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Re: Snort doesn't trigger while the payload size is big (even for ~4-5KB files)
From: Joel Esler <jesler () sourcefire com>
Date: Sun, 5 Dec 2010 12:22:41 -0500

Can you provide your Snort configuration, rule you are trying to write
and a full session pcap of the traffic you are attempting to detect?

On Sunday, December 5, 2010, Sujit Ghosal <thesujit () gmail com> wrote:
Hi All,    I had a similar type of issue some days back to detect any server side/client side vulnerabilities as 
Snort was not detecting even for a single GET or <html> pattern in any requests/responses respectively. Anyways the 
problem is somehow solved. It just suddenly started working (may be I think my firewall was blocking initially, I am 
not fully sure though).

    Now I came through a very bizzare problem. While I am writing a client side signature (lets say some PDF 
vulnerability signatures). If the PDF has less number of bytes (within 500-600 bytes and the whole PDF is of 600 
bytes) and attack pattern comes within those 600 bytes then snort detects that time with my developed rule. But If I 
generate a malformed PDF File through MSF then the malformed objects are being moved to > 600 and the pattern is 
present at last of the PDF file (around at 20000th offset). In such cases snort is not detecting the attack. I 
checked the signature and everything is perfect as I haven't given any such offset limitations inside that rule.

    I gave a look in snort.conf to see the http_preprocessor configs and the checked till how far snort processes the 
data length and it is set to 0. So I think it should work in any case.

Can anyone please guide me on what could be the issue and how I can resolve this?


Best Regards,Sujit


-- 
Joel Esler

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