While the total number of attacks over the first six months of this year was up 28 percent from the previous six months, the number of companies that were hit with at least one severe attack during that time fell 47 percent, according to a report due this week from Riptech Inc., a leading security monitoring company. The company defines severe attacks as a successful exploit of a vulnerable Web server or a compromise of a system by a worm.

Despite the fact that activity from worms such as Nimda, Code Red and SQL Spida accounted for 44 percent of all attacks during the survey period, fewer than 1 percent of respondents said they had suffered a severe attack from one of these worms.

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