As cyberthreats go, none is more pronounced than the menace of new malware being written by amateur hackers and criminals to find vulnerabilities in Apple products, industry reports showed Wednesday.

Apple computers have been famously safe for years from viral attacks and malware infestation that often afflicts computers with Microsoft's Windows-based operating systems.

Industry analysts said this was both because of the way Macs operated and because of relatively smaller numbers involved in the global distribution of the platform. Computers using Windows operating system dominate global market.

Hackers and malware writers are drawn to the possibility of maximum impact for their mischief making and criminal activities and for many years have concentrated on Microsoft Windows, analysts said.

But as the numbers of Apple users grow worldwide, malware writers have begun focusing on writing code that can deliver maximum harm to Macs. The machines are increasingly used outside the art and design communities and in research and science, defense and military fields and security industries requiring high-quality graphic content.

Warnings about risks to Apple were issued as experts said cybercriminals targeted Portable Document Format files to introduce malware into computers. PDF files had been considered safe to download and open, in contrast to text documents written for Microsoft Office or other formats.

Online forums on the growing crisis in this aspect of security said cybercriminals struggled to find ways to extort money from Mac users through Internet-based scams, but had been able to design malware that prevents users from closing or removing the programs after installation.

As Macs grow in popularity, Web security experts said cybercriminals will target them more frequently, said the mxlogic.com, one of the Web sites monitoring cybersecurity issues affecting Mac users.

"Programs have been made available that instruct people how to create malware, which leads the security industry to believe that Macs may be under attack," said mxlogic.com.

In the meantime, cyberattacks through pdf file downloads also pose a major threat, industry sources said.

Earlier this year, software manufacturer Adobe released a security update to patch a virtual hole in its pdf software that cybercriminals exploited to upload malware onto the computers of unsuspecting users.

When a PDF file is opened, Adobe Reader immediately opens and then closes itself while the malware attacks the user's hard drive. The malware uses JavaScript for a majority of its actions and isn't easily detectable, experts warned.

Industry analysis published online suggested nearly 60 percent of all malware infections originated from PDF files in the fourth quarter of 2009.

The U.S. government has made cybersecurity a priority issue and ran a nationwide campaign in 2009 to increase awareness of cybercrime. But industry reports claim a large portion of the cybercriminals are using sites hosted in the United States and many have managed to escape the law.

While popular opinion believes malware often originates in countries in Eastern Europe and Asia, one study attributed to AVG Technology said 40 percent of malware originated on servers hosted within the United States. It pointed out, however, that developers of the malware don't have to be in the United States to have their malicious code hosted on servers based in the United States.

Local governments, corporate institutions and schools in the United States have lost millions of dollars due to cyberattacks, industry sources said.