[Discussion] Suricata rule not giving alerts

[Victor Julien]

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

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