Those results come from a new MessageLabs Intelligence report released by Symantec Hosted Services on Tuesday.
The report also found that the world's most prolific botnet is now Rustock, which pumps out 44 billion spam emails per day. To keep the spam flowing, operators of the leading botnets -- Rustock, Cutwail, and Grum -- continue to innovate.
"With successful and resilient botnet operations established in prior years... cybercriminals experimented with many tactics to keep spam campaigns active and fresh this year," said Paul Wood, MessageLabs Intelligence senior analyst at Symantec Hosted Services. "From leveraging newsworthy events like the FIFA World Cup to taking advantage of the widespread popularity of URL shortening services and social networks... spammers deployed a variety of tricks to bypass spam filters and lure potential victims."
Botnet operators are also getting more practiced at sneaking their malware past security scanners. MessageLabs said it saw 339,600 different strains of malware sent via email in 2010 -- an increase of a hundred-fold from 2009. The sharp rise is due to the emergence of polymorphic malware variants such as Bredolab. Bredolab's polymorphic engine, for example, alters the code it generates when propagating copies of itself, disguising itself to avoid detection by security software.
More than other malware, MessageLabs said that Bredolab has been pushing the state of the art to evade detection through techniques such as including junk code, disabling antivirus, and immobilizing itself when added to a debugging environment for testing to foil researchers.
While high-volume, botnet-distributed attacks are on the rise, so are very small, discrete, and targeted attacks. MessageLabs said that in 2005, it saw perhaps one or two targeted attacks per week. But by the end of 2010, it saw an average of 77 attacks per day.
"Typically, between 200 and 300 organizations are targeted each month, but the industry sector varies and high-seniority job roles are most frequently targeted -- yet often by way of a general or assistant's mailbox," said Wood. "While five years ago, large and well-known organizations were often targeted, today the scope of targeted organizations has expanded and now no organization is safe from attack."