Microsoft Security Bulletin MS05-014

• In a Web-based attack scenario, an attacker would have to host a Web site that contains a Web page that is used to exploit this vulnerability. An attacker could also attempt to compromise a Web site to have it display a Web page with malicious content. An attacker would have no way to force users to visit a Web site. Instead, an attacker would have to persuade them to visit the Web site, typically by getting them to click a link that takes them to the attacker's site or to a site that has been compromised by the attacker.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
• By default, Microsoft Outlook Express 6, Outlook 2000, Outlook 2002, and Outlook 2003 open HTML e-mail messages in the Restricted sites zone. Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed. The Restricted sites zone helps reduce attacks that could try to exploit this vulnerability.
  
  The risk of attack from the HTML e-mail vector can be significantly reduced if you meet all the following conditions:
  • Install the update that is included with Microsoft Security Bulletin MS03-040 or a later Cumulative Security Update for Internet Explorer.
  • Use the Microsoft Outlook E-mail Security Update, use Microsoft Outlook Express 6 or a later version, or use Microsoft Outlook 2000 Service Pack 2 or a later version in its default configuration.
• By default, Internet Explorer on Windows Server 2003 runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability. See the FAQ section of this vulnerability for more information about Internet Explorer Enhanced Security Configuration.

Microsoft has tested the following workarounds. While these workarounds will not correct the underlying vulnerability, they help block known attack vectors. When a workaround reduces functionality, it is identified in the following section.

• Disable “Drag and Drop or copy and paste files” in Internet Explorer

  Disable “Drag and Drop or copy and paste files” in Internet Explorer by following these steps:

  1. Obtain and install the MS04-038 Cumulative Security Update for Internet Explorer. This security update must be installed for these configuration steps to be effective.
  2. Disable the Drag and drop or copy and paste files option in the Internet and Intranet Web content zones. Disable the Drag and drop or copy and paste files option in the Internet and local intranet zone. To do this, follow these steps:
     1. In Internet Explorer, click Internet Options on the Tools menu, and then click the Security tab.
     2. In the Select a Web content zone to specify its security settings box, click Internet, and then click Custom Level.
     3. In the Settings box, locate the Drag and drop or copy and paste files option under Miscellaneous. Make a note of your current setting.
     4. Under Drag and drop or copy and paste files, click Disable, and then click OK.
     5. Click Yes, and then click OK two times.

     Note Repeat these steps for the local intranet zone by clicking Local intranet instead of Internet in step 2. These steps are also outlined in Microsoft Knowledge Base Article 888534 where steps on how to restore your previous drag and drop or copy and paste files setting are outlined.

• Set Internet and Local intranet security zone settings to “High” to prompt before running ActiveX controls and Active Scripting in these zones.

  You can help protect against this vulnerability by changing your settings for the Internet security zone to prompt before running ActiveX controls and Active Scripting. You can do this by setting your browser security to High.

  To raise the browsing security level in Microsoft Internet Explorer, follow these steps:

  1. On the Internet Explorer Tools menu, click Internet Options.
  2. In the Internet Options dialog box, click the Security tab, and then click the Internet icon.
  3. Under Security level for this zone, move the slider to High. This sets the security level for all Web sites you visit to High.

  Note If no slider is visible, click Default Level, and then move the slider to High.

  Note Setting the level to High may cause some Web sites to work incorrectly. If you have difficulty using a Web site after you change this setting, and you are sure the site is safe to use, you can add that site to your list of trusted sites. This will allow the site to work correctly even with the high security setting.

  Alternatively, you can change your settings to prompt before running ActiveX controls only. To do this, follow these steps:

  1. In Internet Explorer, click Internet Options on the Tools menu.
  2. Click the Security tab.
  3. Click Internet, and then click Custom Level.
  4. Under Settings, in the ActiveX controls and plug-ins section, under Run ActiveX controls and plug-ins, click Prompt.
  5. In the Scripting section, under Active Scripting, click Prompt, and then click OK.
  6. Click Local intranet, and then click Custom Level.
  7. Under Settings, in the ActiveX controls and plug-ins section, under Run ActiveX controls and plug-ins, click Prompt.
  8. In the Scripting section, under Active Scripting, click Prompt.
  9. Click OK two times to return to Internet Explorer.

  Impact of Workaround: There are side effects to prompting before running ActiveX controls. Many Web sites that are on the Internet or on an intranet use ActiveX to provide additional functionality. For example, an online e-commerce site or banking site may use ActiveX controls to provide menus, ordering forms, or even account statements. Prompting before running ActiveX controls is a global setting that affects all Internet and intranet sites. You will be prompted frequently when you enable this workaround. For each prompt, if you feel you trust the site that you are visiting, click Yes to run ActiveX controls. If you do not want to be prompted for all these sites, use the "Restrict Web sites to only your trusted Web sites" workaround.

• Restrict Web sites to only your trusted Web sites.

