[Discussion] Suricata rule not giving alerts

Wed Jul 23 07:55:33 UTC 2014

On 07/22/2014 02:38 PM, Anoop Saldanha wrote:
> On Tue, Jul 22, 2014 at 5:34 PM, Jelte <masterjel5000 at hotmail.com> wrote:
>> Anoop Saldanha schreef op 7/22/2014 1:16 PM:
>>> On Tue, Jul 22, 2014 at 3:01 PM, Jelte <masterjel5000 at hotmail.com> wrote:
>>>> Anoop Saldanha schreef op 7/22/2014 4:09 AM:
>>>>> On Tue, Jul 22, 2014 at 3:51 AM, Jelte <masterjel5000 at hotmail.com> wrote:
>>>>>> Anoop Saldanha schreef op 7/21/2014 4:32 AM:
>>>>>>> On Sun, Jul 20, 2014 at 5:50 PM, Jelte <masterjel5000 at hotmail.com> wrote:
>>>>>>>> Jelte schreef op 7/15/2014 12:08 AM:
>>>>>>>>> Victor Julien schreef op 7/14/2014 9:27 AM:
>>>>>>>>>> On 07/14/2014 12:21 AM, Jelte O. wrote:
>>>>>>>>>>> Hello all,
>>>>>>>>>>>
>>>>>>>>>>> I have a rule from the ET rule-set to alert against an attack that is
>>>>>>>>>>> used to exploit a vulnerability in nginx 1.3.9-1.4.0. In order to
>>>>>>>>>>> trigger this rule I loaded an exploit module in Metasploit and fired it
>>>>>>>>>>> on my server.
>>>>>>>>>>>
>>>>>>>>>>> The vulnerability:
>>>>>>>>>>> https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-2028
>>>>>>>>>>> The rule: http://doc.emergingthreats.net/bin/view/Main/2016918
>>>>>>>>>>> The Metasploit module:
>>>>>>>>>>> https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/linux/http/nginx_chunked_size.rb
>>>>>>>>>>>
>>>>>>>>>>> I'll repeat the rule here:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>     alert tcp $EXTERNAL_NET any -> $HTTP_SERVERS $HTTP_PORTS (msg:"ET
>>>>>>>>>>>     WEB_SERVER Possible NGINX Overflow CVE-2013-2028 Exploit Specific";
>>>>>>>>>>>     flow:established,to_server; content:"chunked"; http_header; nocase;
>>>>>>>>>>>     fast_pattern:only; pcre:"/Transfer-Encoding\x3a[^\r\n]*?chunked/Hi";
>>>>>>>>>>>     pcre:"/^[\r\n\s]*?[^\r\n]+HTTP\/1\.\d[^\r\n]*?\r?\n((?!(\r?\n\r?\n)).)*?Transfer-Encoding\x3a[^\r\n]*?Chunked((?!(\r?\n\r?\n)).)*?\r?\n\r?\n[\r\n\s]*?(f{6}[8-9a-f][0-9a-f]|[a-f0-9]{9})/si";
>>>>>>>>>>>     reference:url,www.vnsecurity.net/2013/05/analysis-of-nginx-cve-2013-2028/;
>>>>>>>>>>>     reference:url,github.com/rapid7/metasploit-framework/blob/master/modules/exploits/linux/http/nginx_chunked_size.rb;
>>>>>>>>>>>     classtype:attempted-admin; sid:2016918; rev:6;)
>>>>>>>>>>>
>>>>>>>>>>> My attack did not generate any alerts. However, as soon as I removed the
>>>>>>>>>>> "http_header;" and changed "/Hi" to "/i" (in the first pcre) the rule
>>>>>>>>>>> started generating alerts. From this it seems like the HTTP header is
>>>>>>>>>>> not complete/not recognized by Suricata. However, when I do an extended
>>>>>>>>>>> logging on the HTTP traffic, I do see entries like:
>>>>>>>>>>>
>>>>>>>>>>> 07/13/14-23:45:27.830342 - - Chunked HTTP/1.1 GET mifpudtilvpjqsjl / - 0
>>>>>>>>>>> x.x.x.x:40590 -> y.y.y.y:80
>>>>>>>>>>>
>>>>>>>>>>> My "customformat" for the http-log contains "%{Transfer-Encoding}i",
>>>>>>>>>>> which would actually be the "contents of the defined HTTP Request Header
>>>>>>>>>>> name" according to the documentation (refer to
>>>>>>>>>>> https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Custom_http_logging).
>>>>>>>>>>> <https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Custom_http_logging%29.>
>>>>>>>>>>>
>>>>>>>>>>> I also created packet dumps of both legitimate web traffic and this
>>>>>>>>>>> attack and analyzed the streams in Wireshark. In both dumps there are
>>>>>>>>>>> TCP PDU's which are re-assembled, but in the valid web traffic Wireshark
>>>>>>>>>>> labels the protocol for some of the fully assembled client-to-server
>>>>>>>>>>> packets as HTTP while for the attack there are only TCP packets from the
>>>>>>>>>>> client to the server.
>>>>>>>>>>>
>>>>>>>>>>> I am wondering why the HTTP header is not available. I am not sure if
>>>>>>>>>>> this is caused by Suricata, my OS/network interface or the rule itself.
>>>>>>>>>>> I hope someone can help me out!
>>>>>>>>>> As a first step, I'd suggest walking through this page
>>>>>>>>>> https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Self_Help_Diagrams
