MICROSOFT has issued a "critical" security alert that affects 900 million people using its Internet Explorer web browser.
The computer giant warned of a newly-discovered flaw in Windows that could be exploited by hackers to steal personal details or take over computers.
The glitch is so severe it potentially affects every user of Internet Explorer.
Firefox, Google Chrome and Safari browsers are all unaffected by the threat because, unlike Internet Explorer, they don't support MHTML files, where the problem lies.
The loophole only seems to affect the way Internet Explorer handles some web pages.
Microsoft just said that the bug is inside Windows, presumably because they don't want users to migrate to other browsers.
This means it affects all versions of the operating system currently supported including Windows XP (SP3), Windows Vista, Windows 7, Windows Server 2003 and Windows Server 2008 (R2).