RANSOM_CRYPTESLA.YUYAIK

March 08, 2016
 Analysis by: Pearl Charlaine Espejo
 ALIASES:

Ransom:Win32/Tescrypt.H (Microsoft); Ransom.TeslaCrypt (Malwarebytes); Win32/Filecoder.TeslaCrypt.I (ESET-NOD32)

 PLATFORM:

Windows

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

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

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

It deletes the initially executed copy of itself. It is capable of encrypting files in the affected system.

  TECHNICAL DETAILS

File Size:

389,120 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

24 Feb 2016

Arrival Details

This Trojan 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 Trojan drops the following copies of itself into the affected system and executes them:

  • %Windows%\{random filename}.exe

(Note: %Windows% is the Windows folder, where it usually is C:\Windows on all Windows operating system versions.)

It drops the following files:

  • %User Profile%\Documents\recover_file_{random letters}.txt
  • %Desktop%\RECOVERY.HTM
  • %Desktop%\RECOVERY.png
  • %Desktop%\RECOVERY.TXT
  • {Folders containg encrypted files}\Recovery+{random letters}.png
  • {Folders containg encrypted files}\Recovery+{random letters}.txt
  • {Folders containg encrypted files}\Recovery+{random letters}.html

(Note: %User Profile% is the current user's profile folder, which is usually C:\Documents and Settings\{user name} on Windows 2000, XP, and Server 2003, or C:\Users\{user name} on Windows Vista and 7.. %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.)

Autostart Technique

This Trojan adds the following registry entries to enable its automatic execution at every system startup:

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Run
{random} = "%System%\cmd.exe /c start "" "%Windows%\{random filename}.exe""

Other System Modifications

This Trojan adds the following registry keys:

HKEY_CURRENT_USER\Software\xxxsys

HKEY_CURRENT_USER\Software\{Installation ID}

It adds the following registry entries:

HKEY_CURRENT_USER\Software\{Installation ID}
data = {encryption information}

HKEY_CURRENT_USER\Software\xxxsys
ID = {Installation ID}

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Policies\
System
EnabledLinkConnections = "1"

Other Details

This Trojan connects to the following possibly malicious URL:

  • http://{BLOCKED}i.se/wp-admin/user/bstr.php
  • http://{BLOCKED}bain.com/legacy/language/bstr.php
  • http://{BLOCKED}rkicks.com/wp-includes/images/bstr.php
  • http://{BLOCKED}angym.com/bstr.php
  • http://{BLOCKED}uevo.csuca.org/attachments/wp-content/bstr.php
  • http://infotlogomas.{BLOCKED}kota.go.id/admin/style/bstr.php

It deletes the initially executed copy of itself

It is capable of encrypting files in the affected system.

NOTES:

It appends the extension .mp3 to the file name of the encrypted files.

  SOLUTION

Minimum Scan Engine:

9.800

FIRST VSAPI PATTERN FILE:

12.366.04

FIRST VSAPI PATTERN DATE:

26 Feb 2016

VSAPI OPR PATTERN File:

12.367.00

VSAPI OPR PATTERN Date:

27 Feb 2016

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

Scan your computer with your Trend Micro product and note files detected as RANSOM_CRYPTESLA.YUYAIK

Step 3

Restart in Safe Mode

[ Learn More ]

Step 4

Delete this registry value

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.

  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
    • {random} = "%System%\cmd.exe /c start "" "%Windows%\{random filename}.exe""
  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\system
    • EnableLinkedConnections = "1"

Step 5

Delete this registry key

[ Learn More ]

Important: Editing the Windows Registry incorrectly can lead to irreversible system malfunction. Please do this step only if you know how or you can ask assistance from your system administrator. Else, check this Microsoft article first before modifying your computer's registry.

  • In HKEY_CURRENT_USER\Software
    • xxxsys
  • In HKEY_CURRENT_USER\Software
    • {Installation ID}

Step 6

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.
  • %User Profile%\Documents\recover_file_{random letters}.txt
  • %Desktop%\RECOVERY.HTM
  • %Desktop%\RECOVERY.png
  • %Desktop%\RECOVERY.TXT
  • Recovery+{random letters}.png
  • Recovery+{random letters}.txt
  • Recovery+{random letters}.html

Step 7

Restart in normal mode and scan your computer with your Trend Micro product for files detected as RANSOM_CRYPTESLA.YUYAIK. 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 8

Restore encrypted files from backup.


Did this description help? Tell us how we did.