Microsoft kindly informed its customers about the latest security risks associated with the Open XML file format: The Trojan.Mdropper.AC. Microsoft is investigating public reports of a vulnerability in Excel that could allow remote code execution if a user opens a specially crafted Excel file. At this time, they are aware only of limited and targeted attacks that attempt to use this vulnerability.
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 affected than users who operate with administrative user rights.
In a Web-based attack scenario, an attacker would have to host a Web site that contains an Office file that is used to attempt to exploit this vulnerability. In addition, compromised Web sites and Web sites that accept or host user-provided content could contain specially crafted content that could exploit this vulnerability. An attacker would have no way to force users to visit a malicious 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.
The vulnerability cannot be exploited automatically through e-mail. For an attack to be successful a user must open an attachment that is sent in an e-mail message. Users who have installed and are using the Office Document Open Confirmation Tool for Office 2000 will be prompted with Open, Save, or Cancel before opening a document.
So users probably should be very cautious with .xlsx files sent to them until the risks are contained through security updates.