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Also known as a trojan horse program, this is a deceptive program that performs additional actions without the user's knowledge or permission. It does not replicate.

Disinfection & Removal

F-Secure renames the malware modified HOSTS file. Windows then creates a new file that restores website access. The renamed file, HOSTS.0, can be deleted.

Technical Details

Some malicious software (malware) applications will modify the Windows HOSTS file in an attempt to block access to antivirus vendor web and update servers.

As a result, the websites of several antivirus vendors may become inaccessible and some antivirus programs may stop receiving updates.The Windows HOSTS file typically contains information only about the localhost. Some malware variants add more entries to the HOSTS file attempting to block access to antivirus websites and update servers.


A normal HOSTS file will appear as follows:

# Copyright (c) 1993-1999 Microsoft Corp. 
# # This is a sample HOSTS file used by Microsoft TCP/IP for Windows. 
# # This file contains the mappings of IP addresses to host names. 
Each # entry should be kept on an individual line. The IP address should 
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one # space.
 # # Additionally, comments (such as these) may be inserted on individual 
 # lines or following the machine name denoted by a '#' symbol. 
# # For example: 
# # rhino.acme.com 
# source server 
# x.acme.com 
# x client host localhost

The typical file path is:

  • C:\%windir%\system32\drivers\etc\hosts

A malware modified version of the HOSTS file will contain additional entries:

  • avp.com
  • ca.com
  • customer.symantec.com
  • dispatch.mcafee.com
  • download.mcafee.com
  • f-secure.com
  • kaspersky.com
  • liveupdate.symantec.com
  • liveupdate.symantecliveupdate.com
  • localhost
  • mast.mcafee.com
  • mcafee.com
  • my-etrust.com
  • nai.com
  • networkassociates.com
  • rads.mcafee.com
  • secure.nai.com
  • securityresponse.symantec.com
  • sophos.com
  • symantec.com
  • trendmicro.com
  • update.symantec.com
  • updates.symantec.com
  • us.mcafee.com
  • viruslist.com
  • viruslist.com
  • www.avp.com
  • www.ca.com
  • www.f-secure.com
  • www.kaspersky.com
  • www.mcafee.com
  • www.my-etrust.com
  • www.nai.com
  • www.networkassociates.com
  • www.sophos.com
  • www.symantec.com
  • www.trendmicro.com
  • www.viruslist.com

Websites or servers configured to an IP Address of will loop back to the local machine, making them unreachable.


F-Secure detects malware modified Windows HOSTS files as Trojan:W32/Qhost or Trojan.Win32.Qhost. The detected HOSTS file is renamed to HOSTS.0.

Submit a sample

Wondering if a file or URL is malicious? Submit a sample to our Lab for analysis via the Sample Analysis System (SAS)

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