Flaw in Word fields and Excel external updates may lead to information disclosure (330008)

SYMPTOMS

Microsoft Word and Microsoft Excel provide a mechanism through which data from one document can be inserted into, and updated in, another document. This mechanism, known as field codes in Word and external updates in Excel, can be automated for the user's ease. For example, Word field codes can be used to automatically insert a standard disclaimer paragraph in a legal document. Excel external updates can be used to automatically update a chart in one spreadsheet by using data in a different spreadsheet.

However, it is possible to maliciously use field codes and external updates to secretly steal information from a user. Certain events can trigger the external update and the field code to be updated: for example, when the user saves a document or manually updates the links. Typically, the user is aware of these updates when they occur. However, a specially crafted field code or external update can be used to trigger an update without any indication to the user. This can permit an attacker to create a document that, when opened, updates itself to include the contents of a file from the user's local computer.

For an attacker to take advantage of this vulnerability, the attacker needs to:

1. Create a Word or Excel document that exploits the vulnerability.
2. Deliver it to the user through e-mail or some other method.
3. Entice the user to open the document.

Then, the user must typically return the document to the attacker for the attack to work successfully.

However, note that Microsoft is aware of one case in which it is not even necessary for the user to do this. By the method used in this case, the attacker's document can post information directly to a Web site, although it permits only the first line of the file to be sent.