Worm attacks over IM (instant-messaging) networks increased by 1,700 percent in 2005, while email spam remained constant at about 75 percent to 80 percent of all messages sent, according to an annual report issued by messaging security service provider Postini yesterday.

Postini generated its 2005 report by analysing the messages that come through its global data centres where it filters customers' messages for spam, viruses and other malware. These centres scan a billion messages a day on average, according to Andrew Lochart, Postini's senior director of marketing.

The findings related to IM, which came from the threat centre of Postini partner IM Logic, showed that MSN Messenger took the brunt of IM attacks in 2005, accounting for 57 percent of all worms detected. AOL's IM service accounted for 34 percent of the attacks, and Yahoo's IM network was home to nine percent, the report said.

"Hackers have discovered IM; it's the new playground for them," said Lochart. He added that as IM service providers strike agreements to pass each other's message traffic back and forth, worms will spread back and forth as well.

Postini's report also showed that the proportion of spam to legitimate mail dipped slightly towards the end of 2005, mainly because of an increase in virus- and phish-laden emails.

Discounted medicine and software topped the spam categories in 2005 at 28 percent.

The company's email security service found 2.5 percent of all blocked inbound messages contained viruses, and nearly two percent of outbound email – from companies that subscribe to Postini's outbound filtering service as well – contained viruses.