Mac OS X version 10.6 - Snow Leopard - is the seventh major release of Apple Mac operating system. Apple CEO Steve Jobs first announced Snow Leopard in June of last year.

Cluley argued that whilst the Flash video player is not causing any direct harm to users' Macs, there is a risk that Flash content on the web generally can be problematical.

He said that, whilst Adobe has been urging users to keep themselves up-to-date, Snow Leopard downgrades Flash users in the background. And most people, even if they had been diligent in the past, he explained, would not think to check for that sort of thing.

Cluley advised Mac users who have upgraded to Snow Leopard to double-check that their version of Flash is current and - if not - they should update.

"This should be done as a matter of priority", he said.

"Mac users who have been diligent enough to keep their security up-to-date do not deserve to be silently downgraded", he added.

Cluley went on to say that, in many ways, Adobe is "the new Microsoft" when it comes to security vulnerabilities, with hackers targeting its code looking for ways to infect users.

"That's deeply concerning because it is so widely used by many internet users, whether on Mac or PC", he said.

"Adobe has acknowledged that previous versions of Flash should not be used for security reasons, but Apple is switching users from the version that is considered current to this old one."

"It's vital, therefore, that users ensure they are running the latest version - and that, in the future, operating system manufacturers do not reduce their customers' level of security without warning."