Outlook Web Access (OWA) is a service of Exchange 5.5 Server that allows users to access and manipulate messages in their Exchange mailbox by using a web browser. A flaw exists in the way OWA handles inline script in messages in conjunction with Internet Explorer (IE). If an HTML message that contains specially formatted script is opened in OWA, the script executes when the message is opened. Because OWA requires that scripting be enabled in the zone where the OWA server is located, a vulnerability results because this script could take any action against the user's Exchange mailbox that the user himself was capable of, including sending, moving, or deleting messages. An attacker could maliciously exploit this flaw by sending a specially crafted message to the user. If the user opened the message in OWA, the script would then execute. While it is possible for a script to send a message as the user, it is impossible for the script to send a message to addresses in the user's address book. Thus, the flaw cannot be exploited for mass-mailing attacks. Also, mounting a successful attack requires knowledge of the intended victim's choice of mail clients and reading habits. If the maliciously crafted message were read in any mail client other than a browser through OWA, the attack would fail.

Download locations for this patch:
Microsoft Exchange 5.5: http://www.microsoft.com/Downloads/Release.asp?ReleaseID=34402