Software Engineering Institute

Microsoft Word contains a memory corruption vulnerability that could be triggered when Word opens a document containing a malformed Word Document stream. Although this vulnerability was initially reported to only cause a denial-of-service, it is now believed that it can be used to execute arbitrary code.

Note that this vulnerability is actively being exploited.

More information is available in Microsoft Security Bulletin MS07-024.

By convincing a user to open a specially crafted Word document, a remote, unauthenticated attacker could execute arbitrary code with the privileges of the user running Word. If the user is logged in with administrative privileges, the attacker could take complete control of a vulnerable system.

Apply an update
This vulnerability is addressed by the updates included with Microsoft Security Bulletin MS07-024.

Do not open untrusted Word documents

Do not open unfamiliar or unexpected Word or other Office documents, particularly those hosted on web sites or delivered as email attachments. Please see Cyber Security Tip ST04-010.

Do not rely on file name extension filtering

In most cases, Windows will call Word to open a document even if the document has an unknown file extension. For example, if document.qwer contains the correct file header information, Windows will open document.qwer with Word. Filtering for common extensions (e.g., .doc, and .dot) will not detect all Word documents.

Disable automatic opening of Microsoft Office documents

By default, Microsoft Office 97 and Microsoft Office 2000 will configure Internet Explorer to automatically open Microsoft Office documents. This feature can be disabled by using the Office Document Open Confirmation Tool. Mozilla Firefox users should disable automatic opening of files, as specified in the Securing Your Web Browser document.

Open untrusted Word documents using the Microsoft Office Isolated Conversion Environment (MOICE)

Office 2003 and 2007 users may wish to consider MOICE as a tool to securely preview untrusted Office documents. According to Microsoft:

MOICE uses the 2007 Microsoft Office system converters to convert the Office binary format files into the Office Open XML format. This process helps remove the potential threat that may exist if the document is opened in the binary format. Additionally, MOICE converts incoming files in an isolated environment. This helps protect the computer from a potential threat.
Instructions on how to install and configure MOICE are available in Microsoft KnowledgeBase article 935865 and Microsoft Security Advisory (937696).