AutoRun

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AutoRun and the companion feature AutoPlay are components of the Microsoft Windows operating system that dictate what actions the system takes when a drive is mounted.

AutoRun was introduced in Windows 95 to ease application installation for non-technical users and reduce the cost of software support calls. When an appropriately configured CD-ROM is inserted into a CD-ROM drive, Windows detects the arrival and checks the contents for a special file containing a set of instructions. For a commercial application, these instructions normally initiate installation of the software from the CD-ROM. To maximise the likelihood of installation success, AutoRun also acts when the drive is accessed ("double-clicked") in Windows Explorer (or "My Computer").

Until the introduction of Windows XP, the terms AutoRun and AutoPlay were used interchangeably, developers often using the former term and end users the latter. This tendency is reflected in Windows Policy settings named AutoPlay that change Windows Registry entries named AutoRun, and in the autorun.inf file which causes "AutoPlay" to be added to drives’ context menus. The terminology was of little importance until the arrival of Windows XP and its addition of a new feature to assist users in selecting appropriate actions when new media and devices were detected. This new feature was called AutoPlay and a differentiation between the two terms was created.^[1]

[edit] AutoRun

AutoRun, a feature of Windows Explorer (actually of the shell32 dll) introduced in Windows 95, enables media and devices to launch programs by use of commands listed in a file called autorun.inf, stored in the root directory of the medium.

Primarily used on installation CD-ROMs, the applications called are usually application installers. The autorun.inf file can also specify an icon which will represent the device visually in Explorer along with other advanced features.^[1]

The terms AutoRun and AutoPlay tend to be interchangeably used when referring to the initiating action, the action that detects and starts reading from discovered volumes. The flowchart illustration in the AutoPlay article shows how AutoRun is positioned as a layer between AutoPlay and the Shell Hardware Detection service and may help in understanding the terminology. However, to avoid confusion, this article uses the term AutoRun when referring to the initiating action.

[edit] AutoPlay

AutoPlay in Windows Vista

AutoPlay is a feature introduced in Windows XP which examines removable media and devices and, based on content such as pictures, music or video files, launches an appropriate application to play or display the content.^[1] If available, settings in an autorun.inf file can add to the options presented to the user.

AutoPlay is based on a set of handler applications registered with the AutoPlay system. Each media type (Pictures, Music, Video) can have a set of registered handlers which can deal with playing or display that type of media.

Each hardware device can have a default action occurring on discovery of a particular media type, or the AutoPlay dialog can prompt the user what action to take.

[edit] AutoRun activation

The AutoRun sequence consists of the initial discovery of a new device or new piece of media. Following this, notification of interested parties occurs, of which the Windows Explorer shell is of primary interest.

After checking the Registry settings to see if AutoRun can proceed, parsing of an optional autorun.inf occurs and any necessary actions are taken.

The initial sequence is handled much the same in every version of Windows from Windows 95. However, the way the autorun.inf file is read and acted upon and the level of integration of AutoRun with AutoPlay has changed significantly from the time AutoPlay was introduced in Windows XP until the present handling in Windows Vista.

[edit] Initiation and notification

When a device with AutoRun-compatible drivers receives new media, a "Media Change Notification" event occurs. Windows then notifies interested applications that a device change has occurred. The notification method used can change depending on the device type.

If the device changed is a volume (like a CD) or a port (like a serial port) Windows broadcasts a WM_DEVICECHANGE notification to all top level windows.^[2][3] Windows terms this a "basic" notification. A top level window is one which is a descendant of the desktop.

However, if the device changed is not one of these types an application can use the RegisterDeviceNotification^[4] function to register to receive device notifications.

An article on the CodeProject website, "Detecting Hardware Insertion and/or Removal", with clarifications from a blog by Doran Holan is of particular technical interest here.

