Because of a flaw in Microsoft Corp.’s Word word processor program, documents containing macros can be modified to bypass Word’s security features and make possibly devastating changes to a user’s computer, the company said Friday. A macro is a small script that can be used to automate tasks, such as text formatting on a document.

The vulnerability affects Word 97, 2000, 2002, the Japanese version of Word 98 and Word for Macintosh 98 and 2001. Someone could exploit the vulnerability by performing what Microsoft calls “low-level editing” on a Word document so as to disguise the malicious macros and prevent Word’s macro checker from detecting them, the company said.

Normally, upon opening a document containing macros, Word notifies users of the macros and gives users the option to run or disable them, but in this case the user wouldn’t know that a malicious macro was present or that it had run. Such a macro could take any action that a user could, including changing or deleting files, contacting a Web site, disabling security settings or even reformatting a hard drive, the company said.

Users could access an affected Word document from a floppy disk, a Web page or an e-mail.

Microsoft said the bug only affects Word, but not other Office components, and that users employing the Outlook Express security update, which is included with Word 2002, would be protected from e-mail worms, and thus from a Word document with a malicious macro.

Microsoft has posted a patch, along with a security bulletin, for this vulnerability at

Steven McLeod discovered the flaw.

Microsoft, based in Redmond, Wash., can be reached at