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171 lines
7.0 KiB
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171 lines
7.0 KiB
Markdown
---
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title: Crypto-Virus decrypted
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language: en
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layout: default
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created: 2016-07-08 15:41:41 +0200
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updated: 2016-07-08 15:41:41 +0200
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toc: false
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tags:
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- know-how
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- hacking
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- virus
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- cryptovirus
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- cryptor
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---
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A friend received the following mail from (supposedly) **FedEx International**:
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The attached zip file contained a file `000794681.doc.js`. Since `.js` is a
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known file type in Windows, it would show up as `000794681.doc` and you'd think
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it's a Word file. But clicking on it will run the JavaScript using the *Windows
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Scripting Host*.
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The entry point
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===============
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The JavaScript looks like this ([GIST](https://gist.github.com/mars3142/3fc6a5522fcb752cdcbde3a5c1bca434)):
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The various `g55(number, string)` calls do nothing but sorting the string into
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the number's position in an array. In the last but one line, the snippets in
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the array are joined in the numeric order which turns out to be valid
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JavaScript (or JScript) code again. In the last line, that code is executed.
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If you omit the last line and, instead, output the contents of `h72`, you'll
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get the unobfuscated code of the first stage: [GIST](https://gist.github.com/mbirth/d21bf52a024d0634f731e90dca94d254).
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What this does is basically:
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* Download 5 files (incl. backup servers if one is taken down) into `%TEMP%`
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* `a1.exe` (some Visual Basic program)
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* `a2.exe` (some NSIS installer, unpacks files to `AppData` then waits)
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* `a.exe` (php.exe, PHP runtime)
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* `php4ts.dll` (PHP library)
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* `a.php` (the actual encrypting PHP code)
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* If all 3 PHP files are downloaded:
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* Write message into `a.txt`
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* Register autostart to open `a.txt` after login
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* Register extension `.crypted` to open/display `a.txt` whenever you click on
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a crypted file
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* Copy `a.txt` to your Desktop as `DECRYPT.txt`
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* Run the actual encryption of your files (see next chapter)
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* Display `a.txt`
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* Overwrite the `a.php` (containing the encryption key) with the BitCoin
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address (so it can't be undeleted to get the actual key)
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* Delete `a.exe`, `php4ts.dll` and, of course, the `a.php`
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The files to download are selected by 3 parameters:
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* `ad`, the BitCoin address you should send money to
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* `id`, some identifier?, can be omitted
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* `rnd`, file selector
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The `ad` parameter is used to generate your encryption key (see next chapter).
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The `rnd` parameter selects the file you download. It's made up of the current
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mirror server's number (0..4) and the file to download (1..5). So it starts
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with `01`, `02`, etc. and if that server doesn't answer, it'll continue with
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`11`, `12` .. `15`. The files you get with `01`, `11`, `21`, etc. are the same.
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However, there seem to be some variation with the files returned for `*1`. I'm
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not sure if it's time based or randomly selected each time you start a
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download. When downloading from different servers in a short time, you'll
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mostly get the same file. In rare occasions, one (the last file downloaded)
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was different. I've found 5 different variations which differ in length and
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bytes starting from position 0x43cb6. Could also be random data to confuse
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antivirus products.
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I think the files `a1.exe` and `a2.exe` are there to either confuse some
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antivirus products or just filler material for future use.
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What's also interesting is, that the files are returned with a MIME type of
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`image/png`. Also, the download only works when the user agent string
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contains `Windows NT`, otherwise you'll get an empty (0 bytes) response.
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Encryption code
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===============
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The downloaded PHP script (`a.php`) seems to be freshly obfuscated each time
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you download it. But while the obfuscated version differs every time, the
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deobfuscated code is the same.
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This is how it looks after download:
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After removing the first layer of obfuscation, it turns into this:
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And now it becomes clearer, what's happening here. In `$h772` we have an
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`eval()` command with the code to *evaluate* being encoded in base64.
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In `$e51` there's a regular expression with the `/e` modifier, which enabled
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callbacks in older PHP versions. This means, whenever the parser finds a
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match, a given piece of code is called. And as you might have guessed, that
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code is the one in `$h772`. And to have the parser find something, the subject
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in the `preg_replace` call is exactly the same as the search string in `$e51`.
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So to get to the actual code, we just have to `base64_decode()` the string
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without `eval()`ing it. And this brings us to the final code. You can see all
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iterations in this [GIST](https://gist.github.com/mbirth/11979a35a152478427928dbdf593797b).
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(I've annotated and reformatted the final stage.)
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Fun fact: The base64 encoded code contains proper indentation and Windows line
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endings (CR+LF instead of LF only). They could've saved a lot of space by
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removing all unneccessary spaces and line breaks.
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The code does this:
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* Try all drives from `C:` to `Z:`
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* For each drive, check the contents of the root directory
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* If it's a folder and it's not `winnt`, `windows`, `appdata`, etc., check that
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folder (recursively)
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* If it's a file, check if the extension is `zip`, `rar`, `doc`, etc. and if so,
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encrypt it and add `.crypted` to the filename
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Now the interesting things are right at the beginning of the `Tree($p)` method.
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There are 3 variables defined:
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* `$a` which defines the intended action, `e` is for encryption, `d` for
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decryption
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* `$k` is the key, which is decoded from base64
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* `$s` is a backslash character to not have to mess around with proper escaping
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I noticed that with different `ad` values in the download URL (see previous
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chapter), you'll get different encryption keys in `$k`. `ad` is the BitCoin
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address you are asked to send money to. This means the encryption key is based
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on the BitCoin address.
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E.g., an `ad` value of `17DmGrhMXJcvsmj9tihgTRGAhACynuBmSo` returns a PHP
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script with the key:
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MWKTbqXczBBUtCGOY6rxrB6Q2ECoaLUCGHDI5C54QaQHiP5010q99mPQNqAKkMkCtCicYss0uCCIDHPa5DiMDF6wYajvGFmaKJD4mtscEVSXPLUuduRStiug/kCCoA16swZZvi2c
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If you change `ad` to `27DmGrhMXJcvsmj9tihgTRGAhACynuBmSo`, the key changes to:
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MmSWbqXczBBUtCGOY6rxrB6Q2ECoaLUCGHDI5C54QaQHiP5010q99mPQNqAKkMkCtCicYss0uCCIDHPa5DiMDF6wYajvGFmaKJD4mtscEVSXPLUuduRStiug/kCCoA16swZZvi2c
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^^^
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In other words: This script is generic and can be used for different BitCoin
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accounts which will generate different encryption keys. And you need to wire
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your money to the correct one to get the correct decryption key.
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Decrypting encrypted files
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==========================
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So let's imagine you accidentally run the script and all your important files
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are now encrypted and renamed to `important.doc.crypted`. What can you do?
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Well, we've learned that the encryption key is based on the `ad` value. And that
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you still have in that mail with the bogus zip file. Using that, we can download
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the PHP script again, which contains the actual key.
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After deobfuscating it, you just have to change `$a='e'` to `$a='d'` and run it
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and it should decrypt all your files again. Problem solved.
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