In a blog post on Wednesday, Mikko Hypponen, chief research officer at Finnish security company F-Secure, said he believes "the attack was launched via a convincing e-mail with an exploit-ridden PDF attachment."
The iDefense report says that the December attack on Google and other companies has similarities to a July attack on about 100 companies in the IT sector. That attack used a malicious PDF file to exploit a zero-day vulnerability in Adobe Reader.
"According to sources familiar with the present attack, attackers delivered malicious code used against Google and others using PDFs as e-mail attachments; those same sources also claim that the files have similar characteristics to those distributed during the July attacks," the report states. "In both attacks, the malicious files drop a backdoor Trojan in the form of a Windows DLL."
The report suggests the July and December attacks may be the same attack, meaning that affected companies may have been compromised for months.
In reference to the December attack, an Adobe spokesperson said, "At this time, we have no evidence to suggest that a vulnerability in Adobe Reader was an attack vector in this incident."
Adobe on Tuesday happened to issue a security patch for a vulnerability in its Acrobat and Reader software that had been actively exploited for at least a month.
Alan Paller, director of research at the SANS Institute, said it is likely that the attack was a highly targeted spear-phishing attack that exploited a zero-day vulnerability in the software used by employees at targeted companies.
"It doesn't really matter if the attack targeted Word or Adobe or a video player," said Chris Wysopal, CTO of Veracode. "What's clear is that the set of software on the user's system really isn't being scrutinized and managed the way a machine connected to the Internet, like a Web server or gateway, is."
That's something that needs to change, Wysopal insists.