BKDR_CHOSZ.E
Trojan.Win32.Tvt.ie (Kaspersky), Backdoor.Trojan (Symantec), Troj/Bkdr-I (Sophos)
Windows 2000, Windows XP, Windows Server 2003
Threat Type: Backdoor
Destructiveness: No
Encrypted: Yes
In the wild: Yes
OVERVIEW
Dropped by other malware
This backdoor 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 commands from a remote malicious user, effectively compromising the affected system. However, as of this writing, the said sites are inaccessible.
TECHNICAL DETAILS
131,584 bytes
DLL
Yes
27 Sep 2012
Connects to URLs/IPs, Compromises system security, Collects system information, Logs keystrokes, Downloads files
Arrival Details
This backdoor 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 backdoor drops the following copies of itself into the affected system and executes them:
- {All User's Profile}\Application Data\mspool.dll
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 User's 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:
- {BLOCKED}g.{BLOCKED}l00dy.net:8080
However, as of this writing, the said sites are inaccessible.
SOLUTION
9.200
9.426.04
27 Sep 2012
9.427.00
28 Sep 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 to delete files detected as BKDR_CHOSZ.E. 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 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\Classes\SS
- PROXY
- PROXY
- 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.E. 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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