Users of all current versions of Microsft Corp’s Internet Explorer browser might be vulnerable to having their computers hacked because of a serious security hole in the software that had yet to be fixed.

The Flaw lets criminals commandeer victims’ machines merely by tricking them into visiting websites tainted with malicious programme code. As many as 10,000 sites have been compromised since last week to exploit the browser flaw, according to AntiVirus Software maker Trend Micro Inc.

The sites are mostly Chinese and have been serving up programmes that steal passwords for computer games, which can be sold for moneyon the black market. However, the hole is such that it could be “adopted by more financially motivated criminals for more serious mayhem-that’s the big fear right now”, Paul Ferguson , a Trend Micro security researcher , said.

“Zero-day” vulnerabilities like this are security holes that haven’t been repaired by the software makers. They’re a gold mine for criminals because users have few ways to fight off attacks. The latest vulnerability is noteworthy because Internet Explorer is the default browser for most of the world’s computers. Also, while Microsft says it has detected attacks only against version 7 of Internet explorer, which is the most widely used edition , the company warned that other versions are also potentially vulnerable.