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shellcode编写探究
首发于奇安信攻防社区：https://forum.butian.net/share/1612
前言
shellcode是不依赖环境，放到任何地方都可以执行的机器码。shellcode的应用场景很多，本文不研究shellcode的具体应用，而只是研究编写一个shellcode需要掌握哪些知识。
ShellCode编写原则
1、不能有全局变量
因为我们编写shellcode时，使用的全局变量是自己的进程里面的全局变量，注入到别的进程里，这个地址就没用了。
2、不能使用常量字符串
和第一点原因一样，字符串常量值也是全局变量，注入到别的进程里，根本没有这个字符串。
要使用字符串，需要使用字符数组。
char s[] = {'1','2',0};
3、不能直接调用系统函数
调用系统函数的方式是间接调用(FF15)，需要从IAT表里获取API地址，每个进程的IAT表位置不同，且对方的进程可能没有导入你需要调用的函数的DLL，那么你是不能调用这个系统函数的。
所以我们需要用到 LoadLibrary 和 GetProcAddress 这两个函数，来动态获取系统API的函数指针。
但是 LoadLibrary，GetProcAddress 本身就是系统函数，它们本身就依赖IAT表，咋办呢？
解决方案是这样的：通过FS:[0x30] 找到PEB，然后通过PEB里的LDR链表 [PEB+0x0C]找到 kernel32.dll 的地址，然后我们遍历它的 IAT表，找到 LoadLibrary 和 GetProcAddress 函数。
4、不能嵌套调用其他函数
和前两点道理是一样的，本进程里的函数地址，拿到别的进程的虚拟地址空间是无效的。
TEB/PEB
每个线程都有一个TEB结构来存储线程的一些属性结构，TEB的地址用fs:[0]来获取
在0x30这个地址有一个指针指向PEB结构，PEB就是进程用来记录自己信息的一个结构
完整结构如下
在PEB的0x00c偏移有一个 Ldr _PEB_LDR_DATA结构跟进去
可以得到3个结构如下所示
InLoadOrderModuleList：模块加载的顺序
InMemoryOrderModuleList：模块在内存的顺序
InInitializationOrderModuleList：模块初始化的顺序
思路
我们一般使用api会直接使用LoadLibrary和GetProcessAddress，但是这里肯定会依赖IAT表，所以这里我们就需要自己实现api所完成的功能
TEB -> PEB -> PEB + 0x0C -> Ldr _PEB_LDR_DATA -> InLoadOrderModuleList -> kernel32.dll -> 导出表定位GetProcessAddress -> 通过找到的GetProcessAddress实现LoadLibrary
实现过程
首先我们自己定义几个结构体，因为我们不依赖系统自己实现
typedef struct _UNICODE_STRING {
USHORT Length;
USHORT MaximumLength;
PWSTR Buffer;
} UNICODE_STRING, *PUNICODE_STRING;
typedef struct _PEB_LDR_DATA
{
DWORD Length;
bool Initialized;
PVOID SsHandle;
LIST_ENTRY InLoadOrderModuleList;
LIST_ENTRY InMemoryOrderModuleList;
LIST_ENTRY InInitializationOrderModuleList;
} PEB_LDR_DATA,*PPEB_LDR_DATA;
typedef struct _LDR_DATA_TABLE_ENTRY
{
LIST_ENTRY InLoadOrderLinks;
LIST_ENTRY InMemoryOrderLinks;
LIST_ENTRY InInitializationOrderLinks;
PVOID DllBase;
PVOID EntryPoint;
UINT32 SizeOfImage;
UNICODE_STRING FullDllName;
UNICODE_STRING BaseDllName;
UINT32 Flags;
USHORT LoadCount;
USHORT TlsIndex;
LIST_ENTRY HashLinks;
PVOID SectionPointer;
UINT32 CheckSum;
UINT32 TimeDateStamp;
PVOID LoadedImports;
PVOID EntryPointActivationContext;
PVOID PatchInformation;
} LDR_DATA_TABLE_ENTRY, *PLDR_DATA_TABLE_ENTRY;
typedef HMODULE (WINAPI * PLOADLIBRARY)(LPCSTR);
typedef DWORD (WINAPI * PGETPROCADDRESS)(HMODULE, LPCSTR);
typedef DWORD (WINAPI * PMESSAGEBOX)(HWND, LPCSTR,LPCSTR,UINT);