When Explorer receives notification of a volume change, it performs a number of actions:^[5][6]

1. checks to see if AutoRun has been disabled through the Registry. If AutoRun is disabled for that drive or drive type, Explorer does not proceed further. However, there have been bugs in this area.
2. checks that the root directory of the inserted media contains a file called autorun.inf and if so, reads and parses it. However, see below for an exception.
3. sends a QueryCancelAutoPlay message to the foreground window. An application which has registered its interest in receiving this message using RegisterWindowMessage can respond to this message to suppress AutoRun. The foreground application can also be notified by using the IQueryCancelAutoPlay COM interface^[7] available in Windows XP and later.
4. Alters double-click and contextual menu behaviours. When a user double clicks on the drive icon in Explorer or right clicks to get a context menu, what happens is fully programmable by settings in the autorun.inf file.
5. Adds an autorun.inf controllable icon and descriptive text to the drive icon.
6. Checks to see if the ⇧ Shift key is held down. If it is then Windows Vista will invoke AutoPlay regardless of settings.^[8] Previous versions of Windows will not continue with the process.^[6]
7. Finally, if shift is not held down, either:
   • executes an application on instructions from the open or shellexecute keys optionally present in an autorun.inf's [autorun] section.
   • invokes AutoPlay.
   Which choice is made depends on the version of Windows in use, instructions from the autorun.inf if available and the type of the media discovered. See the next section on inf handling for details.

[edit] Inf handling

[edit] Before AutoPlay

On Windows versions prior to Windows XP, an autorun.inf on any drive type will be parsed and instructions from open or shellexecute followed immediately and silently.^[9] This includes DRIVE_REMOVABLE, DRIVE_FIXED and DRIVE_REMOTE drive types.

AutoRun will work with network drives (the DRIVE_REMOTE drive type) that are mapped to a drive letter. AutoRun will also work with floppy drives that are provided with autorun-compatible drivers.^[6]

The default Registry settings on Windows versions previous to Windows XP (See NoDriveTypeAutoRun), disable Remote and Removable drives from AutoRun initiation, leaving Fixed and CDROM drive types active by default.

[edit] Introducing AutoPlay

With the introduction of AutoPlay in Windows XP, the final stage action (stage 7 above) for some drive types changed from executing an application to invoking AutoPlay. In Windows Vista, the AutoPlay system is integrated into every aspect of media handling and there is no automatic execution of any autorun.inf open or shellexecute instructions.

The default Registry settings added Removable drives to those that initiated AutoRun. From XP onwards, only Unknown and Network drive types are not active for AutoRun.

Under Windows XP, autorun.inf support was removed for DRIVE_REMOVABLE drive types.^[5] Any discovered removable media would be handled by invoking AutoPlay. Note that drives of type DRIVE_FIXED would parse and execute autorun.inf as before.^[10]

Windows XP Service Pack 2 reintroduced autorun.inf support for Removable drives^[10] but handled the open and shellexecute features as a display item within the AutoPlay dialog (See the action key in the [autorun] section).

This handling also now applied to Fixed drive types, an important change in functionality from previous Windows versions.

For drives of type DRIVE_CDROM, Windows XP uses an autorun.inf if one is available, otherwise AutoPlay is invoked. In Windows Vista, a CD-ROM with an autorun.inf simply adds another option to the AutoPlay dialog list.

[edit] The AutoPlay safety net

It would appear that AutoPlay, by transferring control of what were previously automatic and invisible actions to AutoPlay, acts to increase user control and safety. This applies especially under Windows Vista, where all media and devices fall under AutoPlay control.

However it is important to note that:

• a user can instruct AutoPlay to make the autorun.inf choice for them, indeed any choices for them, and bypass any AutoPlay dialog prompting.

• When a user double clicks on the drive icon in Explorer or right clicks to get a context menu, what happens next is fully programmable by the autorun.inf file and is essentially outside AutoPlay's purview. This is true under any Windows operating system.

• disabling AutoRun under Vista may force a user to double click the drive icon to get a contents list thus actually increasing the chance of malware infiltration.

[edit] Registry and Group Policy

AutoRun consults Windows Registry values to decides whether to initiate actions for any particular drive or drive type. These values can be changed using several methods, one of which is using Group Policy.

The primary relevant Registry entry names are NoDriveTypeAutoRun and NoDriveAutoRun. These exist in both per-machine and per-user settings and their location and priority in the Registry are described in further detail below.