  After you set Internet Explorer to require a prompt before it runs ActiveX controls and Active Scripting in the Internet zone and in the Local intranet zone, you can add sites that you trust to Internet Explorer's Trusted sites zone. This will allow you to continue to use trusted Web sites exactly as you do today, while helping to protect you from this attack on untrusted sites. We recommend that you add only sites that you trust to the Trusted sites zone.

  To do this, follow these steps:

  1. In Internet Explorer, click Tools, click Internet Options, and then click the Security tab.
  2. In the Select a Web content zone to specify its current security settings box, click Trusted Sites, and then click Sites.
  3. If you want to add sites that do not require an encrypted channel, click to clear the Require server verification (https:) for all sites in this zone check box.
  4. In the Add this Web site to the zone box, type the URL of a site that you trust, and then click Add.
  5. Repeat these steps for each site that you want to add to the zone.
  6. Click OK two times to accept the changes and return to Internet Explorer.

  Add any sites that you trust not to take malicious action on your computer. One in particular that you may want to add is "*.windowsupdate.microsoft.com" (without the quotation marks). This is the site that will host the update, and it requires an ActiveX control to install the update.

• Read e-mail messages in plain text format if you are using Outlook 2002 or a later version, or Outlook Express 6 SP1 or a later version, to help protect yourself from the HTML e-mail attack vector.

  Microsoft Outlook 2002 users who have applied Office XP Service Pack 1 or a later version and Microsoft Outlook Express 6 users who have applied Internet Explorer 6 Service Pack 1 or a later version can enable this setting and view e-mail messages that are not digitally signed or e-mail messages that are not encrypted in plain text only.

  Digitally signed e-mail messages or encrypted e-mail messages are not affected by the setting and may be read in their original formats. For more information about how to enable this setting in Outlook 2002, see Microsoft Knowledge Base Article 307594.

  For information about this setting in Outlook Express 6, see Microsoft Knowledge Base Article 291387.

  Impact of Workaround: E-mail messages that are viewed in plain text format will not contain pictures, specialized fonts, animations, or other rich content. Additionally:

  • The changes are applied to the preview pane and to open messages.
  • Pictures become attachments so that they are not lost.
  • Because the message is still in Rich Text or HTML format in the store, the object model (custom code solutions) may behave unexpectedly.

What is the scope of the vulnerability?
This vulnerability involves drag and drop events in Internet Explorer. An attacker who successfully exploited this vulnerability could cause an executable file to be saved on the user's system. The user would not receive a dialog box requesting approval for the download. To exploit this vulnerability, an attacker would have to host a malicious Web site that contained a Web page that was designed to exploit this vulnerability and then persuade a user to visit that site. If the user took certain actions on that Web page, executable files of the attacker's choice could be saved in specified locations on the user's system.

What causes the vulnerability?
Drag-and-Drop technology incorrectly validates some dynamic HTML (DHTML) events. This vulnerability permits a file to be downloaded to the user's system after the user clicks a link.

What are DHTML events?
DHTML events are special actions that are provided by the DHTML Object Model. These events can be used in script code to add dynamic content to a Web site. For more information about DHTML events, see the product documentation.

How could an attacker exploit the vulnerability?
An attacker who successfully exploited this vulnerability could save code of their choice to the user's local file system. Although this code could not be run through this vulnerability directly, the operating system might open the file if it is saved to a sensitive location, or a user may open the file inadvertently and cause the attacker's code to run.

What systems are primarily at risk from the vulnerability?
This vulnerability requires that a user is logged on and reading e-mail or visiting Web sites for any malicious action to occur. Therefore, any systems where e-mail is read or where Internet Explorer is used frequently, such as users’ workstations or terminal servers, are at the most risk from this vulnerability. Systems that are not typically used to read e-mail or to visit Web sites, such as most server systems, are at a reduced risk.

I am running Internet Explorer on Windows Server 2003. Does this mitigate this vulnerability?
Yes. By default, Internet Explorer on Windows Server 2003 runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability.

What is Internet Explorer Enhanced Security Configuration?
Internet Explorer Enhanced Security Configuration is a group of preconfigured Internet Explorer settings that reduce the likelihood of a user or of an administrator downloading and running malicious Web content on a server. Internet Explorer Enhanced Security Configuration reduces this risk by modifying many security-related settings. This includes the settings on the Security tab and the Advanced tab in the Internet Options dialog box. Some of the important modifications include the following:

• Security level for the Internet zone is set to High. This setting disables scripts, ActiveX controls, Microsoft Java Virtual Machine (MSJVM), HTML content, and file downloads.
• Automatic detection of intranet sites is disabled. This setting assigns all intranet Web sites and all Universal Naming Convention (UNC) paths that are not explicitly listed in the Local intranet zone to the Internet zone.
• Install On Demand and non-Microsoft browser extensions are disabled. This setting prevents Web pages from automatically installing components and prevents non-Microsoft extensions from running.
• Multimedia content is disabled. This setting prevents music, animations, and video clips from running.

