Microsoft has upgraded the seriousness of one of its March fixes to "critical".
The upgrade is for a patch that 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.
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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 initially claimed that only computers with the "Outlook Today" home page were vulnerable to attack. That limited the scope of the "Outlook Today" is only the home page until an e-mail account is created.
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.
Microsoft noted the discrepancy, in a revised version of its security bulletin.
"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 does not affect the software patch. Microsoft customers who have already installed the security update do not need to take any additional action.
Paul Roberts writes for IDG News Service