[Discussion] Suricata rule not giving alerts

Tue Jul 22 02:09:22 UTC 2014

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?

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