What does the update do?
The update removes the vulnerability by modifying the way that Internet Explorer validates some drag and drop events. The update also includes a list of valid file types that Internet Explorer will allow drag and drop events on.

Can I add or remove file types to this list of allowed file types?
Yes. You can allow Internet Explorer drag and drop events on other file types. You can also further restrict what file types you want Internet Explorer to allow drag and drop events on. To do this, edit the following registry key.

Warning When you do this, be very selective and allow only file types that you must.

Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

To allow drag and drop of additional file types:

1. Click Start, click Run, type Regedit in the Open box, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Internet Settings\AllowedDragImageExts
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type the file extension, for instance, .aif, and then press ENTER.
5. Right-click the newly created DWORD value, change the Value Data field to 1, and then press ENTER.
   
   Note The Value Data field of a newly created registry value is by default 0 (zero).

To disallow drag and drop of certain file types:

1. Click Start, click Run, type Regedit in the Open box, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Internet Settings\AllowedDragImageExts
3. Remove DWORD Value for which you do not wish to allow Internet Explorer drag and drop functionality.
   
   Note It is recommended that you back up this registry key before performing this operation.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
Yes. This vulnerability has been publicly disclosed. This vulnerability has been assigned Common Vulnerability and Exposure number CAN-2005-0053.

Note The update for the “Drag-and-Drop Vulnerability” - CAN-2005-0053 also addresses the following publicly disclosed variations: CAN-2004-0985, CAN-2004-0839, and CAN-2003-1027.

When this security bulletin was issued, had Microsoft received any reports that this vulnerability was being exploited?
Yes. When the security bulletin was released, Microsoft had received information that this vulnerability was being exploited.

Does applying this security update help protect customers from the code that has been published publicly that tries to exploit this vulnerability?
Yes. This security update addresses the vulnerability and its variations that are currently being exploited. The vulnerability that has been addressed has been assigned the Common Vulnerability and Exposure number CAN-2005-0053.

• In a Web-based attack scenario, an attacker would have to host a Web site that contains a Web page that is used to exploit this vulnerability. An attacker could also attempt to compromise a Web site to have it display a Web page with malicious content. An attacker would have no way to force users to visit a Web site. Instead, an attacker would have to persuade them to visit the Web site, typically by getting them to click a link that takes them to the attacker's site or to a site that has been compromised by the attacker.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
• By default, Microsoft Outlook Express 6, Outlook 2000, Outlook 2002, and Outlook 2003 open HTML e-mail messages in the Restricted sites zone. Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed. The Restricted sites zone helps reduce attacks that could try to exploit this vulnerability.

  The risk of attack from the HTML e-mail vector can be significantly reduced if you meet all the following conditions:

  • Install the update that is included with Microsoft Security Bulletin MS03-040 or a later Cumulative Security Update for Internet Explorer.
  • Use Microsoft Outlook Express 6 or a later version in its default configuration, or use Microsoft Outlook 2000 Service Pack 2 or a later version in its default configuration.
• By default, Internet Explorer on Windows Server 2003 runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability. See the FAQ section of this security update for more information about Internet Explorer Enhanced Security Configuration.
• Windows XP Service Pack 2 introduced a security enhancement known as the Local Machine zone lockdown. This security enhancement mitigates this vulnerability. See the FAQ section of this security update for more information about Local Machine zone lockdown.

Microsoft has tested the following workarounds. While these workarounds will not correct the underlying vulnerability, they help block known attack vectors. When a workaround reduces functionality, it is identified in the following section.

• Restrict Web sites to only your trusted Web sites.

  After you set Internet Explorer to require a prompt before it runs ActiveX controls and Active Scripting in the Internet zone and in the Local intranet zone, you can add sites that you trust to Internet Explorer's Trusted sites zone. This will allow you to continue to use trusted Web sites exactly as you do today, while helping to protect you from this attack on untrusted sites. We recommend that you add only sites that you trust to the Trusted sites zone.

  To do this, follow these steps:

  1. In Internet Explorer, click Tools, click Internet Options, and then click the Security tab.
  2. In the Select a Web content zone to specify its current security settings box, click Trusted Sites, and then click Sites.
  3. If you want to add sites that do not require an encrypted channel, click to clear the Require server verification (https:) for all sites in this zone check box.
  4. In the Add this Web site to the zone box, type the URL of a site that you trust, and then click Add.
  5. Repeat these steps for each site that you want to add to the zone.
  6. Click OK two times to accept the changes and return to Internet Explorer.

  Add any sites that you trust not to take malicious action on your computer. One in particular that you may want to add is "*.windowsupdate.microsoft.com" (without the quotation marks). This is the site that will host the update, and it requires an ActiveX control to install the update.

• Install the update that is described in Microsoft Security Bulletin MS04-018 if you are using Outlook Express 5.5 SP2.

  Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed.

  Customers who use one or more of these products could be at a reduced risk from an e-mail-borne attack that tries to exploit this vulnerability unless the user clicks a malicious link in the e-mail message.