[edit] Drive types

The drive types are distinguished by Type Name as follows:^[11]

[edit] Registry terminology

The Windows Registry is a directory which stores settings and options for the operating system. The terminology is somewhat misleading so it is briefly summarised here.

A Registry key is similar to a folder that, in addition to values, each key can contain subkeys which in turn may contain subkeys, and so on.

A Registry value consists of a name-data pair. Microsoft documentation commonly uses the term "entry" as an equivalent term. It also uses "value" for "data" when it is obvious what is meant. To avoid confusion, this article always uses the term "entry" when referring to the name-data pair.

Two Registry keys that are very commonly referred to are HKEY_LOCAL_MACHINE which contains per-machine settings, and HKEY_CURRENT_USER which contains settings for the currently logged-on user. These are almost always abbreviated as HKLM and HKCU respectively. There may be many users of a machine; their settings are stored in HKEY_USERS, HKCU is actually just a link to the appropriate place in HKEY_USERS.

[edit] Changing Registry settings

Registry settings may be changed directly by using the GUI regedit tool or the command line reg.exe utility. Settings can also be placed in a text file^[12], named with a .reg extension type. For example, "mychanges.reg". When the file is double clicked, the settings in the file are entered into the Registry, permissions allowing.

They can be changed indirectly by using Group Policy, applied locally to a single computer with GPEdit.msc or to a domain with gpmc.msc.

It may be necessary to either logout or restart the computer in order for any Registry changes to take effect.

[edit] NoDriveTypeAutoRun

This Registry entry disables or enables the AutoRun feature on all drives of the type specified.^[13] It reflects the setting of the relevant Autoplay Group Policy. Valid data ranges from 0x00 to 0xFF in hexadecimal notation. If the entry is not present, the default data value is either 0x95 or 0x91 depending on the version of Windows used.

The entry data is a bitmapped value, where a bit set to 1 disables AutoRun on a particular type of drive. The bit settings for each type of drive are shown below:

Note that bit number 1 is unused and that the "Unknown" type is represented twice. Setting all bits to 1 would give a hexadecimal value of 0xFF, decimal 255, and would disable AutoRun on all types of drives.

The default setting for this entry depends on the version of Windows being used:^[9][14]

Windows versions before XP and Server 2003 use the value 0x95. This disables AutoRun on Unknown, Network and Removable drives.

Versions after Windows XP, except Windows Server 2003, use the value 0x91, which disables AutoRun on Unknown and Network drives. Removable drives are now enabled, reflecting the added AutoPlay feature in Windows XP.

AutoRun is, by default, enabled for CD and DVD, Fixed and RAM drives on all versions of Windows from Windows 95.

The Microsoft tool TweakUI for XP completely ignores these default values, thus, once used to change AutoRun settings, it enables AutoRun on Unknown and Network drives.

[edit] NoDriveAutoRun

This Registry entry disables or enables the AutoRun feature on individual drives.^[15] It is not associated with a Group Policy and does not exist by default. The data value is taken to be 0 if the entry is not present.

The data is a 32 bit (DWORD) bitmapped value, of which the lower 26 bits are used to represent each of the 26 drive letters from A to Z. Thus the valid data range is from 0x0 to 0x03FFFFFF. The least significant bit (the right most bit) represents drive A, and the 26th bit from the right represents drive Z.

A bit set to 1 disables AutoRun on a particular drive. For example, if the data value is set to 0x8 (1000 binary), AutoRun is disabled on drive D.

[edit] Evaluation order

The NoDriveAutoRun and NoDriveTypeAutoRun Registry entries can exist in two places, the per-user setting (under HKEY_CURRENT_USER) and the per-machine setting (under HKEY_LOCAL_MACHINE). If an entry appears under HKEY_LOCAL_MACHINE, then any corresponding entry under HKEY_CURRENT_USER is completely ignored. The data values are not merged in any way.

When deciding whether to activate AutoRun, both NoDriveAutoRun and NoDriveTypeAutoRun Registry entries are consulted. If either value indicates a drive should be disabled then AutoRun is disabled for that drive.

