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 file(s)/component(s):
- %System%\mas.dll
- %User Temp%\WIN{Random Characters}.tmp
(Note: %System% is the Windows system folder, where it usually is C:\Windows\System32 on all Windows operating system versions.. %User Temp% is the user's temporary folder, where it usually is C:\Documents and Settings\{user name}\Local Settings\Temp on Windows 2000, Windows Server 2003, and Windows XP (32- and 64-bit); C:\Users\{user name}\AppData\Local\Temp on Windows Vista (32- and 64-bit), Windows 7 (32- and 64-bit), Windows 8 (32- and 64-bit), Windows 8.1 (32- and 64-bit), Windows Server 2008, and Windows Server 2012.)
Other System Modifications
This backdoor adds the following registry keys:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\
services\mas
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\
services\mas\Parameters
It adds the following registry entries as part of its installation routine:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\
services\mas\Parameters
ServiceDll = %System%\mas.dll
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\
services\mas\Parameters
Arguments = install 87.117.204.143 80
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows NT\CurrentVersion\Svchost
mas = mas
Other Details
This backdoor connects to the following possibly malicious URL:
- {BLOCKED}.{BLOCKED}.204.143:80
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
Note that not all files, folders, and registry keys and entries are installed on your computer during this malware's/spyware's/grayware's execution. This may be due to incomplete installation or other operating system conditions. If you do not find the same files/folders/registry information, please proceed to the next step.
Step 3
Restart in Safe Mode
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To restart in Safe Mode:
• For Windows 2000 users
- Restart your computer.
- Press F8 when you see the Starting Windows bar at the bottom of the screen.
- Choose the Safe Mode option from the Windows Advanced Options menu then press Enter.
• For Windows XP users
- Restart your computer.
- Press F8 after the Power-On Self Test (POST) routine is done. If the Windows Advanced Options menu does not appear, try restarting then pressing F8 several times when the POST screen appears.
- Choose the Safe Mode option from the Windows Advanced Options menu then press Enter.
• For Windows Server 2003 users
- Restart your computer.
- Press F8 after Windows starts up. If the Windows Advanced Options menu does not appear, try restarting again and pressing F8 several times afterward.
- On the Windows Advanced Option menu, use the arrow keys to select Safe Mode then press Enter.
• For Windows Vista, Windows 7, and Windows Server 2008 users
- Restart your computer.
- Press F8 after the Power-On Self Test (POST) routine is done. If the Advanced Boot Options menu does not appear, try restarting and then pressing F8 several times after the POST screen is displayed.
- On the Advanced Boot Options menu, use the arrow keys to select the Safe Mode option, and then press Enter.
• For Windows 8, Windows 8.1, and Windows Server 2012 users
- Access the Charms bar by moving your mouse pointer to the upper right corner of the screen.
- Move your mouse pointer down and click Settings>Change your PC Settings.
- In the left panel, click General.
- In the right panel, scroll down to the bottom to find the Advanced startup section, then click the Restart now button and wait for the system to restart.
- In the Advanced Startup menu, click Troubleshoot>Advanced Options>Startup Settings>Restart and wait for the system to restart.
- In the Startup Settings menu, press 4 to enable safe mode.
Step 4
Delete this registry value
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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\mas\Parameters
- ServiceDll = %System%\mas.dll
- In HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\services\mas\Parameters
- Arguments = install 87.117.204.143 80
- In HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Svchost
To delete the registry value this malware/grayware created:
- Open Registry Editor.
» For Windows 2000, Windows XP, and Windows Server 2003 users, click Start>Run, type regedit in the text box provided, and then press Enter.
» For Windows Vista, Windows 7, and Windows Server 2008 users, click the Start button, type regedit in the Search input field then press Enter.
» For Windows 8, Windows 8.1, and Windows Server 2012 users, right-click on the lower-left corner of the screen, click Run, type regedit in the text box provided, and then press Enter. - In the left panel, double-click the following:
HKEY_LOCAL_MACHINE>SYSTEM>ControlSet001>services>mas>Parameters - In the right panel, locate and delete the entry:
ServiceDll = %System%\mas.dll - Again In the right panel, locate and delete the entry:
Arguments = install 87.117.204.143 80 - In the left panel, double-click the following:
HKEY_LOCAL_MACHINE>SOFTWARE>Microsoft>Windows NT>CurrentVersion>Svchost - In the right panel, locate and delete the entry:
mas = mas - Close Registry Editor.
Step 5
Delete this registry key
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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\mas
- In HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\services
To delete registry keys this malware/grayware created:
- Open Registry Editor. To do this:
» For Windows 2000, Windows XP, and Windows Server 2003 users, click Start>Run, type regedit in the text box provided, and then press Enter.
» For Windows Vista, Windows 7, and Windows Server 2008 users, click the Start button, type regedit in the Search input field then press Enter.
» For Windows 8, Windows 8.1, and Windows Server 2012 users, right-click on the lower left corner of the screen, click Run, type regedit in the text box provided, and then press Enter. - In the left panel of the Registry Editor window, double-click the following:
HKEY_LOCAL_MACHINE>SYSTEM>ControlSet001>services>mas - Still in the left panel, locate and delete the key:
Parameters - In the left panel of the Registry Editor window, double-click the following:
HKEY_LOCAL_MACHINE>SYSTEM>ControlSet001>services - Still in the left panel, locate and delete the key:
mas - Close Registry Editor.
Step 6
Search and delete these files
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There may be some files that are hidden. Please make sure you check the
Search Hidden Files and Folders checkbox in the "More advanced options" option to include all hidden files and folders in the search result.
- %System%\mas.dll
- %User Temp%\WIN{Random Characters}.tmp
To delete malware/grayware files:
For Windows 2000, Windows XP, and Windows Server 2003:
- Right-click Start then click Search....
- In the File name* input box, type
- %System%\mas.dll
- %User Temp%\WIN{Random Characters}.tmp
- In the Look In drop-down list, select My Computer then press Enter.
- Once located, select the file then press SHIFT+DELETE to delete it.
- Repeat the said steps for all files listed.
*Note: The file name input box title varies depending on the Windows version (e.g. Search for files or folders named or All or part of the file name.).
For Windows Vista, Windows 7, Windows Server 2008, Windows 8, Windows 8.1, and Windows Server 2012:
- Open a Windows Explorer window.
- For Windows Vista, 7, and Server 2008 users, click Start>Computer.
- For Windows 8, 8.1, and Server 2012 users, right-click on the lower left corner of the screen, then click File Explorer.
- In the Search Computer/This PC input box, type:
- %System%\mas.dll
- %User Temp%\WIN{Random Characters}.tmp
- Once located, select the file then press SHIFT+DELETE to delete it.
- Repeat the said steps for all files listed.
*Note: Read the following Microsoft page if these steps do not work on Windows 7.
Step 7
Restart in normal mode and scan your computer with your Trend Micro product for files detected as BKDR_REMEXI.A. 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.
