DENVER -- Users of all current versions of Microsoft Corp.'s Internet Explorer browser might be vulnerable to having their computers hijacked because of a serious security hole in the software that had yet to be fixed Tuesday.

The flaw lets criminals commandeer victims' machines merely by tricking them into visiting Web sites tainted with malicious programming 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 programs that steal passwords for computer games, which can be sold for money on the black market. However, the hole is such that it could be "adopted by more financially motivated criminals for more serious mayhem -- that's a big fear right now," Paul Ferguson, a Trend Micro security researcher, said Monday.

"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 Microsoft 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.

Microsoft said it is investigating the flaw and is considering fixing it through an emergency software patch outside of its normal monthly updates, but declined further comment. The company is telling users to employ a series of complicated workarounds to minimize the threat.

The company is recommending setting the Internet Zone Security setting to "high" and disabling Ole32db.dll in access control lists (ACL) to offer the best defense against attack.

In the Microsoft Malware Protection Center blog, the company wrote:

"The exploit sites we've seen so far drop a wide variety of malware — most commonly password stealers like new variants of game password stealers like Win32/OnLineGames, and Win32/Lolyda; keyloggers like Win32/Lmir; Trojan horse applications like Win32/Helpud along with some previously unseen malware which we generically detect as Win32/SystemHijack."

"We fully expect the variety of malware being dropped by this exploit to broaden as the exploit code starts to circulate around the internet underground."

Many security experts, meanwhile, are urging Internet Explorer users to use another browser, such as Firefox, until a patch for IE is released.