Threat Description



Aliases:Lecna, Backdoor.Win32.Lecna.b


Lecna is a backdoor that allows remote access to infected machine. It can also utilize rootkit techniques to hide its actions. It may be installed via Microsoft Jet Database file using a vulnerability in msjet40.dll.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.


You may wish to refer to the Support Community for further assistance. You also may also refer to General Removal Instructions for a general guide on alternative disinfection actions.

Technical Details

Installation to system

When run, the backdoor copies itself under %SysDir% directory using the name 'iexplore.exe'. Then it creates a mutex named 'MicrosoftZj' for ensuring it will not run multiple copies of the backdoor at the same time. It installs the following registry key to make sure it will be executed next time the system is started:

 "iexplore.exe" = "iexplore.exe"

The backdoor also creates the following registry keys:

 "pid" = %ProcessId%
 "hostid" = %HostId%

%ProcessId% and %HostId% are random values used to identify the backdoor.

If the backdoor is running under Windows NT-based system, it drops and activates a file 'USBTest.sys'. This file is a rootkit component. See more information on section 'Rootkit'. If the system is Win9x-based, the worm tries to hide the process by issuing Win32 API call 'RegisterServiceProcess'.

The backdoor may also leave the following files on infected system:


Filename 'netsvc.exe' is used by the backdoor when downloading and executing additional files (see the section Backdoor below) and 'cmdLine.exe' is used in uninstallation. It is a simple executable that deletes a file given as command line argument. It is embedded in the backdoors body.


After the installation, Lecna starts to communicate with the server part using specially crafted HTTP queries. If needed, it can also make a use of HTTP proxy settings it finds in the registry. The server can instruct the backdoor to execute the following actions:

Basic file operations (copy, delete, rename, find, execute)
 Download/upload files
 Process operations (list, kill)
 Spawn interactive command shell
 Registry operations (create/delete keys/values)
 Enumerate shares on remote computers
 Uninstall the backdoor

The backdoor may also download and execute additional components from several remote servers.


Lecna carries an encrypted driver file 'USBTest.sys' in its body which it drops in directory '%SysDir%\drivers'. The driver is activated using a standard Windows Service Manager API. This driver implements the following rootkit functionality:

Hide network connections
 Hide processes
 Hide files and directories
 Hide registry entries

When the driver is activate, the backdoor instructs it to hide its file, process and registry entry, as well as connections to servers it uses for communication.


Detection Type: PC
Database: 2005-04-21_02

Technical Details: Jarkko Turkulainen; Apr 21th, 2005


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