RIM, maker of the popular BlackBerry smartphone, has warned users of a critical bug that can expose Windows users to attack.

Microsoft has also stepped in to set "kill bits" for the flawed Research in Motion (RIM) software in an update it issued alongside patches for its own software.

A flaw in the Application Web Loader, an ActiveX control for Internet Explorer (IE), could be used by attackers to hijack Windows computers, said RIM's security advisory. The ActiveX control is installed when users download a BlackBerry application from a third-party developer's website.

The US Computer Emergency Response Team (US-CERT) issued an advisory of its own, noting that attackers who convinced users to view a malicious HTML page "may be able to execute arbitrary code with the privileges of the user." In other words, hackers could inject their own malware into a machine, putting it under their control or plundering it for information.

Danish vulnerability tracker Secunia, meanwhile, rated the BlackBerry control as a "highly critical" threat.

RIM has updated the ActiveX control to Version 1.1, and made it available to third-party developers to swap in for the vulnerable application loader. BlackBerry users, however, may find it simpler to disable the older ActiveX control by grabbing the kill bit update from Microsoft.

By setting the kill bit, Microsoft essentially automates the manual process of editing the Windows registry, which in turn blocks IE from running the buggy control.

Microsoft regularly disables third-party ActiveX controls when their owners ask for help. The last time it did so was in October 2008, when it set kill bits for several controls. Tuesday was the third such kill bit update since August.

Also included in Tuesday's update was a kill bit for an ActiveX control used by Akamai, the content mirroring company that runs servers to host client's Internet applications, services and sites. According to Akamai's security warning from April 2008, its download manager contains a vulnerability that can be exploited by attackers to hijack a Windows PC.