• Read e-mail messages in plain text format if you are using Outlook 2002 or a later version, or Outlook Express 6 SP1 or a later version, to help protect yourself from the HTML e-mail attack vector.

  Microsoft Outlook 2002 users who have applied Office XP Service Pack 1 or a later version and Microsoft Outlook Express 6 users who have applied Internet Explorer 6 Service Pack 1 or a later version can enable this setting and view e-mail messages that are not digitally signed or e-mail messages that are not encrypted in plain text only.

  Digitally signed e-mail messages or encrypted e-mail messages are not affected by the setting and may be read in their original formats. For more information about how to enable this setting in Outlook 2002, see Microsoft Knowledge Base Article 307594.

  For information about this setting in Outlook Express 6, see Microsoft Knowledge Base Article 291387.

  Impact of Workaround: E-mail messages that are viewed in plain text format will not contain pictures, specialized fonts, animations, or other rich content. Additionally:

  • The changes are applied to the preview pane and to open messages.
  • Pictures become attachments so that they are not lost.
  • Because the message is still in Rich Text or HTML format in the store, the object model (custom code solutions) may behave unexpectedly.

What is the scope of the vulnerability?
This vulnerability could potentially allow remote code execution. If a user is logged on with administrative user rights, an attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.

Other possible attack scenarios include cross site scripting and partial address bar spoofing.

What causes the vulnerability?
The way that Internet Explorer parses certain types of encoded URLs.

What is URL encoding and what it is used for?
URL encoding is necessary in certain situations where a URL contains characters other than alphanumeric. Characters such as ASCII control characters or other reserved characters are included in a URL without first being encoded might be interpreted in an unintended way by browsers. Some characters, such as “space”, the pound character (“#”), or the percent character (“%”) may be misunderstood if included in URLs and are therefore encoded. For more information about URL encoding, see the Uniform Resource Locators (URL) RFC document: RFC 1738.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could take complete control of the affected system.

How could an attacker exploit the vulnerability?
An attacker could use this vulnerability to cause Internet Explorer to interpret an HTML page in the Internet zone as if it were in another Internet Explorer security zone. Internet Explorer would then apply the security settings for that Internet Explorer security zone to the HTML page instead of the Internet Explorer security zone that the HTML page belongs to. A user would have to click a link for any malicious action to occur. This vulnerability could also be used to attempt cross site scripting attacks or partial address bar spoofing. An attacker could create a Web page that would display a URL of the attacker’s choice in the Address bar, while displaying a different Web site in the browser window. An attacker could use this vulnerability to create a malicious page that spoofs a legitimate site. For example, an attacker could create a Web page that looks like a user's online e-mail Web site. However, this Web page would be hosted on a malicious Web site. An attacker could use this vulnerability to display a legitimate-looking URL in the Address bar. A user might see this URL and mistakenly provide sensitive information to the attacker's Web site.

The address bar spoof in this case is not complete and would, in part, display the attacker’s Web site URL as well.

What are Internet Explorer security zones?
Internet Explorer security zones are part of a system that divides online content into categories or zones that are based on the trustworthiness of the content. Specific Web domains can be assigned to a zone, depending on how much you trust the content of each domain. The zone then restricts the capabilities of the Web content, based on the zone's policy. By default, most Internet domains are treated as part of the Internet zone. By default, the policy of the Internet zone prevents scripts and other active code from accessing resources on the local system.

What systems are primarily at risk from the vulnerability?
This vulnerability requires that a user is logged on and reading e-mail or visiting Web sites for any malicious action to occur. Therefore, any systems where e-mail is read or where Internet Explorer is used frequently, such as users’ workstations or terminal servers, are at the most risk from this vulnerability. Systems that are not typically used to read e-mail or to visit Web sites, such as most server systems, are at a reduced risk.

Are Windows 98, Windows 98 Second Edition, or Windows Millennium Edition critically affected by one or more of the vulnerabilities that are addressed in this security bulletin?
Yes. Windows 98, Windows 98 Second Edition, and Windows Millennium Edition are critically affected by this vulnerability. Critical security updates for these platforms are available, are provided as part of this security bulletin, and can be downloaded only from the Windows Update Web site. For more information about severity ratings, visit the following Web site.

What does the update do?
The update removes the vulnerability by modifying the way that Internet Explorer validates encoded URLs.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
No. Microsoft received information about this vulnerability through responsible disclosure. However, a similar vulnerability has since been publicly disclosed. This update addresses both the privately reported vulnerability, which has been assigned CAN-2005-0054, as well as the independently publicly disclosed variation, which has not been assigned any CAN number.

When this security bulletin was issued, had Microsoft received any reports that this vulnerability was being exploited?
No. Microsoft had seen examples of proof of concept code published publicly but had not received any information to indicate that this vulnerability had been publicly used to attack customers when this security bulletin was originally issued.

