Limiting false-hits with "SMTP RCPT TO overflow" rule

[Jason Haar <Jason.Haar () trimble co nz>]

Let's say I want to write a better "SMTP RCPT TO overflow" rule:

alert tcp $EXTERNAL_NET any -> $SMTP 25 (msg:"SMTP RCPT TO overflow";
flags:A+; content:"rcpt to|3a|"; dsize:>800; reference:cve,CAN-2001-0260;
reference:bugtraq,2283; classtype:attempted-admin; sid:654; rev:1;)

I get a few false-hits on that one. Mainly from security lists obviously :-)
The problem of course is that rule looks for "rct to:", and a large packet
size - something easily met inside the body of a mail message about SMTP.

Would adding perhaps a "offset:0" to this rule fix it? All SMTP RCPT calls
are done as single packets, so that offset would always be right - wouldn't
it?

Agh. PIPELINING may screw that...

-- 
Cheers

Jason Haar

Unix/Special Projects, Trimble NZ
Phone: +64 3 9635 377 Fax: +64 3 9635 417

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