Analysis by: Anthony Joe Melgarejo
ALIASES:

Trojan:Win32/Necurs.gen!A (Microsoft), Backdoor.Necurs (Symantec), BackDoor-FHK (Symantec)

 PLATFORM:

Windows 2000, Windows Server 2003, Windows XP (32-bit, 64-bit), Windows Vista (32-bit, 64-bit), Windows 7 (32-bit, 64-bit)

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

  • Threat Type: Trojan

  • Destructiveness: No

  • Encrypted: Yes

  • In the wild: Yes

  OVERVIEW

Infection Channel: Downloaded from the Internet, Dropped by other malware

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 executes then deletes itself afterward.

It modifies registry entries to disable the Windows Firewall settings. This action allows this malware to perform its routines without being deteted by the Windows Firewall.

  TECHNICAL DETAILS

File Size: 339,968 bytes
File Type: EXE
Memory Resident: Yes
Initial Samples Received Date: 19 Dec 2012
Payload: Connects to URLs/IPs, Drops files

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:

  • %Windows%\Installer\{GUID}\syshost.exe

(Note: %Windows% is the Windows folder, which is usually C:\Windows.)

It creates the following folders:

  • %Windows%\Installer\{GUID}

(Note: %Windows% is the Windows folder, which is usually C:\Windows.)

It executes then deletes itself afterward.

Autostart Technique

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

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\syshost32
ImagePath = ""%Windows%\Installer\{GUID}\syshost.exe" /service"

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

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Run
syshost32 = "%Windows%\Installer\syshost.exe"

It registers as a system service to ensure its automatic execution at every system startup by adding the following registry keys:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\syshost32

Other System Modifications

This Trojan modifies the following registry entries to disable the Windows Firewall settings:

HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\
Services\SharedAcces\Parameters\
FirewallPolicy\DomainProfile
EnableFirewall = "0"

(Note: The default value data of the said registry entry is 1.)

Other Details

This Trojan connects to the following URL(s) to get the affected system's IP address:

  • checkip.dyndns.org

It connects to the following possibly malicious URL:

  • {BLOCKED}7.{BLOCKED}2.178.104:82/cgi-bin/forum.cgi

NOTES:

It drops its component, %System%\drivers\{random}.sys and loads it. Then it deletes the dropped component.

It performs a netsh.exe command that disables Windows Firewall depending on the OS Version. It elevates certain privileges when not in Admin Mode.

It generates random IP addresses and connects through port 123.

  SOLUTION

Minimum Scan Engine: 9.300
FIRST VSAPI PATTERN FILE: 9.612.01
FIRST VSAPI PATTERN DATE: 23 Dec 2012
VSAPI OPR PATTERN File: 9.613.00
VSAPI OPR PATTERN Date: 24 Dec 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 BKDR_NECURS.XSP

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\SYSTEM\CurrentControlSet\Services
    • syshost32

Step 5

Restore this modified 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\SYSTEM\ControlSet001\Services\SharedAcces\Parameters\FirewallPolicy\DomainProfile
    • From: Enable Firewall = "0"
      To: Enable Firewall = "1"

Step 6

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
    • syshost32 = "%Windows%\Installer\{GUID}\syshost.exe"

Step 7

Search and delete this folder

[ 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.
  • %Windows%\Installer\{GUID}

Step 8

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