• In a Web-based attack scenario, an attacker would have to host a Web site that contains a Web page that is used to exploit this vulnerability. An attacker could also attempt to compromise a Web site to display a Web page with malicious content. An attacker would have no way to force users to visit a Web site. Instead, an attacker would have to persuade them to visit the Web site, typically by getting them to click a link that takes them to the attacker's site or to a site that has been compromised by the attacker.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
• By default, Outlook Express 6, Outlook 2002, and Outlook 2003 open HTML e-mail messages in the Restricted sites zone. Additionally, Outlook 2000 opens HTML e-mail messages in the Restricted sites zone if the Outlook E-mail Security Update has been installed. Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed. The Restricted sites zone helps reduce attacks that could try to exploit this vulnerability.

  The risk of attack from the HTML e-mail vector can be significantly reduced if you meet all the following conditions:

  • Apply the update that is included with Microsoft Security Bulletin MS03-040 or a later Cumulative Security Update for Internet Explorer.
  • Use the Microsoft Outlook E-mail Security Update, use Microsoft Outlook Express 6 or a later version, or use Microsoft Outlook 2000 Service Pack 2 or a later version in its default configuration.
• The vulnerability could not be exploited automatically through e-mail. For an attack to be successful, a user must open an attachment this is sent in an e-mail message.

Microsoft has tested the following workarounds. While these workarounds will not correct the underlying vulnerability, they help block known attack vectors. When a workaround reduces functionality, it is identified in the following section.

• Set Internet and Local intranet security zone settings to “High” to prompt before running Active Scripting in these zones.

  You can help protect against this vulnerability by changing your settings for the Internet security zone to prompt before running Active Scripting. You can do this by setting your browser security to High.

  To raise the browsing security level in Microsoft Internet Explorer, follow these steps:

  1. On the Internet Explorer Tools menu, click Internet Options.
  2. In the Internet Options dialog box, click the Security tab, and then click the Internet icon.
  3. Under Security level for this zone, move the slider to High. This sets the security level for all Web sites you visit to High.

  Note If no slider is visible, click Default Level, and then move the slider to High.

  Note Setting the level to High may cause some Web sites to work incorrectly. If you have difficulty using a Web site after you change this setting, and you are sure the site is safe to use, you can add that site to your list of trusted sites. This will allow the site to work correctly even with the high security setting.

  Alternatively, you can change your settings to prompt before running Active Scripting only. To do this, follow these steps:

  1. In Internet Explorer, click Internet Options on the Tools menu.
  2. Click the Security tab.
  3. Click Internet, and then click Custom Level.
  4. Under Settings, in the Scripting section, under Active Scripting, click Prompt, and then click OK.
  5. Click Local intranet, and then click Custom Level.
  6. Under Settings, in the Scripting section, under Active Scripting, click Prompt.
  7. Click OK two times to return to Internet Explorer.

  Impact of Workaround: There are side effects to prompting before running Active Scripting. Many Web sites that are on the Internet or on an intranet use Active Scripting to provide additional functionality. Prompting before running Active Scripting controls is a global setting that affects all Internet and intranet sites. You will be frequently when you enable this workaround. For each prompt, if you feel you trust the site that you are visiting, click Yes to run Active Scripting. If you do not want to be prompted for all these sites, use the "Restrict Web sites to only your trusted Web sites" workaround.

• Restrict Web sites to only your trusted Web sites.

  After you set Internet Explorer to require a prompt before it runs Active Scripting in the Internet zone and in the Local intranet zone, you can add sites that you trust to Internet Explorer's Trusted sites zone. This will allow you to continue to use trusted Web sites exactly as you do today, while helping to protect you from this attack on untrusted sites. We recommend that you add only sites that you trust to the Trusted sites zone.

  To do this, follow these steps:

  1. In Internet Explorer, click Tools, click Internet Options, and then click the Security tab.
  2. In the Select a Web content zone to specify its current security settings box, click Trusted Sites, and then click Sites.
  3. If you want to add sites that do not require an encrypted channel, click to clear the Require server verification (https:) for all sites in this zone check box.
  4. In the Add this Web site to the zone box, type the URL of a site that you trust, and then click Add.
  5. Repeat these steps for each site that you want to add to the zone.
  6. Click OK two times to accept the changes and return to Internet Explorer.

  Add any sites that you trust not to take malicious action on your computer. One in particular that you may want to add is "*.windowsupdate.microsoft.com" (without the quotation marks). This is the site that will host the update, and it requires an ActiveX control to install the update.

• Install the update that is described in Microsoft Security Bulletin MS04-018 if you are using Outlook Express 5.5 SP2.

  Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed.

  Customers who use one or more of these products could be at a reduced risk from an e-mail-borne attack that tries to exploit this vulnerability unless the user clicks a malicious link in the e-mail message.

• Read e-mail messages in plain text format if you are using Outlook 2002 or a later version, or Outlook Express 6 SP1 or a later version, to help protect yourself from the HTML e-mail attack vector.

