Microsoft security software detects and removes this threat.

This threat downloads and runs files on your PC, including a trojan that sends spam emails. It can also steal your email user names and passwords, as well as your FTP credentials, using a plugin detected as PWS:Win32/Fareit.gen!C.

This threat also uses a rootkit and other defensive techniques to avoid detection and removal.

Find out ways that malware can get on your PC.  

What to do now

The following free Microsoft software detects and removes this threat:

You should also run a full scan. A full scan might find other, hidden malware.

Advanced troubleshooting

To restore your PC, you might need to download and run Windows Defender Offline. See our advanced troubleshooting page for more help.

Get more help

You can also ask for help from other PC users at the Microsoft virus and malware community.

If you’re using Windows XP, see our Windows XP end of support page.

Threat behavior


Cutwail tries to drop a device driver into your PC, overwriting the original legitimate driver file. The file name differs depending on your operating system, but Cutwail has been known to use one of the following:

Cutwail then tries to start the corresponding kernel driver by name:

  • Ip6Fw
  • Secdrv
  • NetDetect

This driver tries to restore various system hooks to their original unhooked state. For example, any System Service Descriptor Table (SSDT) hook will be reverted. By doing this, Cutwail is able to circumvent security applications or even other malware which may be installed on your PC.


Steals passwords

Cutwail contains a plugin that can steal FTP passwords. The plugin is detected as PWS:Win32/Fareit.gen!C. A full list of FTP clients that it can steal passwords from is in the Fareit description.

Cutwail can also steal passwords you have stored in your browser. It steals stored passwords from these browsers:

  • Internet Explorer
  • Mozilla Firefox
  • Google Chrome

It can also steal user names and passwords from these email clients:

  • Windows Live Mail
  • Microsoft Outlook
  • Becky! Internet Mail
  • Barca
  • IncrediMail
  • The Bat!
  • Mozilla Thunderbird

Provides advanced stealth functionality

Cutwail drops a second device driver to disk:

It installs the driver via the following registry changes:

In subkey: HKLM\SYSTEM\CurrentControlSet\Services\runtime\
Sets value: "ImagePath"
With data: "\\??\\C:\\WINDOWS\\System32\\drivers\\runtime.sys"
Sets value: "Type"
With data: "0x1"
Sets value: "Start"
With data: "0x3"

It then loads the driver. This driver is able to hide processes for a supplied process id (PID) by directly manipulating the EPROCESS structure.

Cutwail usually downloads an updated version of itself. This updated version drops another driver, which implements additional rootkit functionality. The updater tries to write the device driver to:

It installs this driver via the following registry changes:

In subkey: HKLM\SYSTEM\CurrentControlSet\Services\runtime2\
Sets value: "ImagePath"
With data: "\\??\\C:\\WINDOWS\\System32\\drivers\\runtime2.sys"
Sets value: "Type"
With data: "0x1"
Sets value: "ErrorControl"
With data: "0x1"
Sets value: "Start"
With data: "0x3"

It then loads the driver.

If runtime2.sys already exists, the new device driver is written to the alternate location:

The existing device driver is then instructed to update itself with the new copy.

The driver also creates the following registry keys to ensure that is loaded in safe mode:

  • HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot\Minimal\runtime2.sys
  • HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot\Network\runtime2.sys

This driver then drops a file to:

It creates the following registry entry to ensure that the dropped file is run:

In subkey: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\
Sets value: "startdrv"
With data: "%TEMP%\startdrv.exe"

Inhibits removal

Cutwail is not only able to hide itself; it can prevent the removal of its files and registry entries. To hide and protect its registry entries, it hooks the following functions via SSDT:

  • ZwDeleteValueKey()
  • ZwEnumerateKey()
  • ZwEnumerateValueKey()
  • ZwOpenKey()
  • ZwSetValueKey()

To protect files on disk it implements a file system filter driver. The IRP handlers IRP_MJ_CREATE and IRP_MJ_DIRECTORY_CONTROL are hooked for the FastFAT or NTFS driver objects, depending on the filesystem type.

Downloads and runs files

Cutwail tries to run Internet Explorer if it exists as this file:

It then injects the downloading component into this process, where it runs. Cutwail instructs runtime.sys to hide the iexplore.exe process. After this, runtime.sys is deleted.

The downloading component creates the mutex: k4j.32H_f7z_Z6e.g8G0.

It tries to connect to one of the following remote hosts to download a software bundle.


Cutwail creates a file during the download process, selecting the name randomly from the following list:

Cutwail might also make the following subkey:

  • HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\LastTheme\Last

Executables from within the downloaded software bundle are usually written to disk or injected directly into Internet Explorer. Those which are written to disk are given a random numerical file name and are written to the %TEMP% folder, for example, %TEMP%\1193135.exe.

Analysis by Scott Molenkamp and Shawn Wang


This threat uses advanced stealth (rootkit) functionality to hide its presence. Alerts from your security software may be the only symptom.


Alert level: Severe
This entry was first published on: Nov 29, 2007
This entry was updated on: Sep 15, 2014

This threat is also detected as:
No known aliases