The latest Internet Security Threat Report released today by global internet infrastructure security firm, Symantec Corp, reveals that the number of bot-infected PCs in the world is rising.

According to a bi-annual report, Symantec's Internet Security Threat Report, Volume XI, the number of computers hijacked by malicious hackers to send out spam and viruses has grown almost 30 per cent in the last year.

The rise in the number of infected computers is being attributed to the global rise in internet users

Symantec says today more than six million computers worldwide are part of a "bot network". Worse, a vast majority of computer users are typically unaware that that their PCs have been hijacked.

The current internet threat environment is characterised by an increase in data theft, data leakage, and the creation of targeted, malicious code for the purpose of stealing confidential information that can be used for financial gain.

The worst sufferers are in the US says the security firm. More than a third of all computer attacks in the second half of 2006 originated from PCs in the United States.

While the total number of bot-net PCs rose, the number of servers controlling them dropped by about 25 per cent to 4,700, the report says. According to researchers at Symantec the decrease showed that bot network owners were consolidating to expand their networks, creating a more centralised structure for launching attacks.

As Alfred Huger, vice president of Symantec security response, puts it, online criminals appeared to be adopting more sophisticated means of self-policing. "They're launching denial-of-service attacks on rivals' servers and posting pictures online of competitors' faces," he says, adding, "It's ruthless, highly organised and highly evolved."

Cyber criminals continue to refine their attack methods in an attempt to remain undetected and to create global, cooperative networks to support the ongoing growth of criminal activity.

The semi annual Symantec Internet Security Threat Report (ISTR), Volume XI, covers the six-month period from July 1, 2006, through December 31, 2006. It is based on Symantec data collected from more than 40,000 sensors deployed in more than 180 countries in addition to a database that covers more than 20,000 vulnerabilities affecting more than 30,000 technologies from more than 4,000 vendors.

Symantec also reviews more than 2 million decoy accounts that attract e-mail messages from 20 different countries around the world allowing it to gauge global spam and phishing activity.