Thus in the following example:

the data value taken for NoDriveAutoRun is 0x08, disabling drive D and the data value taken for NoDriveTypeAutoRun is 0x95, disabling removable and network drives. The per-user NoDriveAutoRun entry is never used.

[edit] Group Policy

The Group Policy settings dialog

The only Group Policy settings available for AutoRun affect the NoDriveTypeAutoRun Registry entries. The policy is available on either a per-machine or a per-user basis reflecting the Registry entry location in either HKLM or HKCU.^[13][15]

As described above, a per-machine policy setting will cause the per-user policy setting to be ignored.

When a policy is Enabled, Group Policy will add the NoDriveTypeAutoRun entry to the Registry. If the policy is Disabled or set to Not configured, Group Policy deletes this entry from the Registry. System defaults may then take effect as described in the NoDriveTypeAutoRun section.

The policy names, locations and possible settings vary slightly between Windows versions. The list of settings are relatively short and are always additional to the system default setting.

Therefore, on Windows 2000, enabling the "Disable Autoplay" policy and setting it to "CD-ROM drives", disables AutoRun (as distinct from AutoPlay) for CD-ROM and DVD drives, removable drives, network drives, and drives of unknown type.

This setting cannot be used to enable AutoRun on drives on which it is disabled by default or disable AutoRun for drives not listed. To disable or enable any particular drives or drive types, the Registry must be edited manually.

[edit] Windows Server 2003, Windows XP, and Windows 2000

The per-machine policy location is:

Computer Configuration \ Administrative Templates \ System

The per-user policy location is:

User Configuration \ Administrative Templates \ System

The relevant policy is "Turn off Autoplay". In Windows 2000 the policy is called "Disable Autoplay" instead.

Once the policy is Enabled it can be set to "All drives" or "CD-ROM drives". The latter setting adds CD-ROM drives to the existing list of disabled drive types as described above.

[edit] Windows Vista, Windows Server 2008

The per-machine policy location is:^[16]

Computer Configuration \ Administrative Templates \ Windows Components \ Autoplay Policies

The per-user policy location is:

User Configuration \ Administrative Templates \ Windows Components \ AutoPlay Policies

The relevant policy is "Turn off Autoplay" and can be set for CD-ROM, DVD-ROM and removable drives or all drives.

Two related policies were added in Vista and Server 2008:^[17]

Default behavior for AutoRun

Sets the default behavior for AutoRun commands found in autorun.inf files.

Prior to Windows Vista, when media containing an autorun.inf with open or shellexecute commands was inserted, the default action was to automatically execute the program without user intervention.

The default behaviour in Windows Vista is to invoke AutoPlay. The autorun.inf command is then represented as one of the dialog options. This is the behaviour when the policy is Not configured or Disabled.

If this policy is Enabled, the behaviour can be changed to either:

• Completely disable autorun.inf commands or
• Automatically execute the autorun.inf command as per previous Windows versions.

Don't set the always do this checkbox

If this policy is Enabled, the "Always do this..." checkbox in the AutoPlay dialog will not be set by default when the dialog is shown.

[edit] Altering AutoRun behaviour

[edit] Pressing the Shift key

If the ⇧ Shift key is held down at a certain point in the execution sequence Windows Vista executes AutoPlay regardless of any settings.^[8]

Given that Shift must be held down until Windows checks for it, it may be a considerable amount of time before it becomes effective. The time taken primarily depends on the time to recognise the new hardware and time taken for CD-ROMs to spin up.

[edit] Auto Insert Notification

Certain "Media Change Notification" events may be suppressed by altering certain Registry entries. "Media Change Notification" is the generic term; for CD-ROM drives, the specific term is "Auto Insert Notification".

For CD-ROM drives, changing the value of this Registry entry to 0, will disable Auto Insert Notification for CD-ROM drives only.^[18] A Windows restart will be necessary.

Under Windows 95/98/ME, this setting can be changed under Device Manager, accessible from the System icon in Control Panel.

Auto insert notification under Windows 98

Although the Registry entry is named "AutoRun", it only suppresses the MCN message. The MCN message does trigger AutoRun initiation but it also instructs the Explorer shell to update its views and contents.

