A vulnerability in Microsoft’s Visual Studio 2005 application development tool could let an attacker execute code on a targeted Windows machine, the company has revealed.

Danish security vendor Secunia AsP rated the vulnerability as 'extremely critical' since it hasn't been patched and there are unconfirmed reports it's being exploited, said Thomas Kristensen, chief technology officer at the firm.

Microsoft said proof-of-concept attack code has been published, and the company could eventually issue a patch after its investigation. The software giant typically issues patches on the second Tuesday of the month, although it has been known to push out a patch earlier, depending on the risk of the vulnerability.

"We are aware of the possibility of limited attacks that are attempting to use the reported vulnerability," wrote Christopher Budd, a security program manager on Microsoft's Security Center Response blog.

The vulnerability lies in an Active X control called the WMI Object Broker control, contained in the WmiScriptUtils.dll, Microsoft said. For a successful attack, a user would have to be lured to a website designed to exploit the flaw. Microsoft cautioned against following links in emails from unknown senders, since those could lead to an attacker's site.

Microsoft posted several mitigating factors that would head off attacks. For example, the particular ActiveX control is not included in the default allow list for IE7 (Internet Explorer 7.0).

Also, those running Visual Studio 2005 on Windows Server 2003 and Windows Server 2003 SP1 in their default configurations – with the Enhanced Security Configuration turned on – are not affected, Microsoft wrote.

If an attack was successful, the hacker would only have the same rights as a local user, so Microsoft said those accounts configured with fewer rights than an administrator might cause less of an impact.