Win32/Ryknos is a family of backdoor Trojans that targets computers running certain versions of Microsoft Windows. The Trojan opens a backdoor on the infected computer to receive commands from attackers. If the rootkit WinNT/F4IRootkit is already installed on the target computer, the Trojan uses the rootkit to hide.

What to do now

It is best to use up-to-date antivirus software to remove Win32/Ryknos as well as rootkit WinNT/F4IRootkit from your computer. To recover from these threats using antivirus software, follow these steps: 
  1. Run up-to-date antivirus software.
  2. Take steps to prevent re-infection.

Run up-to-date antivirus software  

You can use the online antivirus scanner at the Microsoft Safety Scanner Web site to detect and remove Win32/Ryknos, WinNT/F4IRootkit, and other malicious software from your computer.

Take steps to prevent re-infection  

Do not reconnect your computer to the Internet until the computer is protected from re-infection. See the "Preventing Infection" section for more information.”

Threat behavior

Win32/Ryknos creates a copy of itself in the Windows system folder. The file name begins with "$sys$" and may be a name like $sys$drv.exe or $sys$xp.exe. The Trojan attempts to enter this name in a registry key to cause the Trojan to run automatically each time Windows starts.
The Trojan takes advantage of the stealth functionality of rootkit WinNT/F4IRootkit if the rootkit is already installed on the target computer. The rootkit hides certain names on the system that are prefixed by $sys$, such as names of files, processes, and registry entries. The Trojan file name, process name, and registry entry name begin with $sys$, so the effect is that the rootkit hides the Trojan from the user.
Win32/Ryknos uses the netsh command to configure the Windows firewall so the Trojan can exchange data over the network. The Trojan can connect to several IRC servers at a time to receive commands from attackers, who can then take complete control of the infected computer.  
Like other Trojans, Win32/Ryknos does not have its own spreading mechanism. It can be distributed in numerous ways, for example, through e-mail, file-sharing, network shares, or file downloads.


There are no readily apparent indications of infection by Win32/Ryknos. The presence of names of files, processes, or registry settings containing the string "$sys$xp" or "$sys$drv" may be a sign of infection by this Trojan, but the rootkit WinNT/F4IRootkit hides these names if it is also installed on the computer.


Take the following steps to help prevent infection on your system:
  • Enable a firewall on your computer.
  • Get the latest computer updates.
  • Use up-to-date antivirus software.

Enable a firewall on your computer

Use a third-party firewall product or turn on the Microsoft Windows XP Internet Connection Firewall.
To turn on the Internet Connection Firewall in Windows XP
  1. Click Start, and click Control Panel.
  2. Click Network and Internet Connections, and click Network Connections. If you do not see Network and Internet Connections, click Switch to Category View.
  3. Highlight a connection that you want to help protect, and click Change settings of this connection.
  4. Click Advanced, and select Protect my computer and network by limiting or preventing access to this computer from the Internet.
  5. Click OK.

Get the latest computer updates

Updates help protect your computer from viruses, worms, and other threats as they are discovered. You can use the Automatic Updates feature in Microsoft Windows XP to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
To turn on Automatic Updates in Windows XP
  1. Click Start, and click Control Panel
  2. Click Performance and Maintenance. If you do not see Performance and Maintenance, click Switch to Category View.
  3. Click System.
  4. Click Automatic Updates, and select Keep my computer up to date.
  5. Select a setting. Microsoft recommends selecting Automatically download the updates, and install them on the schedule that I specify and setting a regular update time.
  6. If you choose to have Automatic Updates notify you in step 5, you will see a notification balloon when new downloads are available to install. Click the notification balloon to review and install updates.

Use up-to-date antivirus software

Most antivirus software can detect and prevent infection by known malicious software. You should always run antivirus software on your computer that is updated with the latest signature files to automatically help protect you from infection. If you don't have antivirus software installed, you can get it from one of several companies. For more information, see

Alert level: Severe
This entry was first published on: Nov 16, 2005
This entry was updated on: Apr 17, 2011

This threat is also detected as:
No known aliases