Microsoft investigates new Office zero-day flaw

Microsoft has confirmed Wednesday that it is looking into reports of a new Office zero-day flaw attackers could exploit to cause a denial of service or run malicious code on targeted Windows machines.

Antivirus giant Symantec released an email advisory on the flaw to customers of its DeepSight threat management system early afternoon US time on 23 May. A couple hours later, a Microsoft spokesperson confirmed the company is investigating the report. At issue is a buffer overflow flaw in Office 2000's UA ActiveX control. Because of the flaw, the application fails to properly check user-supplied data before copying it into a poorly-sized buffer, Symantec said.

"This issue occurs when an excessive amount of data is passed to the 'HelpPopup' method of the 'OUACTRL.OCX' ActiveX control," Symantec said. "Successfully exploiting this issue allows remote attackers to execute arbitrary code in the context of the application using the ActiveX control (typically Internet Explorer). Failed exploit attempts likely result in denial-of-service conditions."

Symantec said the flaw was discovered by a researcher named Shinnai, who has posted exploit code. To successfully exploit the flaw, an attacker must trick an unsuspecting user into accessing a malicious Web page.

The Microsoft spokesperson said Microsoft is not aware of any attacks attempting to use the flaw, but that it will continue to investigate the issue.

"Upon completion of this investigation, Microsoft will take the appropriate action to protect our customers, which may include issuing a security advisory or providing a security update through our monthly release process, depending on customer needs," she added.

For now, Symantec recommends users mitigate the threat by disabling ActiveX scripting in Internet Explorer, or set the kill bit on CLSID:8936033C-4A50-11D1-98A4-00A0C90F27C6.