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Re: Snort doesn't trigger while the payload size is big (even for ~4-5KB files)
From: Sujit Ghosal <thesujit () gmail com>
Date: Tue, 14 Dec 2010 11:53:39 +0530

Hi Joel,
    Below are my snort related details, you have asked for:
Snort version is 2.8.5.1 (Build 114) and is installed in FC13.
Snort Config: http://vim.pastey.net/143877
Pcap: http://www.mediafire.com/?p3wdxosvswt9zmj

As you can see, for the SID: 8888 I am using an addition content match
"|2f|StructTreeRoot|20|" which you will find in the 3rd/4th TCP reassembled
stream in the pcap. And for this my snort rule doesn't trigger. But for the
SID: 9999 I have removed that pattern and the snort rule triggers.

Signatures:
-----------------
## Not Working
alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:"NOT WORKING TCP
Reassembly Check"; flow:established,to_client; content:"PDF-"; nocase;
depth:300; content:"|2f|Linearized|20|"; nocase; distance:0;
content:"|2f|StructTreeRoot|20|"; nocase; distance:0;
classtype:attempted-user; reference:url,www.exploit-db.com/exploits/11111/;
reference:url,www.google.com; sid:8888; rev:1;)

### Working (StructTreeRoot Pattern removed)
alert tcp $EXTERNAL_NET $HTTP_PORTS -> $HOME_NET any (msg:"WORKING Sample
TCP Reassembly Check"; flow:established,to_client; content:"PDF-"; nocase;
depth:300; content:"|2f|Linearized|20|"; nocase; distance:0;
classtype:attempted-user; reference:url,www.exploit-db.com/exploits/11111/;
reference:url,www.google.com; sid:9999; rev:1;)

Best Regards,
Sujit


On Tue, Dec 14, 2010 at 3:49 AM, Joel Esler <jesler () sourcefire com> wrote:

If an alert spans more than one packet, Snort will log the Stream
Reassembled packet.

What version of Snort are you running?  What is the rule you are trying to
write?  What is your Snort configuration?  Can you provide a full session
pcap of the traffic?

J

On Dec 13, 2010, at 5:09 AM, Sujit Ghosal wrote:

Hey Joel,
      I have figured out the issue. Its because of TCP reassembly of
packets. In the first steam of my payload snort was working flawlessly but
if I jump to the second place of tcp reassembled data then it was not
detecting. Well I knew that the problem can be solved using flowbits
keyword, but I wanted a solution where snort can detect those re-assembled
packets as well. Is it possible to command snort to handle tcp re-assembly
without the use of flowbits?

Regards,
Sujit

On Sun, Dec 5, 2010 at 10:52 PM, Joel Esler <jesler () sourcefire com> wrote:

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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