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Dll DIY: 另一种修改方式
时间：2004/10/15 0:58:00来源：本站整理作者：蓝点我要评论(0)
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PEdiy已经被大牛门玩的烂熟，这里来点dll的修改，仅供一笑:)
问题来源于这里：hX*p://www.cpnn.com.cn/，网页文件被加密为btk文件，如果你查看源文件只会看到被加密后的文本。它是怎么作的？
首先浏览网页时，会下载ie的一个插件，文件名是：bk_paper.dll，加密后的文本由这个dll来负责解密，以此保证不能很容易的拷贝出来。
经过简单的分析和跟踪，发现解密出的内容是以明文存在于内存中，这样就有机会将它保存出来。为保存文件至少需要下面几个api: createfile, setfilepointer, writefile, closehandle。观察原文件，并无这几个。如果让我来手工将它们导入到IT表中，肯定是要晕倒~~ 找找其它机会，发现引入表中有：loadlibrary, getprocaddress, freelibrary，呵呵，有机可乘。
记得hying说过，dll 的 entry 会在加载和卸载时各运行一次，那就可以在它加载时再load一个自己写的dll，取得保存函数的地址，并保存起来；在退出时将自己的dll也卸载掉，这样会是很安全；在内存中出现明文的时刻调用我们的保存函数就可以存盘。这样作的好处是对原文件修改的不多，自己定制的dll有很强的灵活性。
下面看看具体作法：
1。编译一个用于对原文件打补丁的代码：
;crack source.asm
.386
.model flat,stdcall
option casemap:none
include \masm32\include\windows.inc
include \masm32\include\user32.inc
include \masm32\include\kernel32.inc
includelib \masm32\lib\kernel32.lib
includelib \masm32\lib\user32.lib
;.data
;hDll dd 0
; dd 0
.code
.RADIX 16
start: ;copy the code to imagebase + 3030
nop
nop
call delta
delta:
pop esi ;
add esi, 0a9 ;imagebase + 30e0
push esi
pop edi ;edi=esi=temp var, offset of 2 dwords
;mov esi, 100030e0
lodsd ;eax=dll handle
test eax, eax ;if "save.dll" is loaded
jnz UnLoadDll
call LoadDll ;if not, load it.
db "save.dll", 0
LoadDll:
call LoadDlla
jmp LoadDllb
LoadDlla:
db 0ff, 25, 0a4, 45, 01, 10 ;use api-LoadLibrary
;call LoadLibrary
LoadDllb:
nop
nop
nop
test eax, eax
jz Next1 ;if error occurs, then returns
stosd ;else save dll handle
call FindFunc
db "SaveIt", 0 ;
FindFunc:
push eax
db 0ff, 15, 9c, 45, 01, 10 ;call GetProcAddress
nop
nop
nop
nop
test eax, eax
jz Next1
stosd ;if no error, save it.
jmp Next1
UnLoadDll:
push eax
db 0ff, 15, 0c0, 45, 01, 10 ;call FreeLibrary
Next1:
nop
push 01
pop eax
ret 0ch
;invoke ExitProcess,NULL
end start
2。上面的编译完成后用winhex将二进制代码复制到要打补丁的位置，我选的是：imagebase+3030。
来看一下修改前后的关键文件，还要记得把.text的节属性改为可写~：
dll entry的原始状态：
:10001ECF 6828390110 push 10013928
:10001ED4 E85EF6FFFF call 10001537
* Referenced by a (U)nconditional or (C)onditional Jump at Addresses:
|:10001EC6(U), :10001ECD(C)
|
:10001ED9 6A01 push 00000001
:10001EDB 58 pop eax ；eax=1
:10001EDC C20C00 ret 000C ；返回
:10001EDF CC int 03
:10001EE0 CC int 03
:10001EE1 CC int 03
:10001EE2 CC int 03
修改后的：
:10001ED4 E85EF6FFFF call 10001537
* Referenced by a (U)nconditional or (C)onditional Jump at Addresses:
|:10001EC6(U), :10001ECD(C)
|
:10001ED9 E952110000 jmp 10003030
:10001EDE CC int 03
...