然后定义shellcode，这里因为kernel32.dll是unicode字符串所以用两字节存储
char szKernel32[] = {'k',0,'e',0,'r',0,'n',0,'e',0,'l',0,'3',0,'2',0,'.',0,'d',0,'l',0,'l',0,0,0}; // Unicode
char szUser32[] = {'u','s','e','r','3','2','.','d','l','l',0};
char szGetProcAddress[] = {'G','e','t','P','r','o','c','A','d','d','r','e','s','s',0};
char szLoadLibrary[] = {'L','o','a','d','L','i','b','r','a','r','y','A',0};
char szMessageBox[] = {'M','e','s','s','a','g','e','B','o','x','A',0};
char szHelloShellCode[] = {'H','e','l','l','o','S','h','e','l','l','C','o','d','e',0};
找到InLoadOrderModuleList存入寄存器
__asm
{
mov eax,fs:[0x30] // PEB
mov eax,[eax+0x0C] // PEB->LDR
add eax,0x0C // LDR->InLoadOrderModuleList
mov pBeg,eax
mov eax,[eax]
mov pPLD,eax
}
找到kernel32.dll，通过遍历的方式来寻找，通过LDR指向DllBase获取基址
// Find Kerner32.dll
while (pPLD != pBeg)
{
pLast = (WORD*)pPLD->BaseDllName.Buffer;
pFirst = (WORD*)szKernel32;
while (*pFirst && *pLast == *pFirst)
pFirst++,pLast++;
if (*pFirst == *pLast)
{
dwKernelBase = (DWORD)pPLD->DllBase;
break;
}
pPLD = (LDR_DATA_TABLE_ENTRY*)pPLD->InLoadOrderLinks.Flink;
}
然后通过指针定位到导出表
// 通过指针定位到导出表
PIMAGE_DOS_HEADER pDosHeader = (PIMAGE_DOS_HEADER)dwKernelBase;
PIMAGE_NT_HEADERS pNTHeader = (PIMAGE_NT_HEADERS)((DWORD)pDosHeader + pDosHeader->e_lfanew);
PIMAGE_FILE_HEADER pPEHeader = (PIMAGE_FILE_HEADER)((DWORD)pDosHeader + pDosHeader->e_lfanew + 4);
PIMAGE_OPTIONAL_HEADER32 pOptionHeader = (PIMAGE_OPTIONAL_HEADER32)((DWORD)pPEHeader + sizeof(/images/shellcode/image_FILE_HEADER));
PIMAGE_SECTION_HEADER pSectionHeader = (PIMAGE_SECTION_HEADER)((DWORD)pOptionHeader + pPEHeader->SizeOfOptionalHeader);
PIMAGE_EXPORT_DIRECTORY pExportDirectory = (PIMAGE_EXPORT_DIRECTORY)((DWORD)dwKernelBase + pOptionHeader->DataDirectory[0].VirtualAddress);
// 导出函数地址表RVA
DWORD *pAddOfFun_Raw = (DWORD*)((DWORD)dwKernelBase + pExportDirectory->AddressOfFunctions);
// 导出函数名称表RVA
WORD *pAddOfOrd_Raw = (WORD*)((DWORD)dwKernelBase + pExportDirectory->AddressOfNameOrdinals);
// 导出函数序号表RVA
DWORD *pAddOfNames_Raw = (DWORD*)((DWORD)dwKernelBase + pExportDirectory->AddressOfNames);
还是通过遍历找到GetProcessAddress，用指针指向这个地址
DWORD dwCnt = 0;
char* pFinded = NULL, *pSrc = szGetProcAddress;
for (; dwCnt < pExportDirectory->NumberOfNames;dwCnt++)
{
pFinded = (char*)((DWORD)dwKernelBase + pAddOfNames_Raw[dwCnt]);
while (*pFinded && *pFinded == *pSrc)
pFinded++, pSrc++;
if (*pFinded == *pSrc)
{
