You would have already heard about the Microsoft Word 2003 Zero-Day vulnerability, which is rated as critical by antivirus vendors. What does ‘Zero-Day’ mean in this context?
Zero-Day usually means information that is not publicly available. Security vulnerability exploits which are unknown to computer security professionals are Zero-Day Vulnerabilities. Zero-day attack is completely unknown to the general public and it is often difficult to defend against. It can remain difficult to detect even after they are launched. The term zero-day exploits is sometimes used to indicate publicly known exploits for which no patches yet exist.
Microsoft is still working on the recent Word 2003 Zero-Day vulnerability (919637) and a patch is scheduled to be released on 13 June, 2006.