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JavaScript anti-analysis tricks: last-modified
October 14, 2009
Writers of malicious JavaScript code have always been keen on developing novel ways to make the analysis of their code harder. One of the most commonly used mechanisms to do so is (no surprise here) simple obfuscation. For example, malware authors commonly encode string literals with custom schemes. A decoding routine then de-scrambles the strings before using them further (for example, as the URL of the next step of an attack or as the CLSID of a vulnerable ActiveX control).
Interestingly, malware authors have also introduced various techniques to make the basic deobfuscation step more difficult, in particular, if performed in an off-line analysis environment, which, for example, examines the pages saved during a crawling session.
One of the earliest trick consists of using the URL of the obfuscated page as a decoding key in the deobfuscation routine. More recently, other techniques have also been used. One I have seen lately uses the time of the last modification of the page in the decoding routine.
Consider, for example, the following script:
<html><body><script>
var gtvwx=true,abwz="",gnru=false,
bfqrv=document.lastModified.split("/"),
dilp=String,
cjltu=bfqrv[2].split(":"),
acinqu=dilp['f#r(o#mZC#h#aZrZC(o,d#e('.replace(/[\(Z,G#]/g,'')],
gnty=bfqrv[0]+"25"+cjltu[2],
ckoxz=window,cklqry=0,klny="",
bfkw=ckoxz['euv9a2lS'.replace(/[S2u9@]/g,'')],
fopv=[150,173,160...90,94,111],
ailmux=function(){
for(var ehlt;cklqry<fopv.length;cklqry++){
klny+=acinqu(fopv[cklqry]-
gnty.substring(cklqry%gnty.length,cklqry%gnty.length+1).charCodeAt(0));
bfkw(klny);
};
ailmux();
</script></body></html>
The code reads the time the page was last modified from the
document.lastModified property. This property is initialized from the
value of the Last-Modified header sent from the web server serving the
page. The script then parses the time and extracts the number of seconds
from the time string into the cjltu variable.
The seconds value is then used to compute the value of the gnty
variable, which is used in the decoding routine to recover the
in-the-clear text from the encoded array fopv..
These are the Wepawet reports for a couple of sites that use this techniques: report for hxxp://www.pipisechka.com/sleep/news.php and report for hxxp://day-evryday.cn/news.php
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Wow, thx for share knowledge :) ill learn java script coz i think so cool