[Snort-sigs] false positive for sid 2087

Matt Kettler mkettler at ...189...
Tue Aug 3 10:48:02 EDT 2004


At 11:15 AM 8/3/2004, Chris Kronberg wrote:
>alert tcp $EXTERNAL_NET any -> $SMTP_SERVERS 25 (msg:"SMTP From comment 
>overflow attempt"; flow:to_server,established; content:"From|3A|"; nocase; 
>content:"<><><><><><><><><><><><><><><><><><><><><><>"; distance:0; 
>content:"|28|"; distance:1; content:"|29|"; distance:1; 
>reference:cve,2002-1337; reference:url,www.kb.cert.org/vuls/id/398025; 
>classtype:attempted-admin; sid:2087; rev:6;)
>
>    This triggers mails like the one included below which has nothing to
>    do with the sendmail vulnerability.
>    I understand that the distance keyword gives a relative offset to
>    the last match, but then I do not understand what distance:0 does.
>    As the vulnerabilty goes for oversized comments in the MAIL FROM
>    and RCPT TO header, is there a possibility to rewrite the rule in
>    the sense that it triggers wenn the content match above does _not_
>    occur before the smtp DATA command?
>    I'm currently playing with the within keyowrd to restrict the
>    search depth, but I don't feel comfortable with that. Any better
>    ideas? I'm interested for solutions for snort 2.0.x and 2.1.x.

Theoretically, your email should not have matched. the "distance:0" should, 
theoretically, force the <> string to immediately follow the From: string. 
However, since it's fundamentally pointless to ever use, a distance of 0 it 
may be buggy.

Really, IMO, the From and the <> strings should be merged into a single 
content check if they are meant to be one string. using distance:0 is 
confusing, and likely to run into boundary conditions in parsers that don't 
deal so well with underflow. (ie: it may be trying to do something like 
"while i < distance-1" a classic programing bug that fails to handle 0 
properly if you're doing unsigned numbers.)

I'd also be tempted to rewrite the whole thing as a single PCRE so I could 
have more flexibility

something like this appears to match the original intent of the rule:
pcre: "/From\:(<>){22}.?\(.?\)/i"

But you could make it more flexible to the number of <> sequences:
pcre: "/From\:(<>){22,50}.?\(.?\)/i"





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