Zero Access

Original Issue Date:- April 15, 2013
Virus Type:- Trojan/botnet
Severity:- High

Win32/Sirefef a.k.a "Zero Access" is a widespread multi-component malware family of rootkits which is affecting the windows operating systems. The threat spreads majorly by exploit kits, use of pirated softwares and other malware downloaders. It uses disk-level hooking to hide itself (hide processes, related files, network activites,) in order to hinder its detection and removal on infected computer. It uses multi-layers self-defense mechanism to protect itself against security related softwares by stopping or deleting any process that attempts to access.

The Trojan is responsible for the following functions:

  • Download and execute arbitrary files
  • Contact foreign hosts
  • Disable security features
  • Modifying browser search engine results
  • Generating pay-per-click revenue
  • Performing Bitcoin mining

Aliases: Trojan-Dropper.Win32.PMax.a (Kaspersky, Trojan.Horse (Symantec), TrojanDropper:Win32/Sirefef.A (Microsoft), Trojan.Win32.Agent.csaf [Kaspersky], Trojan.Cryptor.A [BitDefender]Trojan.Win32.Agent.csaf [F-Secure], Mal/Crot-A [Sophos], Trojan.Agent.csaf (CAT-QuickHeal), Crot.gen.b [McAfee] , TrojWare.Win32.Agent.csaf [Comodo]


The Trojan has been distributed by various ways such as exploit kits ( e.g. Blackhole Kit ), Malware/TrojanDownloaders ( such as TrojanDownloader: Win32/Beebone.gen!A , TrojanDownloader:Win32/Karagany.I and Win32/Dofoil family variants ) , and use of pirated softwares related to 'crack' and 'keygen'. Some of the softwares are:

  • "Download Nokia Dongle.exe"
  • "Facebook Password Cracker.exe"
  • "autocad_2007_full_crack.exe".

The files dropped by Sirefef are as follows:

Location: c:\recycler\ [Redacted]
Files Dropped:

  • "@" - This file contains information that Sirefef can use to find other infected computers
  • "n" - This file contains the malicious code for peer-to-peer (P2P) communication

The registry changes made by the trojan Sirefef to ensure its persistence are as follows:

In subkey: HKLM\Software\Classes\clsid\{5839fca9-774d-42a1-acda-d6a79037f57f}\InprocServer32
Modifies value: "(Default)"
From data: " < system folder > \wbem\wbemess.dll"
With data: " " (For Example : "c:\recycler\ \n" )

To intercept and hijack network traffic, it drops the following files:

  • %windir%\assembly\GAC\desktop.ini or
  • %windir%\assembly\GAC_32\desktop.ini

It stops and attempts to delete Windows security services like Windows Defender (windefend), IP helper (iphlpsvc),Windows Security Center(wscsvc), windows firewall (mpssvc), Base filtering engine (bfe)

On execution Sirefef replaces a randomly-selected system driver with its own malicious copy. Some of the drivers replaced are afd.sys, i8042prt.sys, ipsec.sys, mrxsmb.sys, netbt.sys, raspppoe.sys, serial.sys. It creates a folder to retain the original copy of drivers and also for additional malware components in an encrypted and non accessible manner.

Format for the folder name is as follows:
\$NtUninstallKB $



  • Perform scanning on computer for possible infection with the removal tools mentioned below.
  • Exercise caution while visiting links within emails received from untrusted users or unexpectedly received from trusted users.
  • Do not download and open attachments in emails received from untrusted users or unexpectedly received from trusted users.
  • Exercise caution while visiting links to web pages.
  • Protect yourself against social engineering attacks.
  • Do not visit untrusted websites.
  • Enable firewall at desktop and gateway level.
  • Use strong password and also enable password policies.
  • Avoid downloading pirated software.
  • Keep up-to-date patches and fixes on the operating system and application softwares
  • Keep up-to-date antivirus and antispyware signatures at desktop and gateway level.
  • Selectively disable Java/Flash and javascript.
  • Exercise caution while using external drives, disable autoplay.