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wiki.mbirth.de/know-how/hacking/_posts/2016-07-07-crypto-virus-decrypted.md

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title language layout created updated toc tags
Crypto-Virus decrypted en default 2016-07-08 01:24:11 +0200 2016-07-08 01:24:11 +0200 false
know-how
hacking
virus
cryptovirus
cryptor

A friend received the following mail from (supposedly) FedEx International:

![]({{ site.url }}/assets/cryptovirusmail.png)

The attached zip file contained a file 000794681.doc.js. Since .js is a known file type in Windows, it would show up as 000794681.doc and you'd think it's a Word file. But clicking on it will run the JavaScript using the Windows Scripting Host.

The entry point

The JavaScript looks like this (GIST):

![]({{ site.url }}/assets/codedocjs.png)

The various g55(number, string) calls do nothing but sorting the string into the number's position in an array. In the last but one line, the snippets in the array are joined in the numeric order which turns out to be valid JavaScript (or JScript) code again. In the last line, that code is executed.

If you omit the last line and, instead, output the contents of h72, you'll get the unobfuscated code of the first stage: GIST.

What this does is basically:

  • Download 5 files (incl. backup servers if one is taken down) into %TEMP%
    • a1.exe (some Visual Basic program)
    • a2.exe (some NSIS installer, unpacks files to AppData then waits)
    • a.exe (php.exe, PHP runtime)
    • php4ts.dll (PHP library)
    • a.php (the actual encrypting PHP code)
  • If all 3 PHP files are downloaded:
    • Write message into a.txt
    • Register autostart to open a.txt after login
    • Register extension .crypted to open/display a.txt whenever you click on a crypted file
    • Copy a.txt to your Desktop as DECRYPT.txt
    • Run the actual encryption of your files (see next chapter)
    • Display a.txt
    • Overwrite the a.php (containing the encryption key) with your key's identifier (so it can't be undeleted to get the actual key)
    • Delete a.exe, php4ts.dll and, of course, the a.php

The files to download are selected by 3 parameters:

  • ad, the BitCoin address you should sent money to
  • id, some identifier?, can be omitted
  • rnd, file selector

The ad parameter is used to generate your encryption key (see next chapter).

The rnd parameter selects the file you download. It's made up of the current mirror server's number (0..4) and the file to download (1..5). So it starts with 01, 02, etc. and if that server doesn't answer, it'll continue with 11, 12 .. 15. The files you get with 01, 11, 21, etc. are the same.

However, there seem to be some variation with the files returned for *1. I'm not sure if it's time based or randomly selected each time you start a download. When downloading from different servers in a short time, you'll mostly get the same file. In rare occasions, one (the last file downloaded) was different. I've found 5 different variations which differ in length and bytes starting from position 0x43cb6. Could also be random data to confuse antivirus products.

I think the files a1.exe and a2.exe are there to either confuse some antivirus products or just filler material for future use.

What's also interesting is, that the files are returned with a MIME type of image/png. Also, the download only works when the user agent string contains Windows NT, otherwise you'll get an empty (0 bytes) response.

Encryption code

The downloaded PHP script (a.php) seems to be freshly obfuscated each time you download it. But while the obfuscated version differs every time, the deobfuscated code is the same.

Only with different ad values in the download URL, you'll get different encryption keys. This is the BitCoin address you are asked to send money to. This is also and identifier for your encryption key.

E.g., an ad value of 17DmGrhMXJcvsmj9tihgTRGAhACynuBmSo returns a PHP script with the key:

MWKTbqXczBBUtCGOY6rxrB6Q2ECoaLUCGHDI5C54QaQHiP5010q99mPQNqAKkMkCtCicYss0uCCIDHPa5DiMDF6wYajvGFmaKJD4mtscEVSXPLUuduRStiug/kCCoA16swZZvi2c

If you change ad to 27DmGrhMXJcvsmj9tihgTRGAhACynuBmSo, the key changes to:

MmSWbqXczBBUtCGOY6rxrB6Q2ECoaLUCGHDI5C54QaQHiP5010q99mPQNqAKkMkCtCicYss0uCCIDHPa5DiMDF6wYajvGFmaKJD4mtscEVSXPLUuduRStiug/kCCoA16swZZvi2c
 ^^^

to be continued...