1. Symantec/
  2. Security Response/
  3. W32.Gosys


Risk Level 2: Low

November 11, 2009
July 1, 2013 7:52:54 AM
Also Known As:
W32/Semblant-A [Sophos], Swisyn.v [McAfee], Troj/VBLuiha-A [Sophos]
Infection Length:
192,533 bytes
Systems Affected:
When the worm executes, it creates the following files:
  • %UserProfile%\Application Data\mrsys.exe
  • %UserProfile%\Local Settings\Temporary Internet Files\Content.IE5\4H67CTM7\3picsys[1].gif
  • %UserProfile%\Local Settings\Temporary Internet Files\Content.IE5\GTYN8HUZ\cmsys[1].gif
  • %UserProfile%\Local Settings\Temporary Internet Files\Content.IE5\W9UNG1MR\2picsys[1].gif
  • %UserProfile%\Application Data\stsys.exe
  • %System%\blsys.bln
  • %System%\cmsys.cmn
  • %System%\explorer.exe
  • %Windir%\2clksys1.ptn
  • %Windir%\2clksys2.ptn
  • %Windir%\2clksys3.ptn
  • %Windir%\2clksys4.ptn
  • %Windir%\2dclsys1.ptn
  • %Windir%\2entsys1.ptn
  • %Windir%\2entsys2.ptn
  • %Windir%\2picsys.cpn
  • %Windir%\3clksys1.ptn
  • %Windir%\3clksys2.ptn
  • %Windir%\3clksys3.ptn
  • %Windir%\3clksys4.ptn
  • %Windir%\3dclsys1.ptn
  • %Windir%\3entsys1.ptn
  • %Windir%\3entsys2.ptn
  • %Windir%\3picsys.cpn
  • %Windir%\blsys.bln
  • %Windir%\spoolsv.exe
  • %Windir%\svchost.exe

Next, the worm creates the following registry entries so that it executes whenever Windows starts:
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\{F146C9B1-VMVQ-A9RC-NUFL-D0BA00B4E999}\"StubPath" = "%UserProfile%\Local Settings\Application Data\mrsys.exe MR"
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\{Y479C6D0-OTRW-U5GH-S1EE-E0AC10B4E666}\"StubPath" = "%UserProfile%\Local Settings\Application Data\mrsys.exe MR"
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce\"Explorer" = "c:\windows\system32\explorer.exe RO"
  • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce\"Svchost" = "c:\windows\svchost.exe RO"

It also creates the following registry entries:
  • HKEY_CURRENT_USER\Software\VB and VBA Program Settings\Explorer\Process\"LO" = "0"
  • HKEY_CURRENT_USER\Software\VB and VBA Program Settings\Explorer\Process\"BL" = "[PATH TO EXECUTABLE FILE]"
  • HKEY_CURRENT_USER\Software\VB and VBA Program Settings\Explorer\Process\"NF" = "0"
  • HKEY_CURRENT_USER\Software\VB and VBA Program Settings\Svchost\Process\"BL" = "[PATH TO EXECUTABLE FILE]"

The worm may also modify the following registry entries:
  • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\USBSTOR\"Start" = "3"
  • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\USBSTOR\"Type" = "1"
  • HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\StorageDevicePolicies\"WriteProtect" = "0"

The worm modifies the following registry entry so that it executes whenever Windows starts:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\"Shell" = "%Windir%\explorer.exe, c:\windows\system32\explorer.exe"

It also modifies the following registry entry:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\"ShowSuperHidden" = "0"

Next, the worm downloads an encrypted configuration file from one of the following locations:
  • [http://]cmdexp01.googlecode.com/files/cmsy[REMOVED]
  • [http://]expcmd01.googlecode.com/files/cmsy[REMOVED]
  • [http://]expcmd02.googlecode.com/files/cmsy[REMOVED]
  • [http://]expcmd03.googlecode.com/files/cmsy[REMOVED]
  • [http://]expcmd01.netai.net/cmsy[REMOVED]
  • [http://]expcmd01.byethost15.com/cmsy[REMOVED]
  • [http://]expcmd01.50webs.com/cmsy[REMOVED]
  • [http://]expcmd01.07x.net/cmsy[REMOVED]
  • [http://]expcmd01.zxq.net/cmsy[REMOVED]
  • [http://]expcmd01.atspace.com/cmsy[REMOVED]
  • [http://]expcmd01.ezeserv.com/cmsy[REMOVED]

It also downloads further configuration files from the following locations:
  • [http://]cmdexp01.googlecode.com/files/2pics[REMOVED]
  • [http://]cmdexp01.googlecode.com/files/3pics[REMOVED]

Next, it deletes all files in the following folder:

It then creates a scheduled task to run the following file every day at the current time plus two minutes:

The worm then starts the task scheduler.

Next, the worm opens a back door, which may allow a remote attacker to perform the following actions on the compromised computer:
  • Record keystrokes
  • Update itself
  • Send an email (Using Microsoft's CDO Messaging API)
  • Download files
  • Execute commands

The worm monitors Internet Explorer and Mozilla Firefox windows for the following strings, which correspond to email login pages:
  • Yahoo! Mail: The best web-based email!
  • Gmail: Email from Google

It then steals information from the compromised computer, which may then be sent to one of the following email addresses:
  • expemail@gmail.com
  • expeml01@gmail.com
  • expeml02@gmail.com
  • expeml01@hotmail.com

The worm then spreads by copying itself across all network shares on the compromised computer.

It also attempts to spread by copying itself to all removable drives.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
Writeup By: Stephen Doherty
Summary| Technical Details| Removal

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