The number of hack attacks has fallen for the fourth year in a row, according to the Computer Security Institute (CSI) and FBI. A survey jointly conducted by the two found that the incidence of unauthorised use of computer systems fell again in 2004 - a trend which started in 2001.

According to the 2004 Computer Crime and Security Survey, "just" 53 percent of the 494 US computer security practitioners said they had been subject to a hack attack in the last 12 months, the smallest percentage recorded since 1999. The survey also found that denial of service (DoS) attacks were the most costly and that fewer organisations are reporting computer intrusions to law enforcement.

The survey - the ninth - polled security experts in US corporations, government agencies, universities, financial and medical institutions about a wide range of security issues.

New questions in this year's survey revealed that 15 percent of respondents said wireless networks had been abused and 10 percent experienced the misuse of public Web applications.

DoS attacks were, by far, the most costly, when measured in dollars. Total losses from DoS attacks were reported to be $26 million, with theft of proprietary information the next most costly type of attack - $11.4 million in total. Total losses due to the top 12 kinds of security incidents for the last year were put at $141.4 million.

The CSI-FBI poll follows another, similar survey by CSO magazine earlier this week. In that survey, 15 percent of 476 chief security officers (CSOs) and senior security executives said that their employer lost or had critical documents or corporate information copied without authorisation in the past year. Almost one quarter of those responding to the CSO survey said they could not be sure whether such losses had occurred at their company.