One day after releasing a trio of security patches, Microsoft is upgrading the seriousness of one of those fixes to "critical".

The software update attached to security bulletin number MS04-009 has been rated "critical" by the software giant, after initially being described as an "important" patch on Tuesday. The change follows "continued evaluation" by Microsoft's Security Response Center, a company spokesman said yesterday.

Microsoft defines "critical" bulletins as those concerning software vulnerabilities that, if exploited, "could allow the propagation of an Internet worm without user action". "Important" bulletins concern vulnerabilities that, if exploited, "could result in compromise of the confidentiality, integrity, or availability of users’ data, or of the integrity or availability of processing resources," according to information on the company's website.

The change in severity for MS04-009 came after Microsoft learned of a "new attack scenario discovered after the bulletin’s original release on 9 March," the spokesman said in the e-mail.

Released on Tuesday, MS04-009 fixes a problem with the way the Outlook e-mail software treats URLs (uniform resource locators) that use the "mailto" tag, which allows Web page authors to insert links on Web pages that launch Outlook or other e-mail clients.

A problem with the way Outlook interprets mailto URLs could allow an attacker to use a specially formatted mailto URL to gain access to files on an affected system or insert and run malicious computer code and is rated "important", Microsoft said.

Microsoft initially claimed that only computers with the "Outlook Today" home page were vulnerable to attack. "Outlook Today" is only the home page until an e-mail account is created, Microsoft said.

However, following release of the bulletin, Finnish security researcher Jouko Pynnönen, who discovered the vulnerability, informed the company that malicious hackers could still attack vulnerable Outlook installations even if "Outlook Today" is not the default home page, the spokesman said.

In a revised version of its security bulletin, Microsoft noted the discrepancy. "This vulnerability could also affect users who do not have the 'Outlook Today' folder home page as their default home page in Outlook 2002," the company said.

The change in status doesn't affect the software patch. Microsoft customers who have already installed the security update do not need to take any additional action, Microsoft said.