  Microsoft Outlook 2002 users who have applied Office XP Service Pack 1 or a later version and Microsoft Outlook Express 6 users who have applied Internet Explorer 6 Service Pack 1 or a later version can enable this setting and view e-mail messages that are not digitally signed or e-mail messages that are not encrypted in plain text only.

  Digitally signed e-mail messages or encrypted e-mail messages are not affected by the setting and may be read in their original formats. For more information about how to enable this setting in Outlook 2002, see Microsoft Knowledge Base Article 307594.

  For information about this setting in Outlook Express 6, see Microsoft Knowledge Base Article 291387.

  Impact of Workaround: E-mail messages that are viewed in plain text format will not contain pictures, specialized fonts, animations, or other rich content. Additionally:

  • The changes are applied to the preview pane and to open messages.
  • Pictures become attachments so that they are not lost.
  • Because the message is still in Rich Text or HTML format in the store, the object model (custom code solutions) may behave unexpectedly.

What is the scope of the vulnerability?
This is a remote code execution vulnerability. If a user is logged on with administrative user rights, an attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.

What causes the vulnerability?
The process used by Internet Explorer to validate the buffer used when processing certain DHTML methods?

What are DHTML methods?
The DHTML object model allows for more dynamic content on Web pages. DHTML methods are the methods that the DHTML object model provides. These methods can be used in script code to add dynamic content to a Web site. For more information about DHTML methods, see the product documentation.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could take complete control of the affected system.

How could an attacker exploit the vulnerability?
An attacker could exploit this vulnerability by creating a malicious Web page or an HTML e-mail message and then persuading the user to visit the page or to view the HTML e-mail message. If the user visited the page or viewed the e-mail message, the attacker could access information from other Web sites, could access local files on the system, or could cause malicious code to run in the security context of the locally logged on user.

What systems are primarily at risk from the vulnerability?
This vulnerability requires that a user is logged on and reading e-mail or visiting Web sites for any malicious action to occur. Therefore, any systems where e-mail is read or where Internet Explorer is used frequently, such as users’ workstations or terminal servers, are at the most risk from this vulnerability. Systems that are not typically used to read e-mail or to visit Web sites, such as most server systems, are at a reduced risk.

Are Windows 98, Windows 98 Second Edition, or Windows Millennium Edition critically affected by one or more of the vulnerabilities that are addressed in this security bulletin?
Yes. Windows 98, Windows 98 Second Edition, and Windows Millennium Edition are critically affected by this vulnerability. Critical security updates for these platforms are available, are provided as part of this security bulletin, and can be downloaded only from the Windows Update Web site. For more information about severity ratings, visit the following Web site.

What does the update do?
The update removes the vulnerability by modifying the way that Internet Explorer validates the length of a message before it passes the message to the allocated buffer.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
No. Microsoft received information about this vulnerability through responsible disclosure.

• In a Web-based attack scenario, an attacker would have to host a Web site that contains a Web page that is used to exploit this vulnerability. An attacker could also attempt to compromise a Web site to have it display a Web page with malicious content. An attacker would have no way to force users to visit a Web site. Instead, an attacker would have to persuade them to visit the Web site, typically by getting them to click a link that takes them to the attacker's site or to a site that has been compromised by the attacker.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
• By default, Microsoft Outlook Express 6, Outlook 2000, Outlook 2002, and Outlook 2003 open HTML e-mail messages in the Restricted sites zone. Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed. The Restricted sites zone helps reduce attacks that could try to exploit this vulnerability.

  The risk of attack from the HTML e-mail vector can be significantly reduced if you meet all the following conditions:

  • Install the update that is included with Microsoft Security Bulletin MS03-040 or a later Cumulative Security Update for Internet Explorer.
  • Use Microsoft Outlook Express 6 or a later version in its default configuration, or use Microsoft Outlook 2000 Service Pack 2 or a later version in its default configuration.
• By default, Internet Explorer on Windows Server 2003 runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability. See the FAQ section of this security update for more information about Internet Explorer Enhanced Security Configuration.
• Windows XP Service Pack 2 introduced a security enhancement known as the Local Machine zone lockdown. This security enhancement mitigates this vulnerability. See the FAQ section of this security update for more information about Local Machine zone lockdown.
• Windows XP Service Pack 2 introduced a new security context on all scriptable objects so that access to cached objects (except for ActiveX controls) is blocked. In addition to blocking access when navigating across domains, access is also blocked when navigating within the same domain. In this context, a domain is defined as a fully qualified domain name, or FQDN. A reference to an object is no longer accessible after the context has changed due to navigation. For more information about Internet Explorer Object Caching in Windows XP Service Pack 2, see the product documentation.

Microsoft has tested the following workarounds. While these workarounds will not correct the underlying vulnerability, they help block known attack vectors. When a workaround reduces functionality, it is identified in the following section.

• Set Internet and Local intranet security zone settings to “High” to prompt before running ActiveX controls and Active Scripting in these zones.

