TROJ_NECURS.BGST
W32/Necurs.BXOS!tr (Fortinet), Trojan-Dropper.Win32.Necurs.toi (Kaspersky), Trojan:Win32/Necurs.A (Microsoft)
Windows 2000, Windows Server 2003, Windows XP (32-bit, 64-bit), Windows Vista (32-bit, 64-bit), Windows 7 (32-bit, 64-bit)
Threat Type: Trojan
Destructiveness: No
Encrypted:
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.
TECHNICAL DETAILS
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\{random GUID}\syshost.exe - if logged in as an Administrator
- %Application Data%\{random GUID}\syshost.exe - if not logged in as an Administrator
(Note: %Windows% is the Windows folder, which is usually C:\Windows.. %Application Data% is the current user's Application Data folder, which is usually C:\Documents and Settings\{user name}\Application Data on Windows 2000, XP, and Server 2003, or C:\Users\{user name}\AppData\Roaming on Windows Vista and 7.)
It creates the following folders:
- %Windows%\Installer\{random GUID} - if logged in as an Administrator
- %Application Data%\{random GUID} - if not logged in as an Administrator
(Note: %Windows% is the Windows folder, which is usually C:\Windows.. %Application Data% is the current user's Application Data folder, which is usually C:\Documents and Settings\{user name}\Application Data on Windows 2000, XP, and Server 2003, or C:\Users\{user name}\AppData\Roaming on Windows Vista and 7.)
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
Type = "10"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\syshost32
Start = "2"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\syshost32
ErrorControl = "0"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\syshost32
ImagePath = "%Windows%\Installer\{random GUID}\syshost.exe" /service
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\syshost32
ObjectName = "LocalSystem"
It adds the following registry entries to enable its automatic execution at every system startup:
HKEY_CURRENT_USER\Software\Microsoft\
Windows\CurrentVersion\Run
syshost32 = "%Application Data%\{random GUID}\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 deletes the following files:
- {malware path and file name}.tmp
- %Windows%\Installer\{random GUID}\syshost.exe:Zone.Identifier (if logged in as an Administrator)
- %Application Data%\{random CLSID}\syshost.exe:Zone.Identifier (if not logged in as an Administrator)
(Note: %Windows% is the Windows folder, which is usually C:\Windows.. %Application Data% is the current user's Application Data folder, which is usually C:\Documents and Settings\{user name}\Application Data on Windows 2000, XP, and Server 2003, or C:\Users\{user name}\AppData\Roaming on Windows Vista and 7.)
SOLUTION
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
Restart in Safe Mode
Step 3
Delete this registry key
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
- syshost32
Step 4
Delete this registry value
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
- syshost32 = "%Application Data%\{random GUID}\syshost.exe"
- syshost32 = "%Application Data%\{random GUID}\syshost.exe"
Step 5
Search and delete this folder
- %Windows%\Installer\{random GUID}
- %Application Data%\{random GUID}
Step 6
Restart in normal mode and scan your computer with your Trend Micro product for files detected as TROJ_NECURS.BGST. 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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