Thus, as a side effect only, this disables AutoRun for CD-ROM drives. However, Explorer will now not update its view when a new CD is inserted; it will show the contents of the previous CD until F5 is pressed or View/Refresh is selected from the Explorer menu. This could result in severe confusion for users.

For this reason the Media Change Notification message should not be disabled unless there is absolutely no alternative; AutoRun can be disabled for individual drives using TweakUI, Group Policy or the Registry.

This entry is used to suppress the MCN message for specifically listed type of CD-ROM drive,^[19] primarily CD-ROM changers. The data is a set of device identifiers, which matches those identifiers reported to the system by the devices themselves.

The default value for this entry consists of products identified by Microsoft testing as being unable to support AutoRun. This entry should not be altered from its default.

[edit] Editing Group Policy

AutoRun may be suppressed on particular drives and drive types by using Group Policy as described in the section on the Registry and Group Policy. However, the Group Policy Editor is not available on Home versions of Windows XP^[20] and does not provide any fine-grained drive selection facilities.

However, Group Policy would be the accepted method of disabling AutoRun on an entire Windows domain.

[edit] TweakUI

TweakUI AutoPlay Types

TweakUI is a user interface customisation application produced by Microsoft and is available from their website as part of the PowerToys toolset.

TweakUI has options to disable AutoRun on a drive or drive type basis under the "My Computer/AutoPlay" settings tree. The options here are given in the inverse from normal - TweakUI shows "enable" options while Group Policy and the Registry show "disable" options - and set the corresponding entries in the Registry:

• Settings under Drives set NoDriveAutoRun
• Settings under Types set NoDriveTypeAutoRun

TweakUI AutoPlay Drives

However, TweakUI apparently^[21] sets those Registry entries under the per-user Registry hive, HKEY_CURRENT_USER. In the case of corresponding entries existing under HKEY_LOCAL_MACHINE, the settings would not be effective.

TweakUI is able to change these settings without a reboot being needed. It possibly uses SendMessage to send a setting change (WM_SETTINGCHANGE) broadcast (HWND_BROADCAST) to let Explorer know of a change in policy.^[22]

[edit] Registry files

A Registry setting file can be created that, when executed, makes the desired changes in the Registry.

Windows Registry Editor Version 5.00
 
[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\policies\Explorer]
"NoDriveTypeAutoRun"=dword:000000ff

If the above snippet is saved as a file with a .reg extension, for example "killautorun.reg", and then the file icon double-clicked, the setting will be entered into the Registry, permissions allowing. In the above example, AutoRun would be disabled for all drives and for all users. This example would need to be run as Administrator and a reboot would be needed for the setting to take complete effect.

See the section on the Registry and Group Policy for details on the relevant Registry entries.

[edit] Issues and workarounds

[edit] The AutoRun disable bug

There has been a longstanding issue with Windows failing to handle AutoRun-relevant Registry entries properly.^[23]

When AutoRun is disabled, Windows should not proceed further through the activation sequence than the Registry check. However, it parses any autorun.inf found and does everything except the final action to invoke AutoPlay or execute an application.

This leaves the user open to attack from malware which uses the autorun.inf to alter the double-click and contextual menu behaviours. Double clicking the drive icon will infect the machine. Right Clicking and selecting the "Explore" or "Open" options from the context menu is not a workaround as these menu items can be coopted by the appropriate autorun.inf entries.

This bug has been fixed in security updates issued in July 2008. For Windows Vista and Windows Server 2008 the relevant Knowledge Base Article is 950582^[24] with further details in the security bulletin itself.^[25]

For Windows XP, Windows Server 2003 and Windows 2000 the relevant Knowledge Base Article is 953252^[26] with details and links to the OS specific patches available from that page. Windows 95 and Windows 98 are not affected.

These are fixes which address the Double Click, Contextual Menu and AutoPlay functionality.

These patches also allows reverting to the previous functionality on a per-user and per-machine basis by changing a Registry entry:^[26]

When you install the security update, this Registry entry is created only in the HKEY_LOCAL_MACHINE Registry hive with a default value of 1. This enables the functionality for this update, and for all users.

