Analysis by: Mark Joseph Manahan
ALIASES:

Worm:Win32/Cridex.B (Microsoft), Trojan-Dropper.Win32.Dapato.acyi (Kaspersky), a variant of Win32/Kryptik.AAWT trojan (Eset)

 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 drops an AUTORUN.INF file to automatically execute the copies it drops when a user accesses the drives of an affected system.

It deletes the initially executed copy of itself.

  TECHNICAL DETAILS

File Size: 65,536 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 22 Feb 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 copies of itself into the affected system:

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

(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.)

It creates the following folders:

  • %User Profile%\Application Data\{random folder name 1}

(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.)

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 numbers}.exe = "%User Profile%\Application Data\KB{random numbers}.exe"

Other System Modifications

This worm adds the following registry keys:

HKEY_CURRENT_USER\Software\Microsoft\
Windows Media Center\{random keyname 1}

HKEY_CURRENT_USER\Software\Microsoft\
Windows Media Center\{random keyname 2}

Propagation

This worm creates the following folders in all removable drives:

  • {removable drive}\{random folder name 2}

It drops the following copy(ies) of itself in all removable drives:

  • {random folder name 2}\{random file name}.exe

It drops an AUTORUN.INF file to automatically execute the copies it drops when a user accesses the drives of an affected system.

The said .INF file contains the following strings:

[autorun]
open={random folder name 2}\{random file name}.exe
shell\Open\Command={random folder name 2}\{random file name}.exe
shell\Open\Default=1
shell\Explore\Command={random folder name 2}\{random file name}.exe
shell\Autoplay\command={random folder name 2}\{random file name}.exe

Other Details

This worm connects to the following possibly malicious URL:

  • http://{pseudorandom}.ru:8080/{BLOCKED}x/B1_3n9/in

It deletes the initially executed copy of itself

  SOLUTION

Minimum Scan Engine: 9.300
FIRST VSAPI PATTERN FILE: 8.792.07
FIRST VSAPI PATTERN DATE: 22 Feb 2012
VSAPI OPR PATTERN File: 8.793.00
VSAPI OPR PATTERN Date: 22 Feb 2012

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 WORM_CRIDEX.ICA

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

Step 5

Search and delete these folders

[ Learn More ]
Please make sure you check the Search Hidden Files and Folders checkbox in the More advanced options option to include all hidden folders in the search result.
  • %User Profile%\Application Data\{random folder name 1}
  • {removable drive}\{random folder name 2}

Step 6

Search and delete AUTORUN.INF files created by WORM_CRIDEX.ICA that contain these strings

[ Learn More ]


[AutoRun]
open={random folder  name 2 }\{random file name}.exe
shell\Open\Command={random folder  name 2 }\{random file name}.exe
shell\Open\Default=1
shell\Explore\Command={random folder  name 2 }\{random file name}.exe
shell\Autoplay\command={random folder name 2}\{random file name}.exe

Step 7

Restart in normal mode and scan your computer with your Trend Micro product for files detected as WORM_CRIDEX.ICA. 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 Media Center\{random keyname 1} and HKEY_CURRENT_USER\Software\Microsoft\Windows Media Center\{random keyname 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 be deleted since it won't cause the user system any harm.


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