|
:10003028 B809000280 mov eax, 80020009
:1000302D EBD3 jmp 10003002
:1000302F CC int 03
* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:10001ED9(U)
|
:10003030 90 nop
:10003031 90 nop
:10003032 E800000000 call 10003037
* Referenced by a CALL at Address:
|:10003032
|
:10003037 5E pop esi
:10003038 81C6A9000000 add esi, 000000A9
:1000303E 56 push esi
:1000303F 5F pop edi
:10003040 AD lodsd
:10003041 85C0 test eax, eax
:10003043 7541 jne 10003086
:10003045 E809000000 call 10003053
:1000304A 7361 jnb 100030AD
:1000304C 7665 jbe 100030B3
:1000304E 2E BYTE 02eh
:1000304F 64 BYTE 064h
:10003050 6C insb
:10003051 6C insb
:10003052 00 BYTE 00h
* Referenced by a CALL at Address:
|:10003045
|
* Reference To: KERNEL32.LoadLibraryA, Ord:0000h
|
:10003053 E802000000 Call 1000305A
:10003058 EB06 jmp 10003060
* Reference To: KERNEL32.LoadLibraryA, Ord:0000h
|
:1000305A FF25A4450110 Jmp dword ptr [100145A4]
* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:10003058(U)
|
:10003060 90 nop
:10003061 90 nop
:10003062 90 nop
:10003063 85C0 test eax, eax
:10003065 7426 je 1000308D
:10003067 AB stosd
:10003068 E807000000 call 10003074
:1000306D 53 push ebx
:1000306E 61 popad
:1000306F 7665 jbe 100030D6
:10003071 49 dec ecx
:10003072 7400 je 10003074
* Referenced by a CALL at Address:
|:10003068
|
* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:10003072(C)
|
:10003074 50 push eax
* Reference To: KERNEL32.GetProcAddress, Ord:0000h
|
:10003075 FF159C450110 Call dword ptr [1001459C]
:1000307B 90 nop
:1000307C 90 nop
:1000307D 90 nop
:1000307E 90 nop
:1000307F 85C0 test eax, eax
:10003081 740A je 1000308D
:10003083 AB stosd
:10003084 EB07 jmp 1000308D
* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:10003043(C)
|
:10003086 50 push eax
* Reference To: KERNEL32.FreeLibrary, Ord:0000h
|
:10003087 FF15C0450110 Call dword ptr [100145C0]
* Referenced by a (U)nconditional or (C)onditional Jump at Addresses:
|:10003065(C), :10003081(C), :10003084(U)
|
:1000308D 90 nop
:1000308E 6A01 push 00000001
:10003090 58 pop eax
:10003091 C20C00 ret 000C
:10003094 CC int 03
:10003095 CC int 03
:10003096 CC int 03
:10003097 CC int 03
:10003098 CC int 03
3。修改原文件，在内存中有明码处调用我们的函数来保存：
:1000B263 8BFC mov edi, esp
:1000B265 F3 repz
:1000B266 A5 movsd
:1000B267 8D8D6CE2FFFF lea ecx, dword ptr [ebp+FFFFE26C]
:1000B26D E80960FFFF call 1000127B
:1000B272 8DB535EFFFFF lea esi, dword ptr [ebp+FFFFEF35]
:1000B278 8D7DF4 lea edi, dword ptr [ebp-0C]
:1000B27B A5 movsd
:1000B27C 33C0 xor eax, eax
:1000B27E 3945F0 cmp dword ptr [ebp-10], eax
:1000B281 A5 movsd
:1000B282 7616 jbe 1000B29A
:1000B284 8B4DE8 mov ecx, dword ptr [ebp-18]
:1000B287 8B55EC mov edx, dword ptr [ebp-14]
:1000B28A 8D0CCA lea ecx, dword ptr [edx+8*ecx]
* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:1000B298(C)
|
:1000B28D 8A5405F4 mov dl, byte ptr [ebp+eax-0C]
:1000B291 301401 xor byte ptr [ecx+eax], dl
:1000B294 40 inc eax
:1000B295 3B45F0 cmp eax, dword ptr [ebp-10]
:1000B298 72F3 jb 1000B28D
* Referenced by a (U)nconditional or (C)onditional Jump at Address:
|:1000B282(C)
|
:1000B29A 8D8D40EFFFFF lea ecx, dword ptr [ebp+FFFFEF40]
:1000B2A0 E87462FFFF call 10001519
:1000B2A5 8D8D6CE2FFFF lea ecx, dword ptr [ebp+FFFFE26C]
:1000B2AB E8915DFFFF call 10001041
:1000B2B0 8B45EC mov eax, dword ptr [ebp-14] ;eax指向明文，所以要改一下
:1000B2B3 5F pop edi
:1000B2B4 5E pop esi
:1000B2B5 5B pop ebx
:1000B2B6 C9 leave
:1000B2B7 C20400 ret 0004
:1000B2BA CC int 03
:1000B2BB CC int 03
:1000B2BC CC int 03
:1000B2BD CC int 03
:1000B2BE CC int 03
:1000B2BF CC int 03
:1000B2C0 CC int 03
:1000B2C1 CC int 03
:1000B2C2 CC int 03
修改后的：
:1000B2B4 5E pop esi
:1000B2B5 5B pop ebx
:1000B2B6 C9 leave
:1000B2B7 FF15E4300010 call dword ptr [100030E4] ；懒了点，应该先检查一下是否为空
:1000B2BD C20400 ret 0004
4。自定saveit函数，eax为明文首址，存文件即可，记得寄存器和状态的保护和恢复。
.586
.model flat,stdcall
option casemap:none
include \masm32\include\windows.inc
include \masm32\include\user32.inc
include \masm32\include\kernel32.inc
includelib \masm32\lib\user32.lib
includelib \masm32\lib\kernel32.lib
.data
;.RADIX 16
;MsgTitle db "sss", 0
;MsgContend db "aaa", 0
dByteDone dd 0
.code
.RADIX 16
align 2
DllEntry proc hInstDLL:HINSTANCE, reason:DWORD, reserved1:DWORD
xor eax, eax
inc eax
ret
DllEntry Endp
;
SaveIt proc
pusha
pushf
;save eax
push eax
pop ebx
invoke lstrlen, eax
mov esi, eax
test eax, eax
jz CreateErr
;createfile
xor eax, eax
push eax
push eax
push OPEN_ALWAYS
push eax
push eax
push GENERIC_WRITE
call CreateIt
db "c:\iloveyou.txt", 0
CreateIt:
call CreateFileA ;createfile
mov edi, eax
inc eax
jz CreateErr
dec eax
push FILE_BEGIN
xor eax, eax
push eax
push eax
push edi
call SetFilePointer ;set pointer
;
mov dByteDone,0
push 0
push offset dByteDone
push esi
push ebx
push edi
call WriteFile ;write to file
;
invoke CloseHandle, edi ;close
CreateErr:
popf
popa
ret
SaveIt endp
End DllEntry
因为我很懒，都给它存到一个文件中，后来的内容会覆盖先前的~~手可要快一点。。
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