pGetProcAddress = (PGETPROCADDRESS)(pAddOfFun_Raw[pAddOfOrd_Raw[dwCnt]] + (DWORD)dwKernelBase);
break;
}
pSrc = szGetProcAddress;
}
然后就可以使用pGetProcessAddress实现LoadLibrary和MessageBox
// 通过pGetProcAddress进行调用
pLoadLibrary = (PLOADLIBRARY)pGetProcAddress((HMODULE)dwKernelBase, szLoadLibrary);
pMessageBox = (PMESSAGEBOX)pGetProcAddress(pLoadLibrary(szUser32),szMessageBox);
pMessageBox(NULL,szHelloShellCode,0,MB_OK);
完整代码如下
// shellcode.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <windows.h>
#include <stdio.h>
#include <string.h>
typedef struct _UNICODE_STRING {
USHORT Length;
USHORT MaximumLength;
PWSTR Buffer;
} UNICODE_STRING, *PUNICODE_STRING;
typedef struct _PEB_LDR_DATA
{
DWORD Length;
bool Initialized;
PVOID SsHandle;
LIST_ENTRY InLoadOrderModuleList;
LIST_ENTRY InMemoryOrderModuleList;
LIST_ENTRY InInitializationOrderModuleList;
} PEB_LDR_DATA,*PPEB_LDR_DATA;
typedef struct _LDR_DATA_TABLE_ENTRY
{
LIST_ENTRY InLoadOrderLinks;
LIST_ENTRY InMemoryOrderLinks;
LIST_ENTRY InInitializationOrderLinks;
PVOID DllBase;
PVOID EntryPoint;
UINT32 SizeOfImage;
UNICODE_STRING FullDllName;
UNICODE_STRING BaseDllName;
UINT32 Flags;
USHORT LoadCount;
USHORT TlsIndex;
LIST_ENTRY HashLinks;
PVOID SectionPointer;
UINT32 CheckSum;
UINT32 TimeDateStamp;
PVOID LoadedImports;
PVOID EntryPointActivationContext;
PVOID PatchInformation;
} LDR_DATA_TABLE_ENTRY, *PLDR_DATA_TABLE_ENTRY;
typedef HMODULE (WINAPI * PLOADLIBRARY)(LPCSTR);
typedef DWORD (WINAPI * PGETPROCADDRESS)(HMODULE, LPCSTR);
typedef DWORD (WINAPI * PMESSAGEBOX)(HWND, LPCSTR,LPCSTR,UINT);
DWORD WINAPI ShellCode();
int main(int argc, char* argv[])
{
ShellCode();
getchar();
return 0;
}
DWORD WINAPI ShellCode()
{
PGETPROCADDRESS pGetProcAddress = NULL;
PLOADLIBRARY pLoadLibrary = NULL;
PMESSAGEBOX pMessageBox = NULL;
PLDR_DATA_TABLE_ENTRY pPLD;
PLDR_DATA_TABLE_ENTRY pBeg;
WORD *pFirst = NULL;
WORD *pLast = NULL;
DWORD ret = 0, i = 0;
DWORD dwKernelBase = 0;
char szKernel32[] = {'k',0,'e',0,'r',0,'n',0,'e',0,'l',0,'3',0,'2',0,'.',0,'d',0,'l',0,'l',0,0,0}; // Unicode
char szUser32[] = {'u','s','e','r','3','2','.','d','l','l',0};
char szGetProcAddress[] = {'G','e','t','P','r','o','c','A','d','d','r','e','s','s',0};
char szLoadLibrary[] = {'L','o','a','d','L','i','b','r','a','r','y','A',0};
char szMessageBox[] = {'M','e','s','s','a','g','e','B','o','x','A',0};
