Hackers exploited an unpatched vulnerability in Microsoft's Internet Explorer (IE) browser to break into some of the firms targeted in a widespread attack that compromised Google's and Adobe's corporate networks last year and earlier this month, McAfee said today.

According to Dmitri Alperovitch, vice president of threat research at McAfee, the unpatched vulnerability in IE was the only exploit used to hack into several of the companies attacked starting last month. Other researchers have said that as many as 33 firms, including Google and Adobe, were attacked, their networks compromised and in some cases, data stolen.

Alperovitch said that Microsoft would release additional information about the IE vulnerability in a security advisory later today.

"Microsoft is investigating these reports and will provide more information when it is available," a Microsoft spokesman said in an email.

Techworld reported earlier this week that Google and Adobe, the only two companies to have stepped forward thus far to acknowledge the attacks, were hacked using malicious PDF files that exploited a zero-day vulnerability in Adobe's popular Reader software. According to Mikko Hypponen, the chief technology officer of F-Secure, the exploited flaw was the one of eight that Adobe patched Tuesday. That bug had been public knowledge since mid-December, and had been exploited in targeted attacks -- the very kind that broke into Google, Adobe and a reported 32 other major companies -- since sometime in November.

Alperovitch, however, rejected the idea that a rogue PDF was at the root of the attacks. "We have not seen, in all the organisations we've worked with on this, and there were multiple, any PDF files associated with the attack," he said.

Other sources, who asked to remain anonymous because they were not officially allowed to comment, said that Microsoft would release its advisory around 4pm ET.