[Snort-devel] BUG: corner case involving http_cookie

Will Metcalf william.metcalf at ...2499...
Thu Mar 11 10:02:04 EST 2010


Fails on byte_jump as well, additionally http_header appears to act
the same way.

Regards,

Will

#test 73 http_cookie + byte_jump.
#hmm interesting using http_cookie in combination with byte_jump seems
to fail always. Removing either the byte_jump check or the http_cookie
modifier will cause this sig to fire.  Notice that the byte_jump
#check isn't even relative to the content match.
#
#file oisfsearchnums.pcap
alert tcp any any -> any any (msg:"e6504ae48c99f09df7f58996aacbb6b0 +
http_cookie + byte_jump"; content:"e6504ae48c99f09df7f58996aacbb6b0";
http_cookie; byte_jump:1,596,string,dec; content:"0"; distance:0;
within:1; classtype:bad-unknown; sid:73; rev:1;)



On Wed, Mar 10, 2010 at 4:56 PM, Will Metcalf <william.metcalf at ...2499...> wrote:
> How about this one, using the same pcap. I have both client and server
> flow_depth set to 0.
>
> #test 72 http_cookie + byte_test.
> #hmm interesting using http_cookie in combination with byte_test seems
> to fail always. Removing either the byte_test check or the http_cookie
> modifier will cause this sig to fire.  Notice that the byte_test
> #check isn't even relative to the content match.
> #
> #file oisfsearchnums.pcap
> alert tcp any any -> any any (msg:"e6504ae48c99f09df7f58996aacbb6b0 +
> http_cookie + byte_test"; content:"e6504ae48c99f09df7f58996aacbb6b0";
> http_cookie; byte_test:2,=,12,596,string,dec; classtype:bad-unknown;
> sid:72; rev:1;)
>
>
> Regards,
>
> Will
>
> On Wed, Mar 10, 2010 at 1:54 PM, Will Metcalf <william.metcalf at ...2499...> wrote:
>> I can see the need for the cut-off.  You have to make compromises
>> between accuracy and performance this tends to be the nature of the
>> beast with IDS's,  and sure there will always be evasions abut the
>> dynamic flow_depth thing still isn't making much sense to me and/or at
>> least this behavior should be documented somewhere.  Did I just miss
>> something in the readme?  Btw there is a typo both in the manual and
>> the README regarding client_flow_depth.  "It primarily eliminates
>> Snort fro inspecting larger HTTP Cookies that appear at the end of
>> many client request Headers."
>>
>>
>> Regards,
>>
>> Will
>>
>> On Wed, Mar 10, 2010 at 1:18 PM, Steven Sturges
>> <steve.sturges at ...402...> wrote:
>>> There are different settings for the requests & responses.
>>> See client_flow_depth and server_flow_depth in the readme/manual.
>>>
>>> Setting either of the flow depth values to 0 would inspect
>>> all of the respective request or response, but incurs a
>>> pretty big performance hit.
>>>
>>> And yes, the default (in code) is 300 for both.
>>>
>>> Will Metcalf wrote:
>>>> Ahhh ok so I want to make sure I understand correctly in 2.8.5 the
>>>> default 300 byte cutoff is not only applied to http_responses but also
>>>> http_requests (README.http_inspect) if there are http normalizations.
>>>> Is there not still a potential evasion here based on the tests below
>>>> as packet being normalized or potentially matching on one or more
>>>> rules can be fairly unpredictable?  Would your recommended
>>>> configuration then be to set client_flow_depth to 0 in the
>>>> http_inspect preproc for 2.8.5 users if they wanted to avoid the
>>>> potential evasion?
>>>>
>>>> Regards,
>>>>
>>>> Will
>>>>
>>>> On Wed, Mar 10, 2010 at 12:03 PM, Steven Sturges
>>>> <steve.sturges at ...402...> wrote:
>>>>> Seems that what is really coming into play is the flow depth.
>>>>> That is going to limit how much of the raw data is searched
>>>>> with the pattern matcher.
>>>>>
>>>>> To summarize how it works with 2.8.6:
>>>>>
>>>>> 1) If HTTP normalizes headers and cookies, the amount of raw data
>>>>> searched by fast pattern matcher is determined by client (or server)
>>>>> flow depth settings.
>>>>>
>>>>> 2) If there are no HTTP normalizations, flow depth is not applied, so
>>>>> all of the raw data is searched.
>>>>>
>>>>> With 2.8.5, in 1) above, the flow depth was applied only if there were