char szHelloShellCode[] = {'H','e','l','l','o','S','h','e','l','l','C','o','d','e',0};
__asm
{
mov eax,fs:[0x30] // PEB
mov eax,[eax+0x0C] // PEB->LDR
add eax,0x0C // LDR->InLoadOrderModuleList
mov pBeg,eax
mov eax,[eax]
mov pPLD,eax
}
// Find Kerner32.dll
while (pPLD != pBeg)
{
pLast = (WORD*)pPLD->BaseDllName.Buffer;
pFirst = (WORD*)szKernel32;
while (*pFirst && *pLast == *pFirst)
pFirst++,pLast++;
if (*pFirst == *pLast)
{
dwKernelBase = (DWORD)pPLD->DllBase;
break;
}
pPLD = (LDR_DATA_TABLE_ENTRY*)pPLD->InLoadOrderLinks.Flink;
}
// Kernel32.dll -> GetProcAddress
if (dwKernelBase != 0)
{
// 通过指针定位到导出表
PIMAGE_DOS_HEADER pDosHeader = (PIMAGE_DOS_HEADER)dwKernelBase;
PIMAGE_NT_HEADERS pNTHeader = (PIMAGE_NT_HEADERS)((DWORD)pDosHeader + pDosHeader->e_lfanew);
PIMAGE_FILE_HEADER pPEHeader = (PIMAGE_FILE_HEADER)((DWORD)pDosHeader + pDosHeader->e_lfanew + 4);
PIMAGE_OPTIONAL_HEADER32 pOptionHeader = (PIMAGE_OPTIONAL_HEADER32)((DWORD)pPEHeader + sizeof(/images/shellcode/image_FILE_HEADER));
PIMAGE_SECTION_HEADER pSectionHeader = (PIMAGE_SECTION_HEADER)((DWORD)pOptionHeader + pPEHeader->SizeOfOptionalHeader);
PIMAGE_EXPORT_DIRECTORY pExportDirectory = (PIMAGE_EXPORT_DIRECTORY)((DWORD)dwKernelBase + pOptionHeader->DataDirectory[0].VirtualAddress);
// 导出函数地址表RVA
DWORD *pAddOfFun_Raw = (DWORD*)((DWORD)dwKernelBase + pExportDirectory->AddressOfFunctions);
// 导出函数名称表RVA
WORD *pAddOfOrd_Raw = (WORD*)((DWORD)dwKernelBase + pExportDirectory->AddressOfNameOrdinals);
// 导出函数序号表RVA
DWORD *pAddOfNames_Raw = (DWORD*)((DWORD)dwKernelBase + pExportDirectory->AddressOfNames);
DWORD dwCnt = 0;
char* pFinded = NULL, *pSrc = szGetProcAddress;
for (; dwCnt < pExportDirectory->NumberOfNames;dwCnt++)
{
pFinded = (char*)((DWORD)dwKernelBase + pAddOfNames_Raw[dwCnt]);
while (*pFinded && *pFinded == *pSrc)
pFinded++, pSrc++;
if (*pFinded == *pSrc)
{
pGetProcAddress = (PGETPROCADDRESS)(pAddOfFun_Raw[pAddOfOrd_Raw[dwCnt]] + (DWORD)dwKernelBase);
break;
}
pSrc = szGetProcAddress;
}
}
// 通过pGetProcAddress进行调用
pLoadLibrary = (PLOADLIBRARY)pGetProcAddress((HMODULE)dwKernelBase, szLoadLibrary);
pMessageBox = (PMESSAGEBOX)pGetProcAddress(pLoadLibrary(szUser32),szMessageBox);
pMessageBox(NULL,szHelloShellCode,0,MB_OK);
return 0;
}
成功弹窗
这里我们进反汇编看一下，是有检测堆栈平衡的代码的
在物理机里面查看也是有的
这里关闭一下堆栈平衡的检测，默认情况如下
修改为禁用安全检查
即可生成没有检查堆栈平衡的代码
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