XGEN: Incorrect Attachment Processing in Exchange Server 5.5 Outlook Web Access Can Run Script

Microsoft Outlook Web Access (OWA) is a service of Exchange Server 5.5 and Exchange 2000 Server that a user can use to gain access to an Exchange mailbox by using a Web browser. However, vulnerability exists in the interaction between OWA and Microsoft Internet Explorer for message attachments. If an attachment contains Hypertext Markup Language (HTML) code that includes script, the script is run when the user opens the attachment, regardless of the attachment type. Because OWA requires that scripting be enabled in the zone where the OWA server is located, this script can take action against the user's Exchange mailbox.

An attacker might use this vulnerability to construct an attachment that contains malicious script code. The attacker can then send the attachment in a message to a user. If the user opens the attachment in OWA, the script runs and can take action against the user’s mailbox as if the script is the user, including, under certain circumstances, manipulation of messages or folders.

The following are mitigating factors:

• The vulnerability can only be exploited if the user is using OWA in conjunction with Internet Explorer.
• The vulnerability can only be exploited by attachments that are received by using OWA. In general, an attacker has no way to determine whether a user will open an attachment by using OWA rather than Microsoft Outlook.
• An attacker's ability to exploit this vulnerability requires that the attacker entice the user to open an attachment from an untrusted source. The best practice is not to open any attachment from an unknown or untrusted source.

A supported fix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Apply it only to computers that you determine are at risk of attack. Evaluate your computer's physical accessibility, network and Internet connectivity, and other factors to determine the degree of risk to your computer. See the associated Microsoft Security Bulletin (http://www.microsoft.com/technet/security/bulletin/MS01-030.mspx) to help determine the degree of risk. This fix may receive additional testing. If your computer is sufficiently at risk, Microsoft recommends that you apply this fix now. Otherwise, wait for the next Microsoft Exchange Server 5.5 service pack that contains this fix.

To resolve this problem immediately, download the fix by following the instructions later in this article or contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information about support costs, visit the following Microsoft Web site:NOTE: In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

The following files are available for download from the Microsoft Download Center:
Release Date: June 8, 2001

For additional information about how to download Microsoft Support files, click the following article number to view the article in the Microsoft Knowledge Base: Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help to prevent any unauthorized changes to the file.

The English version of this fix should have the following file attributes or later:

Component: OWA


NOTES

• Because of file dependencies, this fix requires Microsoft Exchange Server version 5.5 Service Pack 4.
• You need to apply this fix to all servers that run the OWA components. With Exchange Server 5.5, you can install the OWA components on a computer that is separate from the Exchange Server. If you install OWA separately, you need to run this patch on the separate computer.
• Sometimes, after applying this fix, users may get a blank window when they open their messages using OWA. If this occurs, please review the following Knowledge Base article
  314532  (http://support.microsoft.com/kb/314532/EN-US/ ) XWEB: Troubleshooting Blank Message Bodies in Outlook Web

Microsoft has confirmed that this problem may cause a degree of security vulnerability in Microsoft Exchange Server version 5.5.

For more information about this security vulnerability, see the following Microsoft Web site: