A security review of the first half of 2005 has reported a sharp rise in the number of Trojan attacks that leave compromised computers under the remote control of malicious hackers.

Security firm McAfee found that the number of recorded backdoor attacks had jumped by almost two thirds, leading to a corresponding rise in botnets of enslaved machines used to send spam or launch distributed denial of service attacks.

"In just the first and second quarters of 2005, the number of exploited machines using backdoor techniques has increased over 63 per cent from the total at the end of 2004," stated Vincent Gullotto, vice president of McAfee's Avert service.

"This often resulted in spyware and adware being downloaded onto affected systems. Thus spyware has continued to be a major problem."

The McAfee review suggests that many people are now using virus toolkits to create variants that can slip past antivirus filters. The company claimed to have found eight Bagel variants in a single day.

McAfee also believes that mobile viruses are a growing threat. The company's researchers found a way to gain complete control of a Bluetooth phone that was supposedly protected with preloaded security software.