Analysis by: Anthony Joe Melgarejo
ALIASES:

Worm:Win32/Cridex.E (Microsoft), W32.Cridex (Symantec), PWS-Zbot.gen.als (McAfee)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Worm

  • Destructiveness: No

  • Encrypted:

  • In the wild: Yes

  OVERVIEW

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

It is injected into all running processes to remain memory resident.

It deletes the initially executed copy of itself.

  TECHNICAL DETAILS

File Size: 87,040 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 09 Oct 2012

Arrival Details

This worm 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 worm drops the following files:

  • %User Temp%\exp{number}.tmp.bat - used to delete the initially executed copy

(Note: %User Temp% is the current user's Temp folder, which is usually C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, XP, and Server 2003.)

It drops the following copies of itself into the affected system:

  • %User Profile%\Application Data\KB{random number}.exe

(Note: %User Profile% is the current user's profile folder, which is usually C:\Windows\Profiles\{user name} on Windows 98 and ME, C:\WINNT\Profiles\{user name} on Windows NT, and C:\Documents and Settings\{user name} on Windows 2000, XP, and Server 2003.)

It is injected into all running processes to remain memory resident.

Autostart Technique

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

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Run
KB{random number}.exe = "%User Profile%\Application Data\KB{random number}.exe"

Other System Modifications

This worm adds the following registry keys:

HKEY_CURRENT_USER\Software\Microsoft\
Windows NT\{random string 1}

HKEY_CURRENT_USER\Software\Microsoft\
Windows NT\{random string 2}

It adds the following registry entries:

HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Internet Settings
GlobalUserOffline = "0"

Other Details

This worm connects to the following possibly malicious URL:

  • http://{BLOCKED}.{BLOCKED}.208.55:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.134.110:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.13.84:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.168.98:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.2.214:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.23.100:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.5.140:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.147.52:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.22.72:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.101.32:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.0.138:8080/mx/5/A/in/
  • http://{BLOCKED}.{BLOCKED}.167.193:8080/mx/5/A/in/

It deletes the initially executed copy of itself

  SOLUTION

Minimum Scan Engine: 9.200
FIRST VSAPI PATTERN FILE: 9.454.03
FIRST VSAPI PATTERN DATE: 09 Oct 2012
VSAPI OPR PATTERN File: 9.455.00
VSAPI OPR PATTERN Date: 10 Oct 2012

Step 1

For Windows XP and Windows Server 2003 users, before doing any scans, please make sure you disable System Restore to allow full scanning of your computer.

Step 2

Restart in Safe Mode

[ Learn More ]

Step 3

Search and delete this file

[ Learn More ]
There may be some component 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 Temp%\exp{number}.tmp.bat

Step 4

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\Microsoft\Windows NT
    • {random string 1}
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows NT
    • {random string 2}

Step 5

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
    • KB{random number}.exe = "%User Profile%\Application Data\KB{random number}.exe"
  • In HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings
    • GlobalUserOffline = "0"

Step 6

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

NOTES:
The created registry keys HKEY_CURRENT_USER\Software\Microsoft\Windows NT\{random hex string 1} and HKEY_CURRENT_USER\Software\Microsoft\Windows NT\{random hex string 2} cannot be identified by the users since there are no reference values in the created keys. The only way these keys can be identified is by comparing the present keys with a backup if the users have one. Note that these keys need not to be deleted since it won't cause the user system any harm.


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