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Re: [Snort-users] Serious problems Snort 2.9 with relative content matches using http_inspect preprocessor and http_uri keyword
From: L0rd Ch0de1m0rt <l0rdch0de1m0rt () gmail com>
Date: Thu, 5 Dec 2013 14:12:14 -0500

Hello.  OH thanks 2 u rmkl, you see the problem that I do!  Snort 2.9.4.1
and previous could have issue and then it is not just me.  Are these
versions supported and can we get a patch?  IF the fix is to upgrade to a
later version then that is not trival and could delay many months or up to
a year or more or never on embedded devices (eps military ones).  Hopefully
supported versions get attention since vendors such as Apple release
updates for their software even if newer versions are available (for an
example, a bug in OS X Lions doesn't make me get suggested to upgrade to
the Mavaricks', it just gets fixed b/c is is still supported),

Cheers,

Lord C.

On Wed, Dec 4, 2013 at 4:08 PM, rmkml <rmkml () yahoo fr> wrote:

Hi L0rd,

ok my testing with your pcap: (in this test, all snort use same config)

-v2.9.5.6 all sigs fire

-v2.9.4.6 all sigs fire

-v2.9.4.5 all sigs fire

-v2.9.4.1 sig three only fire

-v2.9.3.1 sig three only fire

Regards
@Rmkml



On Wed, 4 Dec 2013, L0rd Ch0de1m0rt wrote:

 Thanks Bhagya,

I looked again and the sensor is SNORT v2.9.3.1 (we have a number of
different versions and I access multiples on different consoles so I think
I saw the wrong one when I said 2.9.5.3).  Hopefully v2.9.3.1
is supported still too :) If no, could there be issues with supported
version?

The issue is a problem with holistic recursion on the http_uri buffer.
The engine when looking in the http_uri buffer detects on a content match
that comes before a subsequent relative content match.  But
the enginge does not properly recurse to find all the (initial) content
matches and attempt to match them using the subsequent relative content
match.

I have attached a simple pcap.  I do not have a config readily available
but (althou as you can see below), the info below shows that the
HTTP_INSPECT pre-processor is enabled, working, and inspecting data
for the ports and data in the packets.  Here is the datum:

These rules do *NOT* alert:

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Dont Cry 4 Me
Trojanina"; flow:established, to_server; content:"GET"; http_method;
content:"|2F|"; http_uri; content:"|3A|"; http_uri; distance:1;
within:20;)

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Dont Cry 4 Me
Trojanina"; flow:established, to_server; content:"GET"; http_method;
content:"|2F|"; http_uri; content:"|3A|"; http_uri; distance:1;
within:48;)

Howevers, this one *DOES ALERT*!:

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Dont Cry 4 Me
Trojanina"; flow:established, to_server; content:"GET"; http_method;
content:"|2F|"; http_uri; content:"|3A|"; http_uri; distance:1;
within:49;)

If you count bytes in the packet you can see that clearly the proper
recursion for detection is not taking place.

Pls let me know if you have any question.

Tks & Cheers,

Lord C.


On Fri, Nov 22, 2013 at 11:58 AM, Bhagya Bantwal <bbantwal () sourcefire com>
wrote:



      On Thu, Nov 7, 2013 at 4:42 PM, L0rd Ch0de1m0rt <
l0rdch0de1m0rt () gmail com> wrote:
            Hello.

@Bad Horse: I tried http_raw_uri and it still does not work.  That is
good point about the colon in the uri.  The funny thing is that if I
increase the distance, it WILL work so it seems that
maybe the parser it getting "stuck" on the first content match (0x2F) and
not evaluating everything in full.  To test, I tried this rule and it DID
work! :

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Dont Cry 4 Me
Trojanina"; flow:established, to_server; content:"GET"; http_method;
content:"|2F|"; http_uri; content:"|3A|"; http_uri;
distance:1; within:50;)

@Baggy: I tried a number of versions and the latest was 2.9.5.3 I
believe, is that still supported?


Yes. It is supported. Can you please send your conf and pcap for further
debugging?

Thanks
B

Thanks for everyones' help.

Cheers,

Lord C.


On Thu, Nov 7, 2013 at 12:31 PM, Bhagya Bantwal <bbantwal () sourcefire com>
wrote:
      Hello,

Have you tested with snort version snort 2.9.5.5? With this snort version
I see the alert as expected.

If it still doesn't work, you can send me your pcap & conf and I will
take a look.

Thanks!
B


On Wed, Nov 6, 2013 at 2:01 PM, L0rd Ch0de1m0rt <l0rdch0de1m0rt () gmail com>
wrote:
      Hello,

      Previously I posted on this list with an email subject of,
"distance, within, and negated matches".  Today I bring another issue that
I am having that I believes could be
      related and is non-trivial and super serious.

      When I analyze it I believe that relative (1 byte?) content matches
are not being properly applied in the http_uri buffer.  Other buffers for
the http preprocessor may be
      affected as well but I have not tested them but I won't be suprised
if they are also infected by this bug.

      This is an example of the rule Im using:

      alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Dont Cry
4 Me Trojanina"; flow:established, to_server; content:"GET"; http_method;
content:"|2F|"; http_uri;
      content:"|3A|"; http_uri; distance:1; within:20;)

      Using a simple pcap ("Follow TCP stream" output from Ethereal is
here:)

      GET /iused/2/trust/the.http_preprocessor/sad1/cr1090hs:SN-IF-FF-
HTTP/1.1

      The rule does not alert even though the Snort output shows that the
HTTP data is being properly recognized and processed by the http_inspect
preprocessor. The Snort output shows
      that the specific GET request is being recognized as a HTTP "GET"
request.

      When I remove the http_inspect directives, the rule starts to work,
this is an example:

      alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"Dont Cry
4 Me Trojanina"; flow:established, to_server; content:"GET"; http_method;
content:"|2F|"; content:"|3A|";
      distance:1; within:20;)

      Is this (still?) a known issue?  I have tested this on multiple
different versions of Snort 2.9.

      Cheers,

      Lord C.


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