>>>>> rules searched for one of those HTTP normalized buffers.
>>>>>
>>>>> By default, client flow depth is 0 -- which is what Will is probably
>>>>> hitting in his tests.
>>>>>
>>>>> Alerts w/ combinations of rules & different flow depths in 2.8.6:
>>>>>
>>>>> Default flow depth (0)
>>>>> SID 59, SID 68 enabled
>>>>> 1 alert (sid 59)
>>>>>
>>>>> Default flow depth (0)
>>>>> SID 68 enabled
>>>>> 0 alerts
>>>>>
>>>>> [Same as above for client_flow_depth 300]
>>>>>
>>>>> Client flow depth (1460)
>>>>> SID 59, SID 68 enabled
>>>>> 3 alerts (sid 59, sid 68x2)
>>>>>
>>>>> Client flow depth (1460)
>>>>> SID 68 enabled
>>>>> 2 alerts (sid 68x2)
>>>>>
>>>>> -steve
>>>>>
>>>>> Matt Jonkman wrote:
>>>>>> Appreciate the clarification Steve. But I'm concerned, this will make
>>>>>> hundreds if not a few thousand rules not work correctly in our set and
>>>>>> in vrt/snort gpl. If someone has http_inspect on in a recent snort, but
>>>>>> does NOT have ALL of their http related rules converted to the new form
>>>>>> using the http_* modifiers (which we have none converted) then they're
>>>>>> going to have massive problems, no? I think a lot of people are missing
>>>>>> a lot of things right now.
>>>>>>
>>>>>> Is there a way to make http_inspect not do this? We have a lot of sigs
>>>>>> that won't work on normalized data, for instance the recent trojan sigs
>>>>>> that look at the order of the parameters in an http post for uniqieness.
>>>>>> With the normalized only we can't see that can we?
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>> Matt
>>>>>>
>>>>>> On 3/10/10 10:26 AM, Will Metcalf wrote:
>>>>>>>> 1) Use http_cookie in the rule as you note in the case that works.
>>>>>>> Right so the reason that I cc'd the emerging list is that they are not
>>>>>>> using the http_* modifiers to maintain compatibility with older
>>>>>>> versions of snort.  Shouldn't this buffer at least be available to
>>>>>>> match on via rawbytes  which is what would be consistent with what you
>>>>>>> have done with telnet and dcerpc.  Does this also not add a potential
>>>>>>> evasion method if this is the intended behavior, one that perhaps
>>>>>>> VRT/ET should be made aware of.  I haven't gone through very many of
>>>>>>> the sigs but since the normalized buffer begins with the "Cookie:"
>>>>>>> instead of the value isn't there a now a potential evasion if I can
>>>>>>> get another sig to trip on the same packet as say sid 2136.
>>>>>>>
>>>>>>> Regards,
>>>>>>>
>>>>>>> Will
>>>>>>>
>>>>>>> On Wed, Mar 10, 2010 at 8:15 AM, Steven Sturges
>>>>>>> <steve.sturges at ...402...> wrote:
>>>>>>>> Will--
>>>>>>>>
>>>>>>>> In the 2nd rule the only content is an HTTP cookie.  Without using
>>>>>>>> http_cookie, it would try to match the raw data.
>>>>>>>>
>>>>>>>> With HTTP Inspect enabled, it is separating the headers, cookie,
>>>>>>>> method, etc from the raw data, hence all of the modifier keywords
>>>>>>>> that you can use with content -- can use more than one together.
>>>>>>>>
>>>>>>>> Without specifying http_cookie in the rule and when HTTP Inspect
>>>>>>>> enabled (and cookie inspection enabled in 2.8.6), if the pattern
>>>>>>>> matcher searches any of the HTTP buffers, it doesn't search the raw
>>>>>>>> data.  If no rules use the specific HTTP buffers or there are no HTTP
>>>>>>>> buffers, the pattern matcher will search the raw data, which is why
>>>>>>>> the rule byte itself (without http_cookie) works.
>>>>>>>>
>>>>>>>> This was done to avoid going over the same data twice in the pattern
>>>>>>>> matcher, and it is working as it is designed.
>>>>>>>>
>>>>>>>> Two options:
>>>>>>>>
>>>>>>>> 1) Use http_cookie in the rule as you note in the case that works.
>>>>>>>>
>>>>>>>> 2) Turn off HTTP Inspect (not realistic)
>>>>>>>>
>>>>>>>> Cheers
>>>>>>>> -steve
>>>>>>>>
>>>>>>>> Will Metcalf wrote:
>>>>>>>>> hmmm I don't think so.  Look at first test.  both rules fire.
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>>
>>>>>>>>> Will
>>>>>>>>>
>>>>>>>>> On Tue, Mar 9, 2010 at 10:31 PM, beenph <beenph at ...2499...> wrote:
