The Sans Internet Storm Center on Friday raised its threat level to yellow after observing an increase in the number of Internet-based attempts to exploit a zero-day vulnerability discovered last week.

Dozens of Web sites are exploiting the vector markup language and the number is growing, said Ed Skoudis, a Sans incident handler. The vulnerability, discovered Monday by Sunbelt Software, is also being exploited by an unknown volume of e-mail, he said.

The VML exploit, which allows the remote execution of code, is worrisome to security experts because the only action required for infection is viewing a malicious Web site using Internet Explorer or viewing a specially formatted HTML e-mail with Outlook. So far, there are no reports of trusted Web sites trying to exploit the vulnerability.

"We're anticipating that somebody is going to pop a trusted site and then we'll have a big problem," said Skoudis. "We went to yellow so people will do work-arounds."

Microsoft said earlier this week it was aware of the problem and planned to issue a patch next month during its regularly scheduled update, "or sooner if necessary."

Among the work-arounds is a patch being made available by the Zeroday Emergency Response Team (ZERT). The group also offers a Web page to test whether a machine has successfully been patched. Other temporary remedies include unregistering vgx.dll, the dll file that is targeted in attacks, or simply not using Internet Explorer and Outlook.

Other groups also warned that the VML vulnerability was becoming a bigger target. Websense Security Labs said it has monitored attacks that include Trojan backdoors and code that is designed to steal information from end-user machines. Several Web sites are publishing example code, making it easier for others to pull off an attack, Websense said on its Web site.