The US government and several security vendors warned this week of newly published proof-of-concept code that exploits a serious, unpatched security flaw in Microsoft Internet Explorer.

The warnings come on the heels of a week of security traumas for Microsoft, which on Tuesday published an emergency patch for an IE bug that was being exploited by thousands of websites. Microsoft then acknowledged that an unpatched bug in PowerPoint was being exploited by Internet scammers.

Also this week, the company reissued a third of the patches it originally published in August.

The proof of concept code was published by H.D. Moore, a well-known researcher who co-founded the Metasploit Project. Moore originally publicised the bug involved in July as part of his "Month of Browser Bugs" project, which detailed flaws in IE and other browsers. The bug remains unpatched, according to security companies.

The code targets a flaw in the WebViewFolderIcon ActiveX control, Moore said. The bug works on a fully patched Windows XP SP2 system, and allows attackers to execute malicious code, according to security firm Secunia.

US-CERT, a US government body that monitors IT security issues, issued an alert and recommended disabling ActiveX. Secunia on Thursday gave the flaw its most serious rating, "extremely critical," and recommended allowing only trusted sites to run ActiveX controls.