BKDR_CHOSZ.KG
Windows 2000, Windows XP, Windows Server 2003
![](/vinfo/imgFiles/legend.jpg)
Threat Type: Backdoor
Destructiveness: No
Encrypted: Yes
In the wild: Yes
OVERVIEW
This backdoor may be dropped by other malware.
It executes commands from a remote malicious user, effectively compromising the affected system.
It deletes the initially executed copy of itself.
TECHNICAL DETAILS
Arrival Details
This backdoor may be dropped by the following malware:
Installation
This backdoor drops the following copies of itself into the affected system and executes them:
- All Users' %User Profile%\Application Data\mspool.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 backdoor 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\mspool
Description = "Windows mspool service."
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
DisplayName = "mspool"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
ErrorControl = "1"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
ImagePath = "%SystemRoot%\system32\svchost.exe -k LocalService"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
ObjectName = "LocalSystem"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
Start = "2"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
Type = "10"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Parameters
ServiceDll = "All Users' %User Profile%\Application Data\mspool.dll"
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Parameters
ServiceMain = BofMrf
Other System Modifications
This backdoor adds the following registry keys:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Enum
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Parameters
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Services\mspool\Security
HKEY_LOCAL_MACHINE\Classes\SS
HKEY_LOCAL_MACHINE\Classes\SS\
PROXY
It modifies the following registry entries:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\SvcHost
LocalService = "{Default values} mspool"
(Note: The default value data of the said registry entry is {Default values}.)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Internet Settings\
Cache\Paths
Directory = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5"
(Note: The default value data of the said registry entry is %Temporary Internet Files%\Content.IE5.)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Internet Settings\
Cache\Paths\path1
CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache1"
(Note: The default value data of the said registry entry is %Temporary Internet Files%\Content.IE5\Cache1.)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Internet Settings\
Cache\Paths\path2
CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache2"
(Note: The default value data of the said registry entry is %Temporary Internet Files%\Content.IE5\Cache2.)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Internet Settings\
Cache\Paths\path3
CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache3"
(Note: The default value data of the said registry entry is %Temporary Internet Files%\Content.IE5\Cache3.)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Internet Settings\
Cache\Paths\path4
CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache4"
(Note: The default value data of the said registry entry is %Temporary Internet Files%\Content.IE5\Cache4.)
HKEY_USERS\.DEFAULT\Software\
Microsoft\Windows\CurrentVersion\
Explorer\Shell Folders
Cookies = "%System Root%\Documents and Settings\LocalService\Cookies"
(Note: The default value data of the said registry entry is %System Root%\Documents and Settings\NetworkService\Cookies.)
HKEY_USERS\.DEFAULT\Software\
Microsoft\Windows\CurrentVersion\
Explorer\Shell Folders
Cache = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files"
(Note: The default value data of the said registry entry is %System Root%\Documents and Settings\NetworkService\Local Settings\Temporary Internet Files.)
HKEY_USERS\.DEFAULT\Software\
Microsoft\Windows\CurrentVersion\
Explorer\Shell Folders
History = "%System Root%\Documents and Settings\LocalService\Local Settings\History"
(Note: The default value data of the said registry entry is %System Root%\Documents and Settings\NetworkService\Local Settings\History .)
Backdoor Routine
This backdoor executes the following commands from a remote malicious user:
- Download files
- Get drive information
- Get system information
- Log keystrokes and active window
- Manage files and folders
- Manage processes
- Manage registry
- Manage services
- Perform remote shell
It connects to the following URL(s) to send and receive commands from a remote malicious user:
- http://{BLOCKED}.{BLOCKED}.83.48
- http://{BLOCKED}l.{BLOCKED}uake.com
Other Details
This backdoor deletes the initially executed copy of itself
SOLUTION
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 3
Restart in Safe Mode
Step 4
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
- mspool
- mspool
Step 5
Restore this modified 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_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SvcHost
- From: LocalService = "{Default values} mspool"
To: LocalService = {Default values}
- From: LocalService = "{Default values} mspool"
- In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Cache\Paths
- From: Directory = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5"
To: Directory = %Temporary Internet Files%\Content.IE5
- From: Directory = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5"
- In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Cache\Paths\path1
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache1"
To: CachePath = %Temporary Internet Files%\Content.IE5\Cache1
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache1"
- In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Cache\Paths\path2
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache2"
To: CachePath = %Temporary Internet Files%\Content.IE5\Cache2
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache2"
- In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Cache\Paths\path3
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache3"
To: CachePath = %Temporary Internet Files%\Content.IE5\Cache3
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache3"
- In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\Cache\Paths\path4
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache4"
To: CachePath = %Temporary Internet Files%\Content.IE5\Cache4
- From: CachePath = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\Cache4"
- In HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
- From: Cookies = "%System Root%\Documents and Settings\LocalService\Cookies"
To: Cookies = %System Root%\Documents and Settings\NetworkService\Cookies
- From: Cookies = "%System Root%\Documents and Settings\LocalService\Cookies"
- In HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
- From: Cache = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files"
To: Cache = %System Root%\Documents and Settings\NetworkService\Local Settings\Temporary Internet Files
- From: Cache = "%System Root%\Documents and Settings\LocalService\Local Settings\Temporary Internet Files"
- In HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
- From: History = "%System Root%\Documents and Settings\LocalService\Local Settings\History"
To: History = %System Root%\Documents and Settings\NetworkService\Local Settings\History
- From: History = "%System Root%\Documents and Settings\LocalService\Local Settings\History"
Step 6
Restart in normal mode and scan your computer with your Trend Micro product for files detected as BKDR_CHOSZ.KG. 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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