Analysis by: Jed Valderama
ALIASES:

Backdoor:Win32/Agent (Microsoft), Backdoor.Win32.Agent.pu (Kaspersky), BackDoor-CWA (McAfee)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Backdoor

  • Destructiveness: No

  • Encrypted: No

  • In the wild: Yes

  OVERVIEW

Infection Channel: Dropped by other malware

This backdoor may be dropped by other malware.

It injects its dropped file/component to specific processes.

It executes commands from a remote malicious user, effectively compromising the affected system.

  TECHNICAL DETAILS

Ports: TCP Port 443
File Size: 45,056 bytes
File Type: EXE, DLL
Memory Resident: Yes
Initial Samples Received Date: 02 Aug 2012

Arrival Details

This backdoor may be dropped by the following malware:

  • TROJ_MDROP.ID

Installation

This backdoor drops a copy of itself in the following folders using different file names:

  • %User Temp%\{temporary filename}.tmp

(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 component file(s):

  • %System%\tstdmc.dll - also detected as BKDR_HANMON.A
  • %System%\dmcpyt.dl - also detected as BKDR_HANMON.A

(Note: %System% is the Windows system folder, which is usually C:\Windows\System on Windows 98 and ME, C:\WINNT\System32 on Windows NT and 2000, or C:\Windows\System32 on Windows XP and Server 2003.)

It injects its dropped file/component to the following processes:

  • lsass.exe
  • Explorer.exe
  • iexplore.exe

Autostart Technique

This backdoor registers itself as a system service to ensure its automatic execution at every system startup by adding the following registry entries:

HKLM\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\Svchost
dmon = "DiskMon32"

HKLM\SYSTEM\CurrentControlSet\
Services\DiskMon32
ServiceDLL = "C:\WINDOWS\system32\dmcpyt.dll"

HKLM\SYSTEM\CurrentControlSet\
Services\DiskMon32
Interactive = "dword:00000000"

It adds and runs the following services:

  • "Disk Monitor Services"

Backdoor Routine

This backdoor executes the following commands from a remote malicious user:

  • Send and receive other files.
  • Modify registries.
  • Execute other files.
  • Remote execution of arbitrary code received from the server.

It connects to the following URL(s) to send and receive commands from a remote malicious user:

  • {BLOCKED}.{BLOCKED}.4.173

It posts the following information to its command and control (C&C) server:

  • System LCID
  • Computer Name
  • Username
  • Operating System Version
  • System Info
  • Current Global Memory Status
  • Root Path Volume Information
  • List of disk drives.
  • Local IP Address.

  SOLUTION

Minimum Scan Engine: 9.200
FIRST VSAPI PATTERN FILE: 9.300.02
FIRST VSAPI PATTERN DATE: 02 Aug 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

Scan your computer with your Trend Micro product and note files detected as BKDR_HANMON.A

Step 3

Restart in Safe Mode

[ Learn More ]

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_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Svchost
    • dmon = "DiskMon32"
  • In HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
    • DiskMon32

Step 5

Search and delete the file detected as BKDR_HANMON.A

*Note: Some component files may be 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.

To manually delete a malware/grayware file from the affected system:

  1. Right-click Start then click Search....
  2. In the Named input box, type the name of the file that was detected earlier.
  3. In the Look In drop-down list, select My Computer then press Enter.
  4. Once located, select the file then press SHIFT+DELETE to delete it.

Step 6

Restart in normal mode and scan your computer with your Trend Micro product for files detected as BKDR_HANMON.A. 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 7

Remove malware/grayware files that dropped/downloaded BKDR_HANMON.A

    • TROJ_MDROP.ID


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