Setting the Registry entry data to 0 results in the previous insecure behaviour. If the key is present in both HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE, the HKLM value is used and the HKCU is ignored.

[edit] Workarounds for the AutoRun bug

In the case that the patches referred to above cannot be applied, there are several workarounds available. Two are documented by Microsoft:^[26]

• Disabling the use of USB devices described in Knowledge Base Article 823732^[27]
• Preventing autorun.inf invocation from network shares by following these steps:
  1. Delete any existing autorun.inf file from the root of a mapped network drive.
  2. Do not give anyone Create rights to the root of a mapped network drive.

[edit] Initialisation file mapping

The other workaround involves using initialisation file mapping to create a mapping between the autorun.inf initialisation file and the Registry. This procedure relies on the fact that an autorun.inf file is a standard Windows INI file; so, the appropriate API calls are used by Windows when fetching its settings. These API calls can be redirected using the INI file mapping method.

This method was first mentioned in Nick Brown's blog^[28] and is also referred to in the US-CERT Vulnerability Note.^[23]

Create a subkey called autorun.inf under the HKLM\Software\Microsoft\Windows NT\CurrentVersion\IniFileMapping key. Change the (Default) entry under the new autorun.inf key to the string @SYS:DoesNotExist:

The (Default) Registry entry name is also called (NULL) or, in a Registry file, "@"

Alternatively, create a Registry file with the following contents to make the change:

Windows Registry Editor Version 5.00
 
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\IniFileMapping\Autorun.inf]
@="@SYS:DoesNotExist"

Now whenever Windows tries to read a file called "autorun.inf" using the INI programming calls, it is forbidden from reading from the actual file. Instead, all settings are read from the HKEY_LOCAL_MACHINE\Software\DoesNotExist Registry key. As this key does not exist, it is as if the autorun.inf file contains no settings information. This applies to any autorun.inf in any location and on any drive.

The only drawback is that installation of software from an autorunning install CD or DVD is no longer automatic. It will be necessary to view the CD's autorun.inf file and then execute the appropriate install program manually.

[edit] Patch KB950582

In February 2009 Microsoft issued a hotifx KB950582 to address the problem. It is described in article KB953252 and security advisory 967940.

[edit] MountPoints2

There are a number of websites that assert that a certain Registry key, MountPoints2, contains cached information about every memory stick or other removable device seen so far. The key can be found under the Registry key:

HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer

It is deemed possible that it overrides the NoDriveTypeAutoRun entry when a device the PC already knows about is inserted.

It is currently unknown whether the AutoRun Bug patch fixes any issues with this key. There is no Microsoft documentation available on this Registry entry.

[edit] Other issues

• If you add the computer to an Active Directory domain, the NoDriveTypeAutoRun value may be reset to a default value.^[14] This is due to Group Policy settings in the domain taking effect. This is not a bug.
• Some programs may deliberately change AutoRun Registry settings. Early versions of CD burning software like Roxio have been known to change settings in this way.^[29]
• If the Group Policy "Restrict CD-ROM access to locally logged-on user only" security option under:

  Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options

  is turned on (Enabled), then AutoRun may not function.^[29]

  Windows Installers will also malfunction because "Local System" access to the CD-ROM will be denied.^[30] This Group Policy setting reflects the value of the Registry entry:

  HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon
  Entry name	Data type	Range	Default
  allocatecdroms	REG_SZ	0 or 1	0

  and should be set to 0. 
  

• Real Player 10 interferes with AutoPlay functionality to the extent that it may look as if AutoRun or AutoPlay is not working at all.^[31][32][33]

[edit] Attack vectors

AutoRun functionality has been used as a malware vector for some time. Prior to Windows Vista, the default action with a CD-ROM drive type was to follow any autorun.inf file instructions without prompts or warnings. This makes rogue CD-ROMs one possible infection vector.

In the same category are mixed content CD-ROMs. An audio CD, that a user would not expect to contain software at all, can contain a data section with an autorun.inf. Some companies, such as Sony BMG, have used this vector to install software that attempts to protect against copying of the audio tracks.

U3 enabled flash drives, by emulating a CD-ROM unit, can also cause Windows to execute commands from the autorun.inf found on the emulated CD-ROM.

