That Trojan, which works on PCs running the Vista and XP operating systems, is capable of downloading other malware to the computer.

Like another 0-day bug in Adobe's Reader and Acrobat software, this flaw is being exploited in dozens of targeted attacks, where victims are sent specially crafted messages tailored to make them open the malicious document, according to Vincent Weafer, vice president of Symantec Security Reponse. "We're seeing a lot of them come around different Asian government or industries or defense contractors," he said.

Hackers have increasingly sought to find vulnerabilities in applications as Microsoft has spent much effort into making its Vista OS more secure.