Metasploit - CVE-2012-1823 - Snort Sleeping

[MA Bel <mab_generic () outlook com>]

Hi,


I found a working exploit (reverse shell) where Snort’s
signature fail to trigger an alert. 

In a lab I have 3 physical hosts: one  Snort, one with BackTrack, and one Ubuntu running
Metasploitable in VirtualBox. I use Metasploit to attack the Metasploitable VM,
Snort is in passive (non-inline) mode. 


I came across CVE-2012-1823 (PHP CGI Argument Injection)
which corresponds to three potential snort signatures: 22097, 22063, 22064.
Metasploit has a nice exploit that will give you a reverse shell. It works.


http://www.metasploit.com/modules/exploit/multi/http/php_cgi_arg_injection


SID 22063’s rule attempts to catch the string “auto_prepend_file”
When the Metasploint exploit is launched, WireShark confirms that the string is
indeed sent. I get a reverse shell. I can list directories, move into them,
delete stuff, etc, yet Snort does not generate an alert. Yes, rules are up to
date, activated, etc. The basics are covered.


I decided to strip off all extra parameters and create a
very basic rule: “content:  auto_prepend_file”.
No luck catching the exploit.  I used Scapy to send the “auto_prepend_file” string. Snort
woke up. I used Scapy to send the whole string sent by Metasploit (I did a copy
& paste of what I found in Wireshark). That works, Snort wakes up.


I don’t understand why an http string sent by Scapy
generates an alert whereas the same string sent by Metasploit keeps Snort silent.
I am not event using evasion techniques.


How do I get Snort to catch the exploit? I am worried other rules won't fire when they should.


Thanks in advance.

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