While internet users were troubled by fewer Trojan-based malware attacks last month, there was an increase in mass-mailing worms, according to research by security company Fortinet.

The firm detected over 1,000 pieces of malicious code in February, of which 40 per cent were Trojans and backdoors, 10 per cent mass-mailer worms and 20 per cent internet worms which spread through networks via shares and instant messaging.

Guillaume Lovet, Fortinet's EMEA antivirus team leader, said that February's malware activity showed a slight decrease in the percentage of Trojans, but an increase in the proportion of mass-mailing worms.

"This could be explained by the recent resurgence of MyDoom with the variant MyDoom.BB followed by several minor variants, as well as the recent appearance of Sober.K, with its relatively new, but far-reaching, social engineering strategy," he said.

Significant threats discovered this month included W32/Bropia which Fortinet said is "particularly innovