1. Symantec/
  2. Security Response/
  3. W32.SillyFDC


Risk Level 1: Very Low

February 27, 2007
September 22, 2014 2:39:17 PM
Systems Affected:
W32.SillyFDC is a generic detection for worms that spread by copying themselves to removable drives. Certain variants may also copy themselves to mapped drives, while others may download files or perform other malicious activities.

Background information

AutoRun is the name given to a feature of Windows that allows an executable to run automatically when a drive is accessed. The AutoRun feature was introduced to allow software vendors to distribute media that was configured to run certain programs on insertion of the USB key, CD or DVD. This was useful as it allowed end users to be shielded from the file system details of program installers and the 'multimedia' packages that were popular at the time.

Malicious individuals were quick to take advantage of this infection vector, though, and many worms are now able to copy themselves and an accompanying autorun.inf file to removable drives. This means that the worms are able to spread when the drives are accessed by an unsuspecting user.

How AutoRun works
In order for a program to execute using AutoRun, a configuration file called autorun.inf must be present in the root folder of the drive. This is a simply a text file that contains information that specifies how the file should be displayed, along with options for its execution.

An autorun.inf file may, for example, be created so that the file %DriveLetter%\setup.exe is executed when the disk is inserted into a computer with AutoRun enabled. Options that specify the icon to be used as well as the label for the disk may also be included.

Worms detected as W32.SillyFDC may also propagate in the same way through other drives, due to the fact that the AutoRun feature works in an equivalent manner on removable, fixed and mapped drives. This includes those that are accessed over a network.

Users should note that the AutoRun feature is disabled by default for non-optical removable drives in recent versions of Windows and on systems with certain updates applied. For more information, please see the following resource:

How to prevent a virus from spreading using the 'AutoRun' feature.

How do these threats arrive on a computer?
Worms detected as W32.SillyFDC typically arrive when an infected removable drive is inserted into the computer, or an infected mapped drive is accessed. Infected files may also be downloaded from file-sharing networks.

Typical behavior
When executed, W32.SillyFDC variants typically copy themselves to the system disk of the compromised computer. They then create files or modify the registry so that they run every time Windows starts. Next, the worms copy themselves to removable and sometimes mapped drives, and at the same time create autorun.inf files so that the copied executables run when the drives are accessed.

Extended behavior
Worms that are detected as W32.SillyFDC may also perform actions that include, but need not be limited to, the following:
  • Download files from a preconfigured URL
  • Copy themselves to network shares
  • Lower security settings
  • Alter Safe Mode settings
  • Modify the hosts file
  • Disable Windows Update and System Restore
  • Bypass the Windows firewall
  • Alter Explorer settings
  • Disable the Task Manager, Registry Editor and other system software

Are there any tell-tale signs?
Although these worms are usually designed to run stealthily, users may notice unusual behavior when W32.SillyFDC worms execute. Unusual or unexpected activity when removable drives are inserted may be an indicator of infection.

What can I do to minimize the risks?
Users should disable the AutoRun feature or be sure to upgrade to a recent version of Windows that has AutoRun disabled by default for non-optical removable media.

As a general rule, users should always run up-to-date antivirus software with real-time protection such as Norton Antivirus, Norton Internet Security, Norton 360 or Symantec Endpoint Protection. In addition, a firewall -- or better still, an Intrusion Prevention System (IPS) -- will help to block download activities initiated by these types of malicious programs. Program controls such as those found in Symantec Endpoint Protection can also help to prevent programs such as these from executing in the first place.

How can I find out more?

Advanced users can submit a sample to Threat Expert to obtain a detailed report of the system and file system changes caused by a threat.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
Writeup By: Henry Bell
Summary| Technical Details| Removal

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