Devices like the Huawei E220 HSDPA modem, validly use this method to autoinstall drivers for the modem itself. However plugging in a flash drive from an unknown source is an unwise move. USB Switchblade, and other similar tools, have made U3 flash drive attacks trivial. Given the ease of writing script based attacks, anti-virus software may be ineffective in preventing data and password stealing.

Social Engineering: The Conficker worm in action

With a standard flash drive, social engineering attacks can be employed to entice a user to click on the appropriate item in the AutoPlay dialog. An alluring action string promising free games or pornography would lure many users into the trap. At any time, double clicking on the drive icon will use the autorun.inf automatically, a trap more advanced users could fall into.

Any user can configure AutoPlay to make various decisions for them; by checking the appropriate box in the AutoPlay dialog, running flash drive malware becomes silent and automatic.

AutoRun malware has been extended to use hard drives,^[34] picture frames and other digital devices.^[35] Care in dealing with external devices is a security priority.

Mitigation of malware attacks:

• Do not habitually run with Administrator privileges
• Disable AutoRun (but see the AutoRun disable bug)
• Use Group Policy under Vista to disable autorun.inf commands
• Use initialisation file mapping to nullify autorun.inf sections

[edit] The autorun.inf file

autorun.inf is an ASCII text file located in the root folder of a CD-ROM or other removable media. The structure is that of a classic Windows .ini file, containing information and commands as "key=value" pairs, grouped into sections.^[5] These keys specify:

• The name and the location of a program to call when the media is inserted.

• The name of a file that contains an icon that represents the media in Explorer (instead of the standard drive icon).

• Commands for the menu that appears when the user right-clicks the drive icon.

• The default command that runs when the user double-clicks the drive icon.

• Settings that alter AutoPlay detection routines or search parameters

• Settings that indicate the presence of drivers.

On Windows XP versions previous to Service Pack 2, drives of type DRIVE_REMOVABLE do not have any autorun.inf support.^[5]

From Service Pack 2, removable drives may have an autorun.inf file recognised and used by including an action key. The text from this key may be combined with an icon given by the icon key and this will be used in the AutoPlay dialog presented to the user.^[10]

Following are the sections and keys allowed in a valid autorun.inf.^[10] There also exist architecture specific section types for systems such as Windows NT 4 running on RISC. This are outdated and not described here.

[edit] Sample autorun.inf

This simple autorun.inf file specifies setup.exe as the application to run when AutoRun is activated. The first icon stored within the setup.exe itself will represent the drive in Explorer:

[autorun] 
open=setup.exe 
icon=setup.exe,0
label=My install CD

[edit] [autorun]

The autorun section contains the default AutoRun commands. Valid autorun.inf files must contain this section. Keys allowed are:

action=text
action=@[filepath\]filename,-resourceID

Windows XP SP2 or later; drives of type DRIVE_REMOVABLE and DRIVE_FIXED

Specifies text used in the AutoPlay dialog to represent the program specified in the open or shellexecute keys. The text is expressed as either text or as a resource reference. The icon is displayed next to the text. This item is always first in the AutoPlay dialog and is always selected by default.

If the (action) key does not appear on drives of type:

DRIVE_REMOVABLE		the AutoPlay dialog appears but without additional menu items. Essentially, the autorun.inf is ignored. This makes the action key mandatory for drives of this type.
DRIVE_FIXED		default text is created and used in the AutoPlay dialog.

On all other drive types the key is ignored.

icon=iconfilename[,index]

The name of an file resource containing an icon. This icon replaces the standard drive icon in Windows Explorer. This file must be in the same directory as the file specified by the open key.

label=text

Specifies a text label representing the drive in Windows Explorer.

open=[exepath\]exefile [param1 [param2] ...]

Specifies the path, file name and optional parameters to the application that AutoRun launches when a user inserts a disc in the drive. It is the CreateProcess function that is called by AutoRun.

shellexecute=[filepath\]filename[param1, [param2]...]

Windows 2000, Windows ME or later

Similar to open, but using file association information to run the application. The file name can therefore be an executable or a data file. It is the ShellExecuteEx function that is called by AutoRun.