>>>>>>>>>>
>>>>>>>>> Thanks Will and Victor for your replies! I didn't know of the Self Help
>>>>>>>>> Diagrams yet, unfortunately, because it could have saved me some time on
>>>>>>>>> an earlier issue I had with bad TCP checksums. Anyway, I have traffic in
>>>>>>>>> the stats log and the http log, I already disabled checksum validation
>>>>>>>>> and the startup log says the rule has successfully been loaded (for
>>>>>>>>> testing purposes I have only included this single rule). Also, my server
>>>>>>>>> is not part of a VLAN.
>>>>>>>>>
>>>>>>>>> Offloading is enabled for my NIC. If I issue the "ethtool --show-offload
>>>>>>>>> eth0" command I see that the rx-checksumming, the tx-checksumming,
>>>>>>>>> tcp-segmentation-offload and the generic-segmentation-offload are on.
>>>>>>>>> The NIC does not seem to support turning off the rx-checksumming (I get
>>>>>>>>> "Operation not supported") but I was able to turn off the others. This
>>>>>>>>> didn't have any effect, though. I also created another packet dump of
>>>>>>>>> the attack after disabling these settings and compared this one to the
>>>>>>>>> one I created before turning off the tx-checksumming and segmentation
>>>>>>>>> offloading and they both matched. I already disabled the checksum
>>>>>>>>> validation in Suricata to get rid off the invalid checksum errors I had
>>>>>>>>> in the beginning.
>>>>>>>>>
>>>>>>>>> I went to all the self help diagrams but I still couldn't find the
>>>>>>>>> cause. I did notice that I forgot to mention two important things:
>>>>>>>>>
>>>>>>>>> - Snort (on the same system) does generate an alert for that particular
>>>>>>>>> rule.
>>>>>>>>> - Suricata does generate alerts for other rules in which I filter on
>>>>>>>>> content from the http header. For instance, a HTTP request inside
>>>>>>>>> Firefox generates alerts for a rule that includes 'content:"Mozilla";
>>>>>>>>> http_header; nocase;'.
>>>>>>>>>
>>>>>>>>> It seems like it has to do with the specific exploit that is used in
>>>>>>>>> Metasploit. Refer to
>>>>>>>>> https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/linux/http/nginx_chunked_size.rb
>>>>>>>>>
>>>>>>>>> Do you know anything else I can do for debugging? I have included the
>>>>>>>>> pcap of the attack in the attachment.
>>>>>>>>>
>>>>>>> Does changing the "http_header" to "http_raw_header" and 'H' to 'D'
>>>>>>> make any difference?
>>>>>>>
>>>>>> Thanks, but I already tried that and it didn't work. I double checked,
>>>>>> just to be sure, but even without the pcre's (just the
>>>>>> flow:established,to_server; content:"chunked"; http_raw_header; nocase;)
>>>>>> I get no alerts.
>>>>>>
>>>>>> Maybe someone is able to reproduce the issue locally? just fire the
>>>>>> exploit via Metasploit and make sure that you have the rule enabled.
>>>>>>
>>>>>> I'm eager to find the solution, so any help will be greatly appreciated!
>>>>>>
>>>>> It would be helpful if you could supply the pcap for this?
>>>>>
>>>> I already attached it in a previous email (7/15/2014), but here is it
>>>> again :-)
>>> Can you test if this patch works for you? -
>>>
>>> https://github.com/ironbee/libhtp/pull/78
>>>
>>
>> You saved my day, it works!! Thanks so much :-)
>>
>> If I understand correctly, it was not working before because the code
>> that is specific for handling chunked transfer encoding packages was
>> (incorrectly) not executed because the precondition was not satisfied,
>> the precondition being that the value of the transfer-encoding field
>> equals "chunked". This was not the case because the value actually was
>> "Chunked" (with a capital C) and therefore you have changed the
>> precondition to also include lower-case values of "chunked".
>>
> 
> That's right.
> 
>> Thx again for your time.
> 
> Np.
> 

Thanks guys! Hoping the next libhtp release will address this.

-- 
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Victor Julien
http://www.inliniac.net/
PGP: http://www.inliniac.net/victorjulien.asc
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