Analysis by: Marvelous Pelin
ALIASES:

Ransom.Rensen(Malwarebytes); Ransom:Win32/Rensen.A!rsm(Microsoft)

 PLATFORM:

Windows

 OVERALL RISK RATING:
 DAMAGE POTENTIAL:
 DISTRIBUTION POTENTIAL:
 REPORTED INFECTION:
 INFORMATION EXPOSURE:

  • Threat Type: Ransomware

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  OVERVIEW

Infection Channel: Dropped by other malware, Downloaded from the Internet

This Ransomware arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

  TECHNICAL DETAILS

File Size: 98,816 bytes
File Type: EXE
Memory Resident: No
Initial Samples Received Date: 06 Apr 2017
Payload: Displays message/message boxes, Encrypts files

Arrival Details

This Ransomware arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites.

Installation

This Ransomware drops the following files:

  • %Desktop%\randomiv.bin
  • %Desktop%\randomkey.bin

(Note: %Desktop% is the desktop folder, where it usually is C:\Documents and Settings\{user name}\Desktop in Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\Users\{user name}\Desktop in Windows Vista (32- and 64-bit), Windows 7 (32- and 64-bit), Windows 8 (32- and 64-bit), Windows 8.1 (32- and 64-bit), Windows Server 2008, and Windows Server 2012.)

Other Details

This Ransomware encrypts files with the following extensions:

  • .7z
  • .alz
  • .avi
  • .bas
  • .c
  • .cpp
  • .cs
  • .doc
  • .docx
  • .egg
  • .flac
  • .frm
  • .gif
  • .hwp
  • .jpg
  • .js
  • .mkv
  • .mp3
  • .mp4
  • .pdf
  • .png
  • .ppt
  • .pptx
  • .psd
  • .rar
  • .raw
  • .txt
  • .vb
  • .wav
  • .xls
  • .xlsx
  • .zip

It renames encrypted files using the following names:

  • {original filename and extension}.RENSENWARE

It does the following:

  • It checks if the process "th12" (Touhou 12 - Undefined Fantastic Object) is running. If the Touhou 12 game is running, it checks if the player scored over 20 million in the lunatic level before you can decrypt the encrypted files.

NOTES:

The ransomware displays the following windows showing its ransom note:

It also displays the following for its manual decryption:

  SOLUTION

Minimum Scan Engine: 9.850
FIRST VSAPI PATTERN FILE: 13.326.02
FIRST VSAPI PATTERN DATE: 07 Apr 2017
VSAPI OPR PATTERN File: 13.327.00
VSAPI OPR PATTERN Date: 08 Apr 2017

Step 1

Before doing any scans, Windows XP, Windows Vista, and Windows 7 users must disable System Restore to allow full scanning of their computers.

Step 2

Note that not all files, folders, and registry keys and entries are installed on your computer during this malware's/spyware's/grayware's execution. This may be due to incomplete installation or other operating system conditions. If you do not find the same files/folders/registry information, please proceed to the next step.

Step 3

Scan your computer with your Trend Micro product to delete files detected as RANSOM_HIDDENTEARRENSEN.A. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check this Knowledge Base page for more information.

Step 4

Restart in Safe Mode

[ Learn More ]

Step 5

Search and delete this file

[ Learn More ]
There may be some files that are hidden. Please make sure you check the Search Hidden Files and Folders checkbox in the "More advanced options" option to include all hidden files and folders in the search result.  
  • %Desktop%\randomiv.bin
  • %Desktop%\randomkey.bin

Step 6

Restart in normal mode and scan your computer with your Trend Micro product for files detected as RANSOM_HIDDENTEARRENSEN.A. If the detected files have already been cleaned, deleted, or quarantined by your Trend Micro product, no further step is required. You may opt to simply delete the quarantined files. Please check this Knowledge Base page for more information.

Step 7

Restore encrypted files from backup.


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