[Discussion] Suricata rule not giving alerts

Tue Jul 22 09:31:05 UTC 2014

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 :-)
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