Win32/Hackdef is a family of backdoor Trojans that is distributed in various ways to computers running certain versions of Microsoft Windows. This Trojan is a user-mode rootkit. It creates, alters, and hides Windows system resources on a computer that it has infected, and can hide proxy services and backdoor functionality. It can also conceal use of TCP and UDP ports for receiving commands from attackers.

What to do now

To detect and remove this threat and other malicious software that may be installed in your computer, run a full-system scan with an up-to-date antivirus product such as the following:
For more information on antivirus software, see

Threat behavior

A variant of Win32/Hackdef can be started locally or by a remote process scanning the network for vulnerable computers. It can infect a computer only by gaining access through a local user account that has administrator privileges. After Windows restarts on a computer that is infected with Win32/Hackdef, the Trojan can run under local accounts that do not have administrator privileges.
The variant runs as a process and installs itself as a service. When it runs, it checks for the presence of configuration code that contains parameters for changing settings on the target computer. Settings in the configuration code determine rootkit operations such as creating, altering, and hiding system resources; providing and controlling backdoor functionality; and providing proxy services.
Win32/Hackdef creates mailslots on an infected computer, which function as backdoors to exchange commands and information with attackers. The Trojan creates a separate, private mailslot for each attacker to send commands to control Trojan functionality on the target computer.
Win32/Hackdef uses a driver to run custom code in kernel mode. This driver duplicates process tokens to obtain process-related information so that the rootkit can alter the functionality of processes as they run from memory.
Win32/Hackdef stores original data from multiple Windows system APIs. It infects APIs that are residing in memory locations allocated to various processes. This can include APIs from various DLLs.
If Win32/Hackdef infects a computer through a user account that has administrator privileges, it infects all current and future user sessions. If Win32/Hackdef infects a system through a user account that does not have administrator privileges, it infects current and future sessions of only this user.


Take the following steps to help prevent infection on your computer:
  • Enable a firewall on your computer.
  • Get the latest computer updates for all your installed software.
  • Use up-to-date antivirus software.
  • Limit user privileges on the computer.
  • Use caution when opening attachments and accepting file transfers.
  • Use caution when clicking on links to web pages.
  • Avoid downloading pirated software.
  • Protect yourself against social engineering attacks.
  • Use strong passwords.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.
Get the latest computer updates
Updates help protect your computer from viruses, worms, and other threats as they are discovered. It is important to install updates for all the software that is installed in your computer. These are usually available from vendor websites.
You can use the Automatic Updates feature in Windows to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
Use up-to-date antivirus software
Most antivirus software can detect and prevent infection by known malicious software. To help protect you from infection, you should always run antivirus software, such as Microsoft Security Essentials, that is updated with the latest signature files. For more information, see
Limit user privileges on the computer
Starting with Windows Vista and Windows 7, Microsoft introduced User Account Control (UAC), which, when enabled, allowed users to run with least user privileges. This scenario limits the possibility of attacks by malware and other threats that require administrative privileges to run.
You can configure UAC in your computer to meet your preferences:
Use caution when opening attachments and accepting file transfers
Exercise caution with email and attachments received from unknown sources, or received unexpectedly from known sources.  Use extreme caution when accepting file transfers from known or unknown sources.
Use caution when clicking on links to web pages
Exercise caution with links to web pages that you receive from unknown sources, especially if the links are to a webpage that you are not familiar with, unsure of the destination of, or suspicious of. Malicious software may be installed in your computer simply by visiting a webpage with harmful content.
Avoid downloading pirated software
Threats may also be bundled with software and files that are available for download on various torrent sites. Downloading "cracked" or "pirated" software from these sites carries not only the risk of being infected with malware, but is also illegal. For more information, see 'The risks of obtaining and using pirated software'.
Protect yourself from social engineering attacks
While attackers may attempt to exploit vulnerabilities in hardware or software to compromise a computer, they also attempt to exploit vulnerabilities in human behavior to do the same. When an attacker attempts to take advantage of human behavior to persuade the affected user to perform an action of the attacker's choice, it is known as 'social engineering'. Essentially, social engineering is an attack against the human interface of the targeted computer. For more information, see 'What is social engineering?'.
Use strong passwords
Attackers may try to gain access to your Windows account by guessing your password. It is therefore important that you use a strong password – one that cannot be easily guessed by an attacker. A strong password is one that has at least eight characters, and combines letters, numbers, and symbols. For more information, see

Alert level: Severe
This entry was first published on: Mar 28, 2005
This entry was updated on: Apr 17, 2011

This threat is also detected as:
No known aliases