False positives

[Chris Osicki <osk () gd2 swissptt ch>]

Hi,

I'm trying to reduce the number of false positives I'm getting.
One of the "over sensitive" rule is this one:

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;)

which alarms every time there is a packet containing "rcpt to:"
and the packet payload is more than 800 bytes. Which is not necessarily
the length of recipients list to "rcpt to:".

[fire-proof overalls on]
Having a kind of limited regular expressions or wild-cards and a way
to reference the size of the matched string would be, at least in this 
case, useful. Like  `content: "rcpt to|3a|*|0d 0a|"; msize:>800'
I don't have enough experience with snort to estimate how 
useful could it be to help detect other buffer overflows.
Just wanted to share my thoughts.

Regards,
Chris


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