Analysis by: Abraham Latimer Camba
ALIASES:

Trojan:Win32/Medfos.gen!A (Microsoft), a variant of Win32/Medfos.AM trojan (ESET), Mal/Medfos-A (Sophos)

 PLATFORM:

Windows 2000, Windows XP, Windows Server 2003

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

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted: Yes

  • 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: 123,392 bytes
File Type: DLL
Memory Resident: Yes
Initial Samples Received Date: 11 Jul 2012

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:

  • %User Profile%\Application Data\{random name}.dll

(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.)

Autostart Technique

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

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Run
{random name} = "rundll32.exe "%User Profile%\Application Data\{random name}.dll",HrGetElementImpl"

Other Details

This Trojan connects to the following possibly malicious URL:

  • http://{BLOCKED}8.{BLOCKED}0.131.158/upload005?id={random value}&t={random value}
  • http://{BLOCKED}load.com/file/id=BABQAAEAp3sUAAEFCBcAAAAAAAAAAAAAAAAAAABrDAcGCwAAACQ0-PXhEc3JrLhwgiZvmW8AAFVVVVVVVVVVVVVVVVVVVVURZc0BdoXEWKd7FAB4a3F8dYJOZ3UxXE9PWlMEBQYBAhORIicbl80HAAAAAAA0VgAA&rt=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

It deletes the initially executed copy of itself

  SOLUTION

Minimum Scan Engine: 9.200
FIRST VSAPI PATTERN FILE: 9.250.02
FIRST VSAPI PATTERN DATE: 11 Jul 2012
VSAPI OPR PATTERN File: 9.251.00
VSAPI OPR PATTERN Date: 12 Jul 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 TROJ_MEDFOS.IB

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_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
    • {random name} = "rundll32.exe "%User Profile%\Application Data\{random name}.dll",HrGetElementImpl"

Step 5

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


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