In recent years, a number of security problems with the Internet became apparent. New and current Internet users need to be aware of the likelihood of security incidents and the steps they should take to secure their sites. Before designing a secure system, it is advisable to identify the specific threats against which protection is required. The threats may be classified into interruption, interception, modification, and fabrication. The extents of these threats are examined across four industries in Korea - manufacturing, banking/financial, research institution/university, and distribution/service. Banking/financial firms generally perceive the four categories of threats more seriously than other industries. The companies in the manufacturing industry consider interruption to be the most serious. Research institutions/universities and distribution/service companies regard modification and interruption as critical threats. The appropriate security technique is recommended for each threat. (C) 2001 Elsevier Science B.V. All rights reserved.