A bug in Trend Micro's free online virus scanning service can be used by hackers to hijack Windows PCs running Internet Explorer, security researchers warned.
Attackers able to dupe users into visiting a malicious Web page could exploit a vulnerability in the custom ActiveX control that Trend Micro distributes to users of its free HouseCall service, said Danish bug tracker Secunia ASP in an alert posted Sunday.
HouseCall bills itself as a free scanning tool that checks "whether your computer has been infected by viruses, spyware, or other malware."
"The vulnerability is caused due to a use-after-free error in the HouseCall ActiveX control (Housecall_ActiveX.dll)," said Secunia's warning. "This can be exploited to dereference previously freed memory by tricking the user into opening a Web page containing a specially crafted 'notifyOnLoadNative' callback function."
Trend Micro has fixed the flaw in the ActiveX control and patched the public HouseCall servers, but it noted that the latter has not been extensively tested, and essentially waived responsibility if it turns out not to be sufficient.
"This hot fix was developed as a workaround or solution to a customer-reported problem. As such, this hot fix has received limited testing and has not been certified as an official product update," Trend Micro said in its own advisory, published last Thursday. "Consequently, this hot fix is provided 'as is'. Trend Micro makes no warranty or promise about the operation or performance of this hot fix nor does it warrant that this hot fix is error free."
Users running Microsoft's Internet Explorer -- the only browser that requires the ActiveX control -- should run Version 6.6 of the service, rather than the older HouseCall 6.5, said Secunia.
Companies running HouseCall Server in-house should request the HouseCall 6.6 Hot Fix Build 1285 update through their normal support channels, Trend Micro advised.
Secunia rated the vulnerability as "highly critical," the second-highest ranking in its five-step scoring system.