Updated Hackers who breached the defenses of Google, Adobe Systems and at least 32 other companies used a potent vulnerability in all versions of Internet Explorer to carry out at least some of the attacks, researchers from McAfee said Thursday.

The previously unknown flaw in the IE browser was probably just one of the vectors used in the attacks, McAfee CTO George Kurtz wrote in a blog post. Using a sophisticated spear-phishing campaign, the perpetrators included malicious links exploiting the bug in emails and instant messages sent to employees from at least three of the targeted companies.

Contrary to previous speculation, there was no evidence vulnerabilities in Adobe's Reader or Acrobat applications were used in any of the attacks, Kurtz said. In its own statement, adobe concurred, saying researchers "have not been able to obtain any evidence to indicate that Adobe Reader or other Adobe technologies were used as the attack vector in this incident."

Kurtz said his findings were based on malware samples taken from "three to five" of the targeted companies and he stressed that other zero days or exploits could have been used against other victims.

"In our investigation we discovered that one of the malware samples involved in this broad attack exploits a new, not publicly known vulnerability in Microsoft Internet Explorer," Kurtz wrote. "Our investigation has shown that Internet explorer is vulnerable on all of Microsoft's most recent operating system releases, including Windows 7."

Shortly after the report, Microsoft confirmed the new IE vulnerability was "one of the vectors used in targeted and sophisticated attacks against Google and possibly other corporate networks." A company statement said the attacks were carried out against version 6 of the widely used browser and suggested users protect themselves by enabling security features that have been added to successor versions.

McAfee's report is the latest to shed light on one of the most significant cyberattacks in years. Google first disclosed the "highly sophisticated and targeted attack" on Tuesday, saying it originated in China and targeted its intellectual property. It added that 20 other companies suffered similar assaults, a number that independent researchers soon raised to 34. So far, only Google and Adobe have been identified as victims.

Yahoo, Symantec, Northrop Grumman and Dow Chemical have also been penetrated according to The Washington Post, citing unnamed "congressional and industry sources."

The malware that McAfee researchers analyzed was sent to a highly select group of employees of a handful of companies that Kurtz declined to identify.

"This wasn't something that got blasted to 300,000 people in a corporation," Kurtz said in an interview with The Register. "It was really targeted at senior technology leaders that had access to core pieces of intellectual property, source code, et cetera."

Kurtz has dubbed the attack "Aurora," a reference to the filepath on the attacker's machine that showed up in some of the malware code McAfee researchers analyzed. They believe that is the name the attackers gave to the operation. There was nothing in the binaries that indicated either way whether the code writers spoke Cantonese or Mandarin or were located in China.

The IE vulnerability stems from an invalid pointer reference that when exploited allows an attacker to execute malicious shell code on underlying machines. The malware caused exploited machines to download further malicious scripts that installed a backdoor. The machines then connected to command and control channels that were hosted on servers that resided in the US and Taiwan.

A security feature known as data execution prevention, which prevents data loaded into memory from being executed, will block the particular exploits McAfee has observed. But Kurtz warned the vulnerability exists in all versions of IE except for IE 5.01, service pack 4, and that it would be possible for attackers to work around the protection.

In an advisory, Microsoft recommended people use DEP, which by default is enabled in IE 8 but must be turned on in prior versions. The statement also advised users on Vista and later versions of Windows to run IE in protected mode. The advisory didn't say when an update would be released that patches the vulnerability. ®

This article was updated throughout to add additional details from Kurtz, Microsoft and Adobe.