i want to say thanks for your great post thats so interesting and motivating me so i want to try it to my game.

    After got the address from What Code Access to the this pointer, i have this assembly (below) and call it Original Code.

    Code:
    005B4806   E8 66222200      CALL 007D6A71 //Entry Code Cave
    005B480B   8B4424 1C        MOV EAX,DWORD PTR SS:[ESP+1C]
    005B480F   8BD3             MOV EDX,EBX
    005B4811   C1E2 05          SHL EDX,5
    005B4814   2BD3             SUB EDX,EBX
    005B4816   8D0C50           LEA ECX,DWORD PTR DS:[EAX+EDX*2]
    005B4819   0FBE47 4B        MOVSX EAX,BYTE PTR DS:[EDI+4B]
    005B481D   8B56 04          MOV EDX,DWORD PTR DS:[ESI+4]
    005B4820   8BF0             MOV ESI,EAX
    005B4822   0FAF748A 1C      IMUL ESI,DWORD PTR DS:[EDX+ECX*4+1C]
    005B4827   8B4424 20        MOV EAX,DWORD PTR SS:[ESP+20]
    005B482B   29B0 D4020000    SUB DWORD PTR DS:[EAX+2D4],ESI // Target offset
    005B4831   E9 BB010000      JMP 005B49F1 // Return address
    And try to make JMP so the code will jump to my code cave like this, lets call it Code after Injection :

    Code after Injection :
    Code:
    005B4806   E8 66222200      JMP 00947B8C //Entry Code Cave
    005B480B   8B4424 1C        MOV EAX,DWORD PTR SS:[ESP+1C]
    005B480F   8BD3             MOV EDX,EBX
    005B4811   C1E2 05          SHL EDX,5
    005B4814   2BD3             SUB EDX,EBX
    005B4816   8D0C50           LEA ECX,DWORD PTR DS:[EAX+EDX*2]
    005B4819   0FBE47 4B        MOVSX EAX,BYTE PTR DS:[EDI+4B]
    005B481D   8B56 04          MOV EDX,DWORD PTR DS:[ESI+4]
    005B4820   8BF0             MOV ESI,EAX
    005B4822   0FAF748A 1C      IMUL ESI,DWORD PTR DS:[EDX+ECX*4+1C]
    005B4827   8B4424 20        MOV EAX,DWORD PTR SS:[ESP+20]
    005B482B   29B0 D4020000    SUB DWORD PTR DS:[EAX+2D4],ESI
    005B4831   E9 BB010000      JMP 005B49F1
    And my Code Cave is more like this, lets call it Code Cave :

    Code Cave :
    Code:
    00947B8C   E8 E0EEE8FF      CALL GodsWar.007D6A71
    00947B91   8B4424 1C        MOV EAX,DWORD PTR SS:[ESP+1C]
    00947B95   8BD3             MOV EDX,EBX
    00947B97   C1E2 05          SHL EDX,5
    00947B9A   2BD3             SUB EDX,EBX
    00947B9C   8D0C50           LEA ECX,DWORD PTR DS:[EAX+EDX*2]
    00947B9F   0FBE47 4B        MOVSX EAX,BYTE PTR DS:[EDI+4B]
    00947BA3   8B56 04          MOV EDX,DWORD PTR DS:[ESI+4]
    00947BA6   8BF0             MOV ESI,EAX
    00947BA8   0FAF748A 1C      IMUL ESI,DWORD PTR DS:[EDX+ECX*4+1C]
    00947BAD   8B4424 20        MOV EAX,DWORD PTR SS:[ESP+20]
    00947BB1   83A8 D4020000 00 SUB DWORD PTR DS:[EAX+2D4],0
    00947BB8  ^E9 34CEC6FF      JMP GodsWar.005B49F1
    00947BBD  ^E9 44CCC6FF      JMP GodsWar.00947B8C
    I have try this and the game crashed and disconecting. I need a suggestion or an opinion, just want to learn assembly and disassembly by practicing, im so noob.

    Thanks in advance for your help.


    Warm Regards,


    Donny

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