XWEB: OWA Function Allows Unauthenticated User to Enumerate Global Address List

An information disclosure vulnerability exists in Exchange Server 5.5 that an Internet-based attacker can use to learn the e-mail addresses of users on a corporate Exchange Server 5.5 computer if the server is configured to offer Microsoft Outlook Web Access (OWA).

The vulnerability does not allow the attacker to read, write, or change any of the users' e-mail, or to take any other action against the users. The vulnerability also does not allow the attacker to gain any privileges on the server. The vulnerability's sole effect is to allow the attacker to learn the e-mail names of users on the server. This vulnerability does not affect Microsoft Exchange 2000 Server.

This problem can occur because a function in OWA that queries the Global Address List does not require authentication. Unauthenticated users can call the function and enumerate the mail addresses of users on the server.

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 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 file is available for download from the Microsoft Download Center:
Release Date: September 6, 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

File name	Version
Fumsg.asp	NA

NOTE: Because of file dependencies, this fix requires Microsoft Exchange Server version 5.5 Service Pack 4.

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

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