Reverse Engineering VBX Custom Controls

("function VBRuntime")

by +PopJack

(26 September 1997)

Courtesy of reverser's page of reverse engineering

Well, it was about time... visual basic reversing seems to be VERY neglected lately... a pity, because this language can (slowly) be used to prepare some interesting "quick crack tools", as the +HCU will demontrate next year with two projects connected to this one: Visual Basic tools for reverse engineering and Visual basic tools for protecting your anonymity... as you see there is a lot of work ahead also for our friends "visualbasers"!.
This interesting essay by +PopJack opens the "hunt" for vb controls. We are still awaiting the promised tutorial on OCX reverse engineering by Cat (Ola, amigo, estas muerto?), let's hope that +PopJack will send his essay asap

Reverse Engineering VBX Custom Controls by +PopJack (8 September 1997)
Tools needed: SoftICE for Windows (only sometimes) W32Dasm (only sometimes) A hex editor (I use Hexpert ) Visual Basic (I'm using here version 3) Although cracking Visual Basic programs can be very boring owing to the calls to VBRunxxx.dll, VBX controls are written in C or C++ and normally the protections found in these controls are very simple. VBX Custom controls are used in Visual Basic, and are now being replaced by OCX controls (more on this on the next essay). Some of the protections are listed below: 1) Nag screens that appear when the VBX is loaded in memory. 2) Not allowing you to use the VBX in the VB environment or in run mode (i.e. the exe file). 3) Missing a *.LIC file Let's have a look at each of them. Example 1) The nag screens are easily spoted using W32dasm. I will show you in this example how to remove the nag screen and even make the VBX to show the registered About box. Target: HSlide.vbx Get it at: When you use the custom control it displays a nag screen for some seconds. Since the nag screen appears during a limited amount of time let's try a classical breakpoint: bpx settimer. This is where we land :0001.2032 56 push si :0001.2033 6A01 push 0001 :0001.2035 68E803 push 03E8 :0001.2038 6A00 push 0000 :0001.203A 6A00 push 0000 :0001.203C 9AFFFF0000 call USER.SETTIMER ;HERE! :0001.2041 C706F6070000 mov word ptr [07F6], 0 ;initialize counter :0001.2047 EB1F jmp 2068 :0001.2049 90 nop * Referenced by a Jump at Address:0001.1FCF(C) | :0001.204A FF06F607 inc word ptr [07F6] :0001.204E 833EF6070F cmp word ptr [07F6], F ;have we been here for 15 times? * Referenced by a Jump at Address:0001.1FDE(U) | :0001.2053 7C13 jl 2068 ;no, go away and hold nag on :0001.2055 8B760E mov si, [bp+0E] :0001.2058 56 push si :0001.2059 6A01 push 0001 :0001.205B 9AFFFF0000 call USER.KILLTIMER :0001.2060 56 push si :0001.2061 6A00 push 0000 :0001.2063 9AF21C0000 call USER.ENDDIALOG ;ok, turn off nag screen * Referenced by a Jump at Addresses:0001.1FD6(U), :0001.2053(C) | :0001.2068 33C0 xor ax, ax :0001.206A 1F pop ds :0001.206B 5E pop si :0001.206C C9 leave :0001.206D CA0A00 retf 000A The thing to do to take care of the delay is very simple isn't it? Lets do it :0001.2041 C706F6070E00 mov word ptr [07F6], 000E :0001.2047 90 nop :0001.2048 90 nop :0001.2049 90 nop Remember that our teacher +ORC told us not to use sucessive nops, but in this case there is no problem. Now the Aboutbox, which is really more interesting from the point of vue of reverse engineering. First some general info on VBX initialization: proc LIBMAIN is called only NCE upon VBX first load, and each instance of the custom control must call VBINITCC to initialize itself. So the VBINITCC is normally a good place to start tracing. Let's have a look at the dissambled file. Cortesy of W32dasm. +++++++++++++++++++ ENTRY TABLE FUNCTIONS ++++++++++++++++++ Number of Entry Table Functions = 0010 (decimal) Addr:0001.2BD2 Ord:0000d Type:00h Name: HSLIDE1 Addr:0001.2BD2 Type:00h Name: Visual Basic Horizontal Slider Custom Control Addr:0001.1382 Ord:0001d Type:FFh Name: CTLPROC {Exported} Addr:0001.20D6 Ord:0002d Type:FFh Name: LIBMAIN {Exported} Addr:0001.1FB2 Ord:0003d Type:FFh Name: COPYRIGHTPROC {Exported} Addr:0001.1D06 Ord:0004d Type:FFh Name: ABOUTPOPUPWNDPROC {Exported} Addr:0001.2070 Ord:0005d Type:FFh Name: VBINITCC {Exported} Addr:0001.3022 Ord:0006d Type:FFh Name: ___EXPORTEDSTUB {Exported} Addr:0001.1CD2 Ord:0007d Type:FFh Name: ABOUTDLGPROC {Exported} Addr:0001.20C4 Ord:0008d Type:FFh Name: VBTERMCC {Exported} Name: DialogID_00CF, # of Controls=009, Caption:"About Slider Controls" 001 - ControlID:0001, Control Class:"" Control Text:"OK" 002 - ControlID:FFFF, Control Class:"" Control Text:"" 003 - ControlID:001C, Control Class:"" Control Text:"P‚Slider Controls - HSLIDE.VBX" ......... cutted to save space 009 - ControlID:FFFF, Control Class:"" Control Text:"" Name: DialogID_00D1, # of Controls=014, Caption:"About Slider Controls (Sample)" 001 - ControlID:0001, Control Class:"" Control Text:"OK" 002 - ControlID:FFFF, Control Class:"" Control Text:"" 003 - ControlID:0018, Control Class:"" Control Text:"P‚Slider Controls - HSLIDE.VBX" ......... cutted to save space 006 - ControlID:FFFF, Control Class:"" Control Text:"This is an unregistered version of this control. It is not crippled in any way" Ok...So we have two dialogs. A good and a bad one. Searching for About (DialogID_00D1) we find it inside the Copyrightproc: * Referenced by a Jump at Address:0001.1D1A(C) | :0001.1D3E FF360008 push word ptr [0800] :0001.1D42 6A00 push 0000 * Possible Reference to Dialog: DialogID_00D1 | :0001.1D44 68D100 push 00D1 ; About Slider Controls (unregistered) :0001.1D47 68541D push SEG ADDR of Segment 0001 :0001.1D4A 68D21C push 1CD2 ;AboutdlgProc (see the ENTRY TABLE FUNCTIONS) :0001.1D4D 6A00 push 0000 :0001.1D4F 6A00 push 0000 :0001.1D51 9AF235BE1D call 0001.35F2 :0001.1D56 EBDD jmp 1D35 Stop! Time to think! If you did looked at the ENTRY TABLE FUNCTIONS things become clear. The AboutdlgProc will call the dialog indicated in the previous pushes. So to have a registered about box we replace :0001.1D44 68D100 push 00D1 ;About Slider Controls (unregistered) by :0001.1D44 68CF00 push 00CF ;About Slider Controls (registered) That's all! We are now an happy registered owner of hslide.vbx Example 2) In Visual Basic there are a set of functions to access the windows API - the so called VB API. The custom controls can determine the state of the VB application by calling a function - VBRuntime. This function resides both in VB.exe itself and in the VB runtime dll - VBRunxxx. I did some work to identify where in the code does this function returns its value. Here follows what I found: i) In the VB environment you find it at SS:8D4C. In design time it returns 0, in runmode (still in the environment) it returns 2. ii) In the exe file the runtime state value returned from the VBRunXXX.dll is in SS:6FE6 and has a value of 1. Target: CT_radio.vbx (part of the Component Toolbox) Get it at : (used to be Gamesman Inc) So how do we cut the mustard? I use bpx localinit just to get inside the custom control. Then I set a bpm ss:8D4C or bpm ss:6FE6. This is where we land: Exported fn(): VBINITCC - Ord:0006h :0001.0EA9 B8FFFF mov ax, SEG ADDR of Segment 0002 :0001.0EAC 45 inc bp :0001.0EAD 55 push bp :0001.0EAE 8BEC mov bp, sp :0001.0EB0 1E push ds :0001.0EB1 8ED8 mov ds, ax :0001.0EB3 83EC1A sub sp, 001A :0001.0EB6 56 push si :0001.0EB7 57 push di :0001.0EB8 8B7608 mov si, [bp+08] ;holds VB Version :0001.0EBB 8B7E06 mov di, [bp+06] ;HERE!,holds run mode from VBRun300.dll :0001.0EBE 0BFF or di, di :0001.0EC0 741E jz 0EE0 :0001.0EC2 6A00 push 0000 ;you don't have a license :0001.0EC4 1E push ds ;to use this control outside :0001.0EC5 68DE02 push 02DE ;the VB environment :0001.0EC8 1E push ds :0001.0EC9 680F03 push 030F :0001.0ECC 6A10 push 0010 :0001.0ECE 9AFFFF0000 call USER.MESSAGEBOX :0001.0ED3 33C0 xor ax, ax :0001.0ED5 5F pop di :0001.0ED6 5E pop si :0001.0ED7 8D66FE lea sp, [bp-02] :0001.0EDA 1F pop ds :0001.0EDB 5D pop bp :0001.0EDC 4D dec bp :0001.0EDD CA0400 retf 0004 So we jump always to the good location by replacing :0001.0EC0 741E jz 0EE0 by :0001.0EC0 EB1E jmp 0EE0 There are