Heavy Obfuscation != Malicious

Malicious actors use obfuscation to evade detection, but programmers use it also for various reasons, like restricting reuse or bypassing ad blockers. Obfuscation should increase your attention to an alert, but it’s not always indicative of hostile activity.

Here’s an example.

IDS triggered on a rule for a common exploit kit, specifically on an eval statement. The code was:
(function(){var z=””;var b=”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

;for(var i=0;ieval(eval(‘String.fromCharCode(‘+z+’)’));})();



Changing the eval to print, and running it through Rhino, we get this:

[jstebelton@bwyissrv05 ~]$ rhino 10
(function(){function h(e){try{cookieArray=[];for(var b=/^\s?incap_ses_/,d=document.cookie.split(“\x3b”),c=0;cres;g=new Date;g.setTime(g.getTime()+2E4);document.cookie=”\x5f\x5f\x5f\x75\x74\x6d\x76\x63\x3d”+e+(“\x3b\x20\x65\x78\x70\x69\x72\x65\x73\x3d”+g.toGMTString())+”\x3b\x20\x70\x61\x74\x68\x3d\x2f”}function l(e){for(var b=[],d=0;d“\x3d”+k)}break;case “\x76\x61\x6c\x75\x65”:try{b[b.length]=encodeURIComponent(c+”\x3d”+eval(c).toString())}catch(h){b[b.length]=encodeURIComponent(c+”\x3d”+h)}break;case “\x70\x6c\x75\x67\x69\x6e\x73″:try{p=navigator.plugins;pres=””;for(a in p)pres+=(p[a].description+”\x20″).substring(0,20);b[b.length]=encodeURIComponent(“\x70\x6c\x75\x67\x69\x6e\x73\x3d”+pres)}catch(l){b[b.length]=encodeURIComponent(“\x70\x6c\x75\x67\x69\x6e\x73\x3d”+l)}break;case “\x70\x6c\x75\x67\x69\x6e”:try{for(i in a=navigator.plugins,a)if(f=a[i].filename.split(“\x2e”),2==f.length){b[b.length]=encodeURIComponent(“\x70\x6c\x75\x67\x69\x6e\x3d”+f[1]);break}}catch(m){b[b.length]=
encodeURIComponent(“\x70\x6c\x75\x67\x69\x6e\x3d”+m)}}}return b=b.join()}var m=[[“\x6e\x61\x76\x69\x67\x61\x74\x6f\x72″,”\x65\x78\x69\x73\x74\x73\x5f\x62\x6f\x6f\x6c\x65\x61\x6e”],[“\x6e\x61\x76\x69\x67\x61\x74\x6f\x72\x2e\x76\x65\x6e\x64\x6f\x72″,”\x76\x61\x6c\x75\x65”],[“\x6f\x70\x65\x72\x61″,”\x65\x78\x69\x73\x74\x73\x5f\x62\x6f\x6f\x6c\x65\x61\x6e”],[“\x41\x63\x74\x69\x76\x65\x58\x4f\x62\x6a\x65\x63\x74″,”\x65\x78\x69\x73\x74\x73\x5f\x62\x6f\x6f\x6c\x65\x61\x6e”],[“\x6e\x61\x76\x69\x67\x61\x74\x6f\x72\x2e\x61\x70\x70\x4e\x61\x6d\x65″,”\x76\x61\x6c\x75\x65”],[“\x70\x6c\x61\x74\x66\x6f\x72\x6d”,”\x70\x6c\x75\x67\x69\x6e”],[“\x77\x65\x62\x6b\x69\x74\x55\x52\x4c”,”\x65\x78\x69\x73\x74\x73\x5f\x62\x6f\x6f\x6c\x65\x61\x6e”],[“\x6e\x61\x76\x69\x67\x61\x74\x6f\x72\x2e\x70\x6c\x75\x67\x69\x6e\x73\x2e\x6c\x65\x6e\x67\x74\x68\x3d\x3d\x30″,”\x76\x61\x6c\x75\x65”],[“\x5f\x70\x68\x61\x6e\x74\x6f\x6d”,”\x65\x78\x69\x73\x74\x73\x5f\x62\x6f\x6f\x6c\x65\x61\x6e”]];try{h(l(m)),document.createElement(“\x69\x6d\x67”).src=”\x2f\x5f\x49\x6e\x63\x61\x70\x73\x75\x6c\x61\x5f\x52\x65\x73\x6f\x75\x72\x63\x65\x3f\x53\x57\x4b\x4d\x54\x46\x53\x52\x3d\x31\x26\x65\x3d”+Math.random()}catch(n){img=document.createElement(“\x69\x6d\x67”),img.src=”\x2f\x5f\x49\x6e\x63\x61\x70\x73\x75\x6c\x61\x5f\x52\x65\x73\x6f\x75\x72\x63\x65\x3f\x53\x57\x4b\x4d\x54\x46\x53\x52\x3d\x31\x26\x65\x3d”+
n}})();

We have hex encoding as the output of the function, so we can use a hex decoder to get the final output:

encodeURIComponent(“plugin=”+m)}}}return b=b.join()}var m=[[“navigator”,”exists_boolean”],[“navigator.vendor”,”value”],[“opera”,”exists_boolean”],[“ActiveXObject”,”exists_boolean”],[“navigator.appName”,”value”],[“platform”,”plugin”],[“webkitURL”,”exists_boolean”],[“navigator.plugins.length==0″,”value”],[“_phantom”,”exists_boolean”]];try{h(l(m)),document.createElement(“img”).src=”/_Incapsula_Resource?SWKMTFSR=1&e=”+Math.random()}catch(n){img=document.createElement(“img”),img.src=”/_Incapsula_Resource?SWKMTFSR=1&e=”+
n}})();

Incapsula is a DDoS protection security vendor.

This is a Security Bloggers Network syndicated blog post authored by JeffSoh. Read the original post at: JeffSoh on NetSec