RANSOM_CRYPTESLA.K

December 09, 2015
 Analysis by: Homer Pacag
 ALIASES:

Trojan.FileCoder (MalwareBytes); Ransom:Win32/Tescrypt.A (Microsoft)

 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 modifies the Internet Explorer Zone Settings.

It deletes itself after execution.

  TECHNICAL DETAILS

File Size:

352,256 bytes

File Type:

EXE

Initial Samples Received Date:

03 Dec 2015

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:

  • %Application Data%\{5 random letters}-a.exe

(Note: %Application Data% is the Application Data folder, where it usually is C:\Documents and Settings\{user name}\Application Data on Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\Users\{user name}\AppData\Roaming on 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 System Modifications

This Trojan adds the following registry keys:

HKEY_CURRENT_USER\Software\zsys

HKEY_CURRENT_USER\Software\{installation ID}

HKEY_CIRRENT_USER\S

It adds the following registry entries:

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

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

HKEY_CURRENT_USER\Software\zsys
ID = {installation ID}

HKEY_CURRENT_USER\S
Acronis = "%Application Data%\{5 random letters}-a.exe"

Web Browser Home Page and Search Page Modification

This Trojan modifies the Internet Explorer Zone Settings.

Dropping Routine

This Trojan drops the following files:

  • %User Profile%\Documents\recover_file_{9 random letters}.txt
  • %Desktop%\Howto_RESTORE_FILES.txt
  • %Desktop%\Howto_RESTORE_FILES.html
  • %Desktop%\Howto_RESTORE_FILES.bmp

(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.)

Other Details

This Trojan connects to the following URL(s) to get the affected system's IP address:

  • http://myexternalip.com/raw

It connects to the following possibly malicious URL:

  • http://leboudoirdesbrunettes.com/wp-content/uploads/misc.php
  • http://woodenden.com/sysmisc.php
  • http://apotheke-stiepel.com/tmp/misc.php
  • http://schriebershof.de/tmp/misc.php
  • http://regiefernando.meimages/slideshow/sysmisc.php

It opens the following files:

  • dropped %Desktop%\Howto_RESTORE_FILES.txt
  • dropped %Desktop%\Howto_RESTORE_FILES.html
  • dropped %Desktop%\Howto_RESTORE_FILES.bmp

(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.)

It deletes itself after execution.