Denial of service attacks are falling out of favour with black hat hackers because using compromised machines to send spam is a more lucrative - and less risky - way of making money illicitly.

Networks of compromised PCs can be used for purposes including relaying junk mail or flooding targeted websites with spurious traffic.

Symantec reckons the noticeable fall in denial of service attacks it witnessed in the second half of 2006 is down to the growing difficulty in launching such attacks, and getting victims to pay up even if these assaults are successful. Stealthier misuse of compromised PCs - such as sending spam - poses far less risk, the security firm argues.

Symantec recorded an average of 5,213 denial of service (DoS) attacks per day in the second half of 2006, down from 6,110 in the first half of last year. The US was the target of most DoS attacks accounting for 52 per cent of the worldwide total.

"DoS attacks are loud and risky. Whenever a bot-network owner carries out a denial of service attack they run the risk of losing some of their bots. This could happen either because an attacking computer is identified and disinfected, or if it is simply blocked by its ISP from accessing the network," Symantec researcher Yazan Gable notes in a posting to Symantec's Security Response Weblog.

Gable adds that the "up-front" costs in setting up a botnet before any hope of payment, as well as the possible loss of an entire bot network if a command and control server is identified, also act as a deterrent.

"It is likely that bot network owners are now moving away from DoS extortion and towards more lucrative ventures like spam. Not surprisingly, we saw a noted increase in spam volumes in the last six months of 2006," he added. ®

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