>>>>>>>>>> I will try a wild guess, what is your event_queue size like?
>>>>>>>>>>
>>>>>>>>>> Its probably a bug or something  that need clarification regarding
>>>>>>>>>> http_cookie and http_inspect, but mabey http_cookie enable a modifier
>>>>>>>>>> in http_inspect that alter alerting behavior when event_queue is at 1
>>>>>>>>>> (since i guess both "alerts" are part of the same normalized http
>>>>>>>>>> stream)
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> -elz
>>>>>>>>>> ps: didin't run the pcap and rules test.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On Tue, Mar 9, 2010 at 11:15 PM, Will Metcalf <william.metcalf at ...300.....2499...> wrote:
>>>>>>>>>>> failing to use the http_cookie modifier on a rule where there is
>>>>>>>>>>> another rule that matches the same packet makes a rule that should
>>>>>>>>>>> fire fail.
>>>>>>>>>>>
>>>>>>>>>>> src/snort -V
>>>>>>>>>>>
>>>>>>>>>>>   ,,_     -*> Snort! <*-
>>>>>>>>>>>  o"  )~   Version 2.8.5.3 (Build 124)
>>>>>>>>>>>   ''''    By Martin Roesch & The Snort Team:
>>>>>>>>>>> http://www.snort.org/snort/snort-team
>>>>>>>>>>>           Copyright (C) 1998-2009 Sourcefire, Inc., et al.
>>>>>>>>>>>           Using PCRE version: 7.8 2008-09-05
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> src/snort -k none -q -A console -c etc/snort.conf -l ./ -r oisfsearchnums.pcap
>>>>>>>>>>>
>>>>>>>>>>> #this combo works
>>>>>>>>>>> #alert tcp any any -> any any (msg:"http_client_body";
>>>>>>>>>>> content:"searchword="; uricontent:"/index.php"; nocase;
>>>>>>>>>>> classtype:bad-unknown; sid:59; rev:1;)
>>>>>>>>>>> #alert tcp any any -> any any (msg:"http_cookie match ";
>>>>>>>>>>> content:"e6504ae48c99f09df7f58996aacbb6b0=120e494ce857d6ceeef89f9678d4d703";
>>>>>>>>>>> http_cookie; classtype:bad-unknown; sid:68; rev:1;)
>>>>>>>>>>> #
>>>>>>>>>>> #03/07-21:19:54.242506  [**] [1:59:1] http_client_body [**]
>>>>>>>>>>> [Classification: Potentially Bad Traffic] [Priority: 2] {TCP}
>>>>>>>>>>> 192.168.100.17:38111 -> 96.43.130.5:80
>>>>>>>>>>> #03/07-21:19:54.242506  [**] [1:68:1] http_cookie match  [**]
>>>>>>>>>>> [Classification: Potentially Bad Traffic] [Priority: 2] {TCP}
>>>>>>>>>>> 192.168.100.17:38111 -> 96.43.130.5:80
>>>>>>>>>>> #03/07-21:19:54.364173  [**] [1:68:1] http_cookie match  [**]
>>>>>>>>>>> [Classification: Potentially Bad Traffic] [Priority: 2] {TCP}
>>>>>>>>>>> 192.168.100.17:38111 -> 96.43.130.5:80
>>>>>>>>>>>
>>>>>>>>>>> #the second rule does not fire
>>>>>>>>>>> #alert tcp any any -> any any (msg:"http_client_body + depth";
>>>>>>>>>>> content:"searchword="; uricontent:"/index.php"; nocase;
>>>>>>>>>>> classtype:bad-unknown; sid:59; rev:1;)
>>>>>>>>>>> #alert tcp any any -> any any (msg:"http_cookie match";
>>>>>>>>>>> content:"e6504ae48c99f09df7f58996aacbb6b0=120e494ce857d6ceeef89f9678d4d703";
>>>>>>>>>>> classtype:bad-unknown; sid:68; rev:1;)
>>>>>>>>>>> #
>>>>>>>>>>> #03/07-21:19:54.242506  [**] [1:59:1] http_client_body + depth [**]
>>>>>>>>>>> [Classification: Potentially Bad Traffic] [Priority: 2] {TCP}
>>>>>>>>>>> 192.168.100.17:38111 -> 96.43.130.5:80
>>>>>>>>>>>
>>>>>>>>>>> #this rule fires when used on it's own.
>>>>>>>>>>> #alert tcp any any -> any any (msg:"http_cookie match";
>>>>>>>>>>> content:"e6504ae48c99f09df7f58996aacbb6b0=120e494ce857d6ceeef89f9678d4d703";
>>>>>>>>>>> classtype:bad-unknown; sid:68; rev:1;)
>>>>>>>>>>> #
>>>>>>>>>>> #03/07-21:19:54.242506  [**] [1:68:1] http_cookie match [**]
>>>>>>>>>>> [Classification: Potentially Bad Traffic] [Priority: 2] {TCP}
>>>>>>>>>>> 192.168.100.17:38111 -> 96.43.130.5:80
>>>>>>>>>>> #03/07-21:19:54.364173  [**] [1:68:1] http_cookie match [**]
>>>>>>>>>>> [Classification: Potentially Bad Traffic] [Priority: 2] {TCP}
>>>>>>>>>>> 192.168.100.17:38111 -> 96.43.130.5:80
>>>>>>>>>>>
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>>>>>>> proactively, and fine-tune applications for parallel performance.
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>




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