Microsoft Corp upgraded a recent security warning to "critical" after discovering new ways in which an attacker could run malicious software on a vulnerable computer, the world's largest software maker said on Wednesday.
The software flaw, which affects the two latest versions of Microsoft's Outlook e-mail, calendar and contacts programme, were initially rated as "important" in Microsoft's monthly security bulletin issued on Tuesday.
Redmond, Washington-based Microsoft said that it reissued its latest security bulletin with a "critical" rating, which means that software should download and install a patch to correct the problem as quickly as possible.
"This change is based on information concerning a new attack scenario discovered after the bulletin's original release on March 9th," Microsoft said in a statement.
The number of critical security bulletins issued by Microsoft this year so far is a fraction of the number issued last year, as the company intensifies efforts to boost the security and reliability of its products.