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Discovered: December 07, 2012
Updated: January 25, 2013 9:13:25 AM
Type: Trojan
Systems Affected: Windows

Backdoor.Necurs is a Trojan horse that opens a back door on the compromised computer. The Trojan may also disable antivirus products as well as download and install additional malware.

Antivirus Protection Dates

  • Initial Rapid Release version December 07, 2012 revision 008
  • Latest Rapid Release version May 22, 2019 revision 021
  • Initial Daily Certified version December 07, 2012 revision 020
  • Latest Daily Certified version May 23, 2019 revision 002
  • Initial Weekly Certified release date December 12, 2012

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Technical Description

When the Trojan is executed, it creates the following files:

  • %Windir%\Installer\[RANDOM GUID]\syshost.exe
  • %System%\drivers\[RANDOM CHARACTERS FILE NAME].sys
The Trojan then creates the following registry entry:

Then it creates the following services:
Service Name: syshost32
Image Path: %Windir%\[RANDOM GUID]\syshost.exe
Startup Type: Automatic

Display Name: syshost.exe
Image Path: %System%\drivers\[RANDOM DRIVER NAME].sys

Next, the Trojan loads the following driver:
%System%\drivers\[RANDOM CHARACTERS].sys

The Trojan then uses the driver to disable antivirus products installed on the compromised computer.

It then copies itself to the following location:

Next, the Trojan deletes the following file when the compromised computer restarts:

The Trojan may also delete the following alternative data stream:
%Windir%\Installer\[RANDOM GUID]\syshost.exe:Zone.Identifier

The Trojan may then connect to the following remote locations:

It may then open a back door on the compromised computer, and download potentially malicious files.

This Trojan is virtual machine (VM) aware, and will fail to load in a VM environment.


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.


You may have arrived at this page either because you have been alerted by your Symantec product about this risk, or you are concerned that your computer has been affected by this risk.

Before proceeding further we recommend that you run a full system scan . If that does not resolve the problem you can try one of the options available below.

If you are a Norton product user, we recommend you try the following resources to remove this risk.

Removal Tool

If you have an infected Windows system file, you may need to replace it using the Windows installation CD .

How to reduce the risk of infection
The following resources provide further information and best practices to help reduce the risk of infection.

If you are a Symantec business product user, we recommend you try the following resources to remove this risk.

Identifying and submitting suspect files
Submitting suspicious files to Symantec allows us to ensure that our protection capabilities keep up with the ever-changing threat landscape. Submitted files are analyzed by Symantec Security Response and, where necessary, updated definitions are immediately distributed through LiveUpdate™ to all Symantec end points. This ensures that other computers nearby are protected from attack. The following resources may help in identifying suspicious files for submission to Symantec.

Removal Tool

If you have an infected Windows system file, you may need to replace it using the Windows installation CD .

How to reduce the risk of infection
The following resource provides further information and best practices to help reduce the risk of infection.
Protecting your business network

The following instructions pertain to all current Symantec antivirus products.

1. Restart the computer in Safe Mode
For instructions, read the document, How to start the computer in Safe Mode .

2. Disable suspicious device driver
  1. Open Device Manager, click View > select Show Hidden Devices.
  2. Scroll to and expand Non-Plug and Play Drivers.
  3. Look for the driver named syshost.
  4. Right-click on the suspicious driver and select Properties.
  5. Click the Driver tab > Stop > Yes to disable the driver.
  6. Click OK.

3. Remove the service
  1. Click Start > Run.
  2. Type cmd, and then click OK.
  3. Type sc qc syshost32.
  4. Take note of the [BINARY PATH NAME].
  5. Type sc delete syshost32.
  6. Type del [BINARY PATH NAME].

4. Restart the computer in Normal Mode and perform a full system scan

How to run a full system scan using your Symantec product

5. Restoring settings in the registry
Many risks make modifications to the registry, which could impact the functionality or performance of the compromised computer. While many of these modifications can be restored through various Windows components, it may be necessary to edit the registry. See in the Technical Details of this writeup for information about which registry keys were created or modified. Delete registry subkeys and entries created by the risk and return all modified registry entries to their previous values.

Writeup By: Alan Neville