Microsoft Warns of New Security Flaw

Microsoft Corp. said all versions of the Windows operating system released since 1996 have a serious security flaw that could prevent users from being able to encrypt e-mail and use some Web sites.

The world’s largest software maker called the vulnerability “critical,” its most serious rating for such flaws, in an e-mailed security bulletin.

The company encouraged users to download a software patch to correct the glitch immediately. Microsoft is working to improve the security of its software after holes allowed worms and viruses to enter Microsoft programs several times in the last two years, angering customers.

The flaw could allow a hacker to delete so-called digital certificates from a user’s system.

Shares of Redmond, Wash.-based Microsoft rose $1.20 to $50.58 on Nasdaq.