TROJ_CUTWAIL.YYT

March 07, 2015
 Analysis by: RonJay Kristoffer Caragay
 ALIASES:

TrojanDownloader:Win32/Cutwail.BF (Microsoft); Trojan.Win32.Cutwail.fgx (Kaspersky); Mal/Wonton-AN (Sophos);

 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.

  TECHNICAL DETAILS

File Size:

149,504 bytes

File Type:

EXE

Memory Resident:

Yes

Initial Samples Received Date:

16 Feb 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:

  • %System%\{random 1}.exe
  • %User Profile%\{random 1}.exe

(Note: %System% is the Windows system folder, where it usually is C:\Windows\System32 on all Windows operating system versions.. %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.)

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 1} = "%User Profile%\{random 1}.exe"

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Run
{random 1} = %System%\{random 1}.exe"

Other System Modifications

This Trojan adds the following registry keys:

HKEY_LOCAL_MACHINE\Software\WinNTData

HKEY_CURRENT_USER\Software\WinNTData

It adds the following registry entries:

HKEY_LOCAL_MACHINE\SOFTWARE\WinNTData
prx = "{hex values}"

HKEY_CURRENT_USER\Software\WinNTData
prx = "{hex values}"

Other Details

This Trojan connects to the following possibly malicious URL:

  • {BLOCKED}.{BLOCKED}.2.53
  • {BLOCKED}.{BLOCKED}.10.90
  • {BLOCKED}.{BLOCKED}.36.4
  • {BLOCKED}.{BLOCKED}.230.10
  • {BLOCKED}.{BLOCKED}.79.201
  • {BLOCKED}.{BLOCKED}.4.12
  • {BLOCKED}.{BLOCKED}.148.17
  • {BLOCKED}.{BLOCKED}.5.241
  • {BLOCKED}.{BLOCKED}.128.30
  • {BLOCKED}.{BLOCKED}.14.129
  • {BLOCKED}.{BLOCKED}.0.4
  • {BLOCKED}.{BLOCKED}.83.42
  • {BLOCKED}.{BLOCKED}.27.33
  • {BLOCKED}.{BLOCKED}.32.84
  • {BLOCKED}.{BLOCKED}.254.15
  • {BLOCKED}.{BLOCKED}.159.131

It deletes the initially executed copy of itself

  SOLUTION

Minimum Scan Engine:

9.700

FIRST VSAPI PATTERN FILE:

11.484.01

FIRST VSAPI PATTERN DATE:

17 Feb 2015

VSAPI OPR PATTERN File:

11.485.00

VSAPI OPR PATTERN Date:

18 Feb 2015

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

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 1} = "%User Profile%\{random 1}.exe"
  • In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
    • {random 1} = "%System%\{random 1}.exe"

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_LOCAL_MACHINE\Software
    • WinNTData
  • In HKEY_CURRENT_USER\Software
    • WinNTData

Step 6

Restart in normal mode and scan your computer with your Trend Micro product for files detected as TROJ_CUTWAIL.YYT. 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.


Did this description help? Tell us how we did.