Microsoft Acknowledges Security Flaw in Web-based Exchange

Microsoft Corp. released a security patch Wednesday for a vulnerability affecting users who access their Exchange e-mail accounts over the Web.

In Exchange Server, Microsoft allows administrators to configure the server so that users can access their mailboxes via the Web. The option is called Outlook Web Access, and allows users to open a Web page that looks similar to an Outlook e-mail client.

But because of the way Outlook Web Access (OWA) and Internet Explorer interact, an attacker could take control of a user's mailbox -- possibly manipulating messages and folders.

Microsoft's bulletin explains how the vulnerability arises: "If an attachment contains HTML code including script, the script will be executed when the attachment is opened, regardless of the attachment type. Because OWA requires that scripting be enabled in the zone where the OWA server is located, this script could take action against the user's Exchange mailbox."

Microsoft says an attacker could use the flaw to make an attachment that contains malicious script code and send the attachment in a message to the user.

Mitigating factors cited by Microsoft are that the vulnerability can only be exploited when users are accessing their Exchange mailboxes via the Web, and that attackers have no technical way of knowing whether users will be using their full client or the Web-based client.

Microsoft's full security bulletin can be found at

The security issue is Microsoft's 30th this year.

About the Author

Scott Bekker is editor in chief of Redmond Channel Partner magazine.