The attack was used to hack into networks at 33 companies, including Adobe, security experts say. Typically such hacks involve several such attacks, but the IE bug is the only one definitively linked to the hacking incident, which security experts say originated in China.
In a security advisory released on Thursday, Microsoft said IE 6 users on Windows XP are most at risk from the flaw, but that other users could be affected by modified versions of the attack.
Microsoft said it is developing a fix, but it did not say when it expects to patch the issue. The company is slated to release its next set of security updates on February 9.
A Google spokesman confirmed that the Internet Explorer attack was used against Google and that the company then reported the issue to Microsoft.
Google learned of the issue in December and, after discovering the server used to control the hacked computers, notified other companies affected by the hack. Apparently convinced that the infiltration was sanctioned by the Chinese government, Google has threatened to effectively pull its business out of China.
McAfee released a description of the attack Thursday, saying that the people hit with the attack were probably "targeted because they likely had access to valuable intellectual property".
Microsoft condemned the attack, but said it had no indication that its corporate network or mail products were hit.
Although the IE attack has only been seen in "targeted and limited" incidents, Microsoft may release an emergency patch for the product, the company said.
The malicious code that hit Google "attacks IE6 on XP exclusively", but is thwarted by the Data Execution Prevention (DEP) technology used by Windows XP, said Dan Kaminsky, director of penetration testing with IOActive. However, he added, the bug could be leveraged in attacks that affected more recent versions of IE, running on Windows XP, he added. Vista and Windows 7 use a more advanced protection technology called ASLR (address space layout randomization) that makes exploiting this bug extremely difficult.