  You can help protect against this vulnerability by changing your settings for the Internet security zone to prompt before running ActiveX controls and Active Scripting. You can do this by setting your browser security to High.

  To raise the browsing security level in Microsoft Internet Explorer, follow these steps:

  1. On the Internet Explorer Tools menu, click Internet Options.
  2. In the Internet Options dialog box, click the Security tab, and then click the Internet icon.
  3. Under Security level for this zone, move the slider to High. This sets the security level for all Web sites you visit to High.

  Note If no slider is visible, click Default Level, and then move the slider to High.

  Note Setting the level to High may cause some Web sites to work incorrectly. If you have difficulty using a Web site after you change this setting, and you are sure the site is safe to use, you can add that site to your list of trusted sites. This will allow the site to work correctly even with the high security setting.

  Alternatively, you can change you settings to prompt before running ActiveX controls only. To do this, follow these steps:

  1. In Internet Explorer, click Internet Options on the Tools menu.
  2. Click the Security tab.
  3. Click Internet, and then click Custom Level.
  4. Under Settings, in the ActiveX controls and plug-ins section, under Run ActiveX controls and plug-ins, click Prompt.
  5. In the Scripting section, under Active Scripting, click Prompt, and then click OK.
  6. Click Local intranet, and then click Custom Level.
  7. Under Settings, in the ActiveX controls and plug-ins section, under Run ActiveX controls and plug-ins, click Prompt.
  8. In the Scripting section, under Active Scripting, click Prompt.
  9. Click OK two times to return to Internet Explorer.

  Impact of Workaround: There are side effects to prompting before running ActiveX controls. Many Web sites that are on the Internet or on an intranet use ActiveX to provide additional functionality. For example, an online e-commerce site or banking site may use ActiveX controls to provide menus, ordering forms, or even account statements. Prompting before running ActiveX controls is a global setting that affects all Internet and intranet sites. You will be prompted frequently when you enable this workaround. For each prompt, if you feel you trust the site that you are visiting, click Yes to run ActiveX controls. If you do not want to be prompted for all these sites, use the "Restrict Web sites to only your trusted Web sites" workaround.

• Restrict Web sites to only your trusted Web sites.

  After you set Internet Explorer to require a prompt before it runs ActiveX controls and Active Scripting in the Internet zone and in the Local intranet zone, you can add sites that you trust to Internet Explorer's Trusted sites zone. This will allow you to continue to use trusted Web sites exactly as you do today, while helping to protect you from this attack on untrusted sites. We recommend that you add only sites that you trust to the Trusted sites zone.

  To do this, follow these steps:

  1. In Internet Explorer, click Tools, click Internet Options, and then click the Security tab.
  2. In the Select a Web content zone to specify its current security settings box, click Trusted Sites, and then click Sites.
  3. If you want to add sites that do not require an encrypted channel, click to clear the Require server verification (https:) for all sites in this zone check box.
  4. In the Add this Web site to the zone box, type the URL of a site that you trust, and then click Add.
  5. Repeat these steps for each site that you want to add to the zone.
  6. Click OK two times to accept the changes and return to Internet Explorer.

  Add any sites that you trust not to take malicious action on your computer. One in particular that you may want to add is "*.windowsupdate.microsoft.com" (without the quotation marks). This is the site that will host the update, and it requires an ActiveX control to install the update.

• Install the update that is described in Microsoft Security Bulletin MS04-018 if you are using Outlook Express 5.5 SP2.

  Outlook Express 5.5 Service Pack 2 opens HTML e-mail messages in the Restricted sites zone if Microsoft Security Bulletin MS04-018 has been installed.

  Customers who use any of these products could be at a reduced risk from an e-mail-borne attack that tries to exploit this vulnerability unless the user clicks a malicious link in the e-mail message.

• Read e-mail messages in plain text format if you are using Outlook 2002 or a later version, or Outlook Express 6 SP1 or a later version, to help protect yourself from the HTML e-mail attack vector.

  Outlook 2002 users who have applied Office XP Service Pack 1 or a later version and Outlook Express 6 users who have applied Internet Explorer 6 Service Pack 1 can enable this setting and view e-mail messages that are not digitally signed or e-mail messages that are not encrypted in plain text only.

  Digitally signed e-mail messages or encrypted e-mail messages are not affected by the setting and may be read in their original formats. For more information about enabling this setting in Outlook 2002, see Microsoft Knowledge Base Article 307594.

  For information about this setting in Outlook Express 6, see Microsoft Knowledge Base Article 291387.

  Impact of Workaround: E-mail messages that are viewed in plain text format will not contain pictures, specialized fonts, animations, or other rich content. Additionally:

  • The changes are applied to the preview pane and to open messages.
  • Pictures become attachments so that they are not lost.
  • Because the message is still in Rich Text or HTML format in the store, the object model (custom code solutions) may behave unexpectedly.

What is the scope of the vulnerability?
This is a cross-domain vulnerability that could allow information disclosure or remote code execution. If a user is logged on with administrative privileges, an attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full privileges. Users whose accounts are configured to have fewer privileges on the system could be less impacted than users who operate with administrative privileges.