also more than 20 controls that use this same scheme. Example 3) Normally custom controls come with a license file that enables you to use it in the VB environment, and you should not distribute it along with the final exe file. Let's see one such case: a custom control for managing serial communications. Target : PDQCOMM2.VBX from Crescent Software Get it at: (although the control is really made by - Usually LIC files are acessed using Openfile so it is a nice entry point for us. The control must verify if it is being used or not inside the VB environment. Only in the first case should it look for the LIC file. The file validation is so simple that I made a valid license file using Softice 1.5 in 2 minutes! Let's have a look at the code around the openfile call. :0001.03C2 C80A0100 enter 010A, 00 :0001.03C6 57 push di :0001.03C7 56 push si :0001.03C8 BEFFFF mov si, FFFF :0001.03CB 33C0 xor ax, ax :0001.03CD 8946F8 mov [bp-08], ax :0001.03D0 8946F6 mov [bp-0A], ax :0001.03D3 9AD06D7800 call 0001.6DD0 :0001.03D8 50 push ax :0001.03D9 8D867EFF lea ax, [bp-0082] :0001.03DD 16 push ss :0001.03DE 50 push ax :0001.03DF 6A78 push 0078 :0001.03E1 9A29030000 call KERNEL.GETMODULEFILENAME ;looking for VB.exe :0001.03E6 8BF8 mov di, ax ; if it isn't loaded then ax=0 :0001.03E8 0BF8 or di, ax :0001.03EA 7503 jne 03EF ; jump to verify license file :0001.03EC E9E900 jmp 04D8 ....... cutted to save space * Possible StringData Ref from Data Seg 003 ->"COM4WIN2.LIC" | :0001.043B 684A00 push 004A :0001.043E 8D86F6FE lea ax, [bp-010A] :0001.0442 16 push ss :0001.0443 50 push ax :0001.0444 6A00 push 0000 :0001.0446 9AFFFF0000 call KERNEL.OPENFILE :0001.044B 8BF0 mov si, ax :0001.044D 83FEFF cmp si, FFFF ;LIC file found? :0001.0450 7503 jne 0455 ;yes go ahead :0001.0452 E98300 jmp 04D8 :0001.0455 C746F60000 mov word ptr [bp-0A], 0000 :0001.045A 50 push ax :0001.045B 8D46F6 lea ax, [bp-0A] :0001.045E 16 push ss :0001.045F 50 push ax :0001.0460 6A01 push 0001 ; read 1 byte from begining :0001.0462 9A82040000 call KERNEL._LREAD :0001.0467 48 dec ax ; 1 byte read? :0001.0468 7568 jne 04D2 :0001.046A 8B7EF8 mov di, [bp-08] :0001.046D 8976FA mov [bp-06], si :0001.0470 837EF61A cmp word ptr [bp-0A], 001A ; is byte = 1A ? :0001.0474 7413 je 0489 ; good! :0001.0476 037EF6 add di, [bp-0A] :0001.0479 56 push si :0001.047A 8D46F6 lea ax, [bp-0A] :0001.047D 16 push ss :0001.047E 50 push ax :0001.047F 6A01 push 0001 :0001.0481 9A95040000 call KERNEL._LREAD :0001.0486 48 dec ax :0001.0487 74E7 je 0470 :0001.0489 83C705 add di, 0005 :0001.048C 56 push si :0001.048D 8D46F6 lea ax, [bp-0A] :0001.0490 16 push ss :0001.0491 50 push ax :0001.0492 6A02 push 0002 :0001.0494 9AAC040000 call KERNEL._LREAD ; read 2 more bytes :0001.0499 3D0200 cmp ax, 0002 ; 2 bytes read? :0001.049C 753A jne 04D8 :0001.049E 397EF6 cmp [bp-0A], di ;is 2nd byte= 05? :0001.04A1 7535 jne 04D8 ;ah ah! Bad guy! :0001.04A3 56 push si :0001.04A4 8D46F6 lea ax, [bp-0A] :0001.04A7 16 push ss :0001.04A8 50 push ax :0001.04A9 6A02 push 0002 :0001.04AB 9AFFFF0000 call KERNEL._LREAD ;still another 2 bytes :0001.04B0 3D0200 cmp ax, 0002 :0001.04B3 7405 je 04BA ;I read 2 bytes, move on :0001.04B5 C746F60001 mov word ptr [bp-0A], 0100 :0001.04BA 56 push si :0001.04BB 9AEB040000 call KERNEL._LCLOSE ;as the name says :0001.04C0 8B4604 mov ax, [bp+04] ;I placed a chosen value in the LIC file :0001.04C3 3946F6 cmp [bp-0A], ax ;to avoid the jump :0001.04C6 7210 jb 04D8 :0001.04C8 B80100 mov ax, 0001 ;LIC file is valid! ...ret :0001.04D8 6A00 push 0000 ..... cutted to saved space :0001.04EF 33C0 xor ax, ax ;get the bad flag zero ...ret Of course we can crack the control instead of making a valid license file. The problem is that some applications will install new versions of the same control on your system... and then you will have to crack again and again. That's all for now. Hope you had fun. Once again thank you reverser+ for making available so much knowledge for us. See you +PopJack
(c) +PopJack 1997. All rights reversed
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