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Category: Malware

Type: Trojan

Aliases: Trojan.Win32.Qhost


This is network worm with backdoor capabilities, which spreads itself under Win32 systems. The worm was reported in-the-wild in July-August, 2000. The worm itself is Win32 executable file and about 120K long, written in MS Visual C++.


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

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Technical Details

Some malicious 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.