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National Cyber Alert System
Technical Cyber Security Alert TA09-020A archive

Microsoft Windows Does Not Disable AutoRun Properly

Original release date: January 20, 2009
Last revised: January 29, 2009
Source: US-CERT

Systems Affected

  • Microsoft Windows

Overview

Disabling AutoRun on Microsoft Windows systems can help prevent the spread of malicious code. However, Microsoft's guidelines for disabling AutoRun are not fully effective, which could be considered a vulnerability.


I. Description

Microsoft Windows includes an AutoRun feature, which can automatically run code when removable devices are connected to the computer. AutoRun (and the closely related AutoPlay) can unexpectedly cause arbitrary code execution in the following situations:

  1. A removable device is connected to a computer. This includes, but is not limited to, inserting a CD or DVD, connecting a USB or FireWire device, or mapping a network drive. This connection can result in code execution without any additional user interaction.

    Map Network Drive

  2. A user clicks the drive icon for a removable device in Windows Explorer. Rather than exploring the drive's contents, this action can cause code execution.

    Windows Explorer Device Icon

  3. The user selects an option from the AutoPlay dialog that is displayed when a removable device is connected.

    AutoPlay

Malicious software, such as W32.Downadup, is using AutoRun to spread. Disabling AutoRun, as specified in the CERT/CC Vulnerability Analysis blog, is an effective way of helping to prevent the spread of malicious code.

The Autorun and NoDriveTypeAutorun registry values are both ineffective for fully disabling AutoRun capabilities on Microsoft Windows systems. Setting the Autorun registry value to 0 will not prevent newly connected devices from automatically running code specified in the Autorun.inf file. It will, however, disable Media Change Notification (MCN) messages, which may prevent Windows from detecting when a CD or DVD is changed. According to Microsoft, setting the NoDriveTypeAutorun registry value to 0xFF "disables Autoplay on all types of drives." Even with this value set, Windows may execute arbitrary code when the user clicks the icon for the device in Windows Explorer.


II. Impact

By placing an Autorun.inf file on a device, an attacker may be able to automatically execute arbitrary code when the device is connected to a Windows system. Code execution may also take place when the user attempts to browse to the software location with Windows Explorer.


III. Solution

Disable AutoRun in Microsoft Windows

To effectively disable AutoRun in Microsoft Windows, import the following registry value:

    REGEDIT4
    [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\IniFileMapping\Autorun.inf]
    @="@SYS:DoesNotExist"

To import this value, perform the following steps:

  1. Copy the text
  2. Paste the text into Windows Notepad
  3. Save the file as "autorun.reg"
    Note: In certain circumstances, Notepad may automatically add a .txt extension to saved files. To ensure that the file is saved with the proper extension, select All Files in the "Save as type:" section of the "Save As" dialog.
  4. Navigate to the file location
  5. Double-click the file to import it into the Windows registry

Microsoft Windows can also cache the AutoRun information from mounted devices in the MountPoints2 registry key. We recommend restarting Windows after making the registry change so that any cached mount points are reinitialized in a way that ignores the Autorun.inf file. Alternatively, the following registry key may be deleted:

    HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\MountPoints2

Once these changes have been made, all of the AutoRun code execution scenarios described above will be mitigated because Windows will no longer parse Autorun.inf files to determine which actions to take. Further details are available in the CERT/CC Vulnerability Analysis blog. Thanks to Nick Brown and Emin Atac for providing the workaround and to Aryeh Goretsky for pointing out a possible issue with Notepad appending a .txt file extension.

Update:

Microsoft has provided support document KB953252, which describes how to correct the problem of NoDriveTypeAutoRun registry value enforcement. After the update is installed, Windows will obey the NoDriveTypeAutorun registry value. Note that this fix has been released via Microsoft Update to Windows Vista and Server 2008 systems as part of the MS08-038 Security Bulletin. Windows 2000, XP, and Server 2003 users must install the update manually. Our testing has shown that installing this update and setting the NoDriveTypeAutoRun registry value to 0xFF will disable AutoRun as well as the workaround described above.


IV. References


Feedback can be directed to US-CERT.

Produced 2009 by US-CERT, a government organization. Terms of use


Revision History

January 20, 2009: Initial release
January 21, 2009: Added reference and details for Microsoft KB953252
January 29, 2009: Added information about Notepad and double file extensions

Last updated January 29, 2009
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