Sunday, February 3, 2019

Unknowncrypter, the crypter twin of Fudcrypt: another Crypter-as-a-Service for Java and JS

Last week I wrote about a Crypter-as-a-Service named Fudcrypt which obfuscates Java RATs in VBS scripts. However, this is not the only service being used by threat actors to deliver encrypted Java RATs.

A few days ago there was a new "The Story of Manuel’s Java RAT"  which basically linked two different campaigns using same JRAT malware. In one of the campaigns there were JAR files attached to the emails, whereas in the other campaign the attachment were JS (JavaScript) dropping a JAR file. The JS campaign is the interesting part as this is linked to a Crypter-as-a-Service named Unknowcrypter.

Unknowcrypter and Fudcrypt are strongly linked and either the actor creating both services is the same or they are working together.

Unknowncrypter is announced in several underground forums as a crypter for Java and JS. There is even in Youtube some videos on how to use the crypter.

In order to use the service and encrypt Java/JS files it is necessary to use a windows application and a valid account.

The sample from the campaign "The Story of Manuel’s Java RAT5b7192be8956a0a6972cd493349fe2c58bc64529aa1f62b9f0e2eaebe65829be 
is the perfect candidate to be dissected. 

Looking at the JS code, the first interesting part is the "obfuscation" function for the payload. 

The variable hp_pavilion is the final payload which it is base64 encoded.

It basically uses the same substitution mechanism than fudcrypt, but written in JS instead of VBS and using different characters and strings.  

Below is the snippet code from fudcrypt:

After decoding the payload stored in hp_pavilion, there is again another round of base64 encoded strings, which are stored in LongText1 and LongText

LongText1 is just a base64 encoded and the output is a JS script which I will analyse later.
LongText is the malicious payload, the Java RAT, which it is base64 encoded but some characters needs to be swapped, in this case "#@>" with "A".  

This is the exact same behaviour than in Fudcrypt:

Then, the script checks if Java Run Environment is installed. If not, this is download from  hxxp://www.thegoldfingerinc[.]com/images/

 Fudcrypt does exactly the same and in the same manner. See the code from fudcrypt

Lastly, the Java RAT is made persistent via the Registry key "HKCU\\Software\\Microsoft\\Windows\\CurrentVersion\\Run\\ntfsmgr which it is the same exactly registry key name used by fudcrypt.

The remaining part is the script linked to the LongText1. This is script, after deobfuscation,  leads to a JS script which is a worm name vjw0rm.

 Although this code is not used, it has C2 capabilities.

To summarise: unknowncrypter and fudcrypter are the same crypter service but using to different script languages as output for the obfuscation. The only difference is the C2 capabilities which are gathered by two different malware, one for JS while the other is for VBS.

The analysis of the JavaRAT payload, which it is Adwind, is here