In a report on robot networks activity for the January 1 - June 30 period, Symantec found an average of 10,300 bots online per day, up from 5,000 around December 2004.
Bot networks are compromised computers on which attackers install software that listens and responds to outside commands, usually at the cracker's discretion.
The increase in bot activity follows the release of Microsoft's XP Service Pack 2 in August 2005. Prior to this release, the average bots online per day was around 30,000 (July 2004).
"It is likely that bot network owners have been required to modify their attack methods in order to maintain viability in the face of these changes," Symantec's report said.
The report found that denial of service (DoS) and distributed denial of service (DDoS) attacks averaged 927 per day, up 68% from previous reports.
"This increase in DoS activity is largely due to the corresponding increase in bot network activity. It may be related, at least in part, to financial motivation, as DoS attacks have been reported in extortion attempts," the report said.
The report was compiled with the help of 24,000 sensors monitoring network activity in over 180 countries.
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