Trojan:Win32/Sirefef.C is the detection for a component of the Win32/Sirefef family- a multi-component family of malware that moderates your Internet experience by changing search results and generating pay-per-click advertising revenue for its controllers. The family consists of multiple parts that perform different functions, such as downloading updates and additional components, hiding existing components, or performing a payload.

What to do now

Win32/Sirefef is a dangerous threat that uses advanced stealth techniques to hinder its detection and removal. If you are infected with Sirefef, we recommend you take the following steps to remove it.

Download and run the Microsoft Safety Scanner

Before you begin you will need:

  • A PC that is not infected and is connected to the Internet. You will use this PC to download a copy of the Microsoft Safety Scanner
  • A blank CD, DVD or USB drive. You will use this CD, DVD or USB drive to run the Scanner on your infected PC
  1. Download a copy of the Microsoft Safety Scanner from a clean, uninfected PC
  2. Save a copy of the Scanner on a blank CD, DVD, or USB drive
  3. Restart the infected PC
  4. Insert the CD, DVD, or USB drive into your infected PC and run the Scanner
  5. Let the Scanner clean your PC and remove any infections it finds

After running the scanner, make sure your antivirus software is up-to-date. You can update Microsoft security software by downloading the latest definitions.

The following Microsoft products detect and remove this threat:

Note that as part of the cleaning, our software might change some Windows services back to their default settings. If you had previously changed these settings, you might need to change them again.

The services that are reset include:

  • BFE – Base Filtering Engine
  • Iphlsvc – IP helper Service
  • MSMpSvc – Microsoft Antimalware service – MSE/FEP/SCEP
  • Sharedaccess – Internet Connection Sharing
  • WinDefend – Microsoft Antimalware service
  • Wscsvc - Windows Security Center

Threat behavior

Trojan:Win32/Sirefef.C is the detection for a component of the Win32/Sirefef family.
Trojan:Win32/Sirefef.C is injected into any process that loads the system file "mswsock.dll"; see also the description for Trojan:WinNT/Sirefef.C elsewhere in the encyclopedia.
Downloads arbitrary files
Trojan:Win32/Sirefef.C downloads a configuration file from a remote server. At the time of writing, details about this file were not available.
Redirects webpages
The trojan also attempts to redirect traffic to malicious websites, such as "".
Analysis by Chun Feng


There are no common symptoms associated with this threat. Alert notifications from installed antivirus software may be the only symptoms.


Alert level: Severe
First detected by definition: 1.75.984.0
Latest detected by definition: 1.173.2181.0 and higher
First detected on: Feb 19, 2010
This entry was first published on: Jan 17, 2011
This entry was updated on: Sep 02, 2013

This threat is also detected as:
  • Win-Trojan/Agent.36864.BUQ (AhnLab)
  • Trojan-Spy.Win32.Agent.bdka (Kaspersky)
  • W32/Suspicious_Gen2.EXVQM (Norman)
  • Trojan.Sirefef!E7JTVcoretQ (VirusBuster)
  • Trojan horse PSW.Agent.AEZK (AVG)
  • BackDoor.Maxplus.6 (Dr.Web)
  • Win32/Sirefef.E (ESET)
  • Trojan-Spy.Win32.Agent (Ikarus)
  • Trj/Downloader.MDW (Panda)