Microsoft has issued the first security update for the beta version of its Windows Vista operating system due out later this year. 

The patch, rated critical, covers a similar WMF flaw that hit the company's other operating systems last week. The vulnerability is in the Graphics Rendering Engine and could allow an attacker to gain control of a target machine.

The flaw highlights the extent to which older code is being used in the new operating system, since the problems with the Graphics Rendering Engine occur in every version of Windows since 3.0 which was released 15 years ago.

Microsoft chairman Bill Gates initially suggested that Vista, then codenamed Longhorn, would be a largely new operating system.

"It's not too serious," said Greg Day, security analyst for McAfee. "Windows XP has over 45 million lines of code and it would be an enormous job to rewrite from scratch. It's a case of better the devil you know."