After years of writing viruses and worms for operating systems and software running on Internet servers, hackers found some new areas to target in 2005, according to an influential report on security trends published on Tuesday.

Over the past year, attackers have been switching their focus to network devices, backup software and even the security software designed to protect computers, according to the 2005 SANS Top 20 list of the most critical Internet security vulnerabilities, said Alan Paller, director of research with the SANS Institute, a training organization for computer security professionals.

"In the past 12 to 15 months... attackers have made a massive shift to attack applications," Paller said in an e-mail interview. "Automated patching started making it harder to find new vulnerable systems, so they went after applications that users are just not patching."

"Other more sophisticated attackers, looking for new targets, found they could use vulnerabilities in network devices to set up listening posts where they could collect critical information that would get them into the sites they wanted," he added.

The SANS Top 20 list has been published annually since 2000. It is compiled by representatives from a variety of computer security organizations including the U.S. Computer Emergency Response Team (US-CERT), the British Government's National Infrastructure Security Co-Ordination Centre (NISCC) and the SANS Internet Storm Center.

The list is designed to give security professionals a quick sense of the industry's consensus on which commonly targeted security vulnerabilities require their most immediate attention. It has traditionally focused on Windows and Unix vulnerabilities, as well as problems with some server-side applications.

The focus on new client applications and networking products has happened because so many server-side and operating system bugs have been fixed, forcing security researchers to look elsewhere for bugs, said Gerhard Eschelbeck, chief technology officer and vice president of engineering with Qualys, and a contributor to this year's list. "A lot of the low-hanging fruit has been identified now," he said. "We really reached a tipping point earlier this year, where people started to look aggressively at client-side applications."

Security researchers also started looking at vulnerabilities in networking products, thanks in part to a controversial presentation by security researcher Michael Lynn at this year's Black Hat 2005 conference in Las Vegas. Cisco Systems sued Lynn after he discussed security problems in the Internetwork Operating System (IOS) software that is used by Cisco's routers.

Networking products appeared on the SANS list for the first time this year, with Cisco vulnerabilities taking three of the 20 slots. The list also includes nine common application vulnerabilities, two Unix problems and six Windows issues, all of which "deserve immediate attention from security professionals," according to SANS.

The SANS Top 20 is visible here: http://www.sans.org/top20/