UseAutoPlay=1

Windows XP or later; drives of type DRIVE_CDROM

Use AutoPlay rather than AutoRun with CD-ROMs. The action taken on CD-ROM insertion will depend on the version of Windows being used.

On versions of Windows earlier than XP, this key has no effect and actions specified by open or shellexecute are performed.

On Windows XP and later, the user will be presented with the AutoPlay dialog and any actions specified by open or shellexecute are ignored.

shell\verb\command=[exepath\]exefile [param1 [param2] ...]

Adds a custom command to the drive's shortcut menu. verb is a string with no embedded spaces. verb is also the text that will appear in the shortcut menu unless specifically altered to some other text. See below for an example.

shell\verb=MenuText

Optionally specify the text displayed in the shortcut menu for the verb above. Use an ampersand (&) to select a hotkey for the menu. See below for an example.

shell=verb

Defines the menu command referred to by shell\verb as the default command in the shortcut menu. The default command is the command executed when the drive icon is double-clicked. If missing, the default menu item will be "AutoPlay", which launches the application specified by the open entry.

Example:

shell\readme\command=notepad readme.txt shell\readme=Read &Me shell=readme

[edit] [Content]

Windows Vista or later

The Content section allows authors to communicate the type and intent of content to AutoPlay without AutoPlay having to examine the media.

Valid keys are: MusicFiles, PictureFiles, VideoFiles. Each key can be set to indicate true or false values and values are not case sensitive.

true	(or 1, y, yes, t)	:	display the handlers associated with that content type
false	(or 0, n, no, f)	:	do not display the handlers associated with that content type

Example:

[Content]
MusicFiles=Y
PictureFiles=0
VideoFiles=false

[edit] [ExclusiveContentPaths]

Windows Vista or later

Limits AutoPlay's content search to only those folders listed, and their subfolders. The folder names are always taken as absolute paths (a path from the root directory of the media) whether or not a leading slash is used.

Example:

[ExclusiveContentPaths]
\pictures
\music
more music\special

[edit] [IgnoreContentPaths]

Windows Vista or later

AutoPlay's content search system will not scan the folders listed, nor their subfolders. IgnoreContentPaths takes precedence over ExclusiveContentPaths so if a path given in a [IgnoreContentPaths] section is a subfolder of a path given in an [ExclusiveContentPaths] section it is still ignored.

[edit] [DeviceInstall]

Windows XP or later

This section is used to indicate where driver files may be located. This prevents a lengthy search through the entire contents of a CD-ROM. Windows XP will fully search:

• floppy disks in drives A or B
• CD/DVD media less than 1 GB in size.

without this section present. All other media should include this section to have Windows XP autodetect any drivers stored on that media.

The section is not used with AutoRun or AutoPlay and is only referred to during a driver installation phase. The only valid key is:

DriverPath=directorypath

which lists a path Windows will search for driver files. All subdirectories of that path are also searched. Multiple key entries are allowed.

If no DriverPath entry is provided in the [DeviceInstall] section or the DriverPath entry has no value, then that drive is skipped during a search for driver files.

Example:

[DeviceInstall]
DriverPath=drivers\video 
DriverPath=drivers\audio

[edit] See also

• AutoPlay

[edit] References

[edit] External links

• Autorun Protector: 2 way protection from autorun worms., Raymond Lin
• Autorun and Autoplay: screwed by terminology, Michael Horowitz
• AutoRun and AutoPlay Reference, Microsoft, MSDN Library
• Memory stick worms, Nick Brown's blog
• Dan McCloy's Autorun Reference Guide
• Security Watch Island Hopping: The Infectious Allure of Vendor Swag, TechNet Magazine
• Figure 4: querycancelautoplay example code, Microsoft Systems Journal, September 1998
• AutoPlay Repair Wizard, Microsoft Download Center
• Test your defenses against malicious USB flash drives, Computerworld blog, January 24, 2009
• The best way to disable Autorun for protection from infected USB flash drives, Computerworld blog, January 30, 2009
• How to correct "disable Autorun registry key" enforcement in Windows, Microsoft KB967715