[Discussion] Suricata rule not giving alerts

Tue Jul 22 12:38:23 UTC 2014

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.

-- 
-------------------------------
Anoop Saldanha
http://www.poona.me
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