What causes the vulnerability?
The process by which certain URLs that are found in Channel Definition Format (CDF) files are validated by the Internet Explorer cross-domain security model.

What are Channel Definition Format files?
Channel Definition Format (CDF) files can be used to organize a set of related Web pages into a logical hierarchy. A channel is a Web site described by a Channel Definition Format (CDF) file. The CDF file defines a hierarchy of the pages that are included in the channel. Besides defining the resources in the channel, the CDF file also specifies how each item will be used or displayed, and when the channel should be updated. For more information about CDF files, see the product documentation.

What are Internet Explorer security zones?
Internet Explorer security zones are part of a system that divides online content into categories or zones, based on the trustworthiness of the content. Specific Web domains can be assigned to a zone, depending on how much trust is put in the content of each domain. The zone then restricts the capabilities of the Web content, based on the zone's policy. By default, most Internet domains are treated as part of the Internet zone. By default, the policy of the Internet zone prevents scripts and other active code from accessing resources on the local system.

What might an attacker use the vulnerability to do?
An attacker who successfully exploited this vulnerability could run malicious script code in the Local Machine security zone in Internet Explorer. This could allow an attacker to take complete control of the affected system.

How could an attacker exploit the vulnerability?
An attacker could exploit this vulnerability by creating a malicious Web page or an HTML e-mail message and then persuading the user to visit this page or to view the HTML e-mail message. When the user visited the page or viewed the e-mail message and interacted with the Web page or HTML email message, the attacker could access information from other Web sites, local files on the system, or cause script to run in the security context of the Local Machine zone.

What systems are primarily at risk from the vulnerability?
This vulnerability requires that a user view a Web site for malicious action to occur and accept a prompt for adding an Internet Explorer favorite to their system. Therefore, any systems where Internet Explorer is used frequently, such as workstations or terminal servers, are at the most risk from this vulnerability. Systems that are not typically used to visit Web sites, such as most server systems, are at a reduced risk.

I am running Internet Explorer on Windows Server 2003. Does this mitigate this vulnerabilitiy?
Yes. By default, Internet Explorer on Windows Server 2003 runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability.

What is Internet Explorer Enhanced Security Configuration?
Internet Explorer Enhanced Security Configuration is a group of preconfigured Internet Explorer settings that reduce the likelihood of a user or of an administrator downloading and running malicious Web content on a server. Internet Explorer Enhanced Security Configuration reduces this risk by modifying many security-related settings. This includes the settings on the Security tab and the Advanced tab in the Internet Options dialog box. Some of the important modifications include the following:

• Security level for the Internet zone is set to High. This setting disables scripts, ActiveX controls, Microsoft Java Virtual Machine (MSJVM), HTML content, and file downloads.
• Automatic detection of intranet sites is disabled. This setting assigns all intranet Web sites and all Universal Naming Convention (UNC) paths that are not explicitly listed in the Local intranet zone to the Internet zone.
• Install On Demand and non-Microsoft browser extensions are disabled. This setting prevents Web pages from automatically installing components and prevents non-Microsoft extensions from running.
• Multimedia content is disabled. This setting prevents music, animations, and video clips from running.

I am running Internet Explorer on Windows XP Service Pack 2. Does this mitigate this vulnerability?
Yes. Windows XP Service Pack 2 introduced a security enhancement known as the Local Machine zone lockdown that mitigates this vulnerability.

What is the Local Machine zone lockdown?
In Windows XP Service Pack 2, all local files and content that are processed by Internet Explorer has additional security applied to it in the Local Machine zone. This feature restricts HTML in the Local Machine zone. This feature also restricts HTML that is hosted in Internet Explorer. These restrictions help mitigate attacks where the Local Machine zone is used as an attack vector to load malicious HTML code.

Because of this change, ActiveX script in local HTML pages that are viewed inside Internet Explorer will not run. Also, script in local HTML pages that is viewed inside Internet Explorer prompts the user for permission to run.

Are Windows 98, Windows 98 Second Edition or Windows Millennium Edition critically affected by this vulnerability?
No. Although Windows 98, Windows 98 Second Edition, and Windows Millennium Edition do contain the affected component, the vulnerability is not critical. For more information about severity ratings, visit the following Web site.

What does the update do?
The update removes the vulnerability by making sure that the Internet Explorer cross-domain security model is enforced when Internet Explorer uses Channel Definition Format (CDF) files.

When this security bulletin was issued, had this vulnerability been publicly disclosed?
Yes. This vulnerability has been publicly disclosed. It has been assigned Common Vulnerability and Exposure number CAN-2005-0056.

When this security bulletin was issued, had Microsoft received any reports that this vulnerability was being exploited?
No. Microsoft had seen examples of proof of concept code published publicly but had not received any information to indicate that this vulnerability had been publicly used to attack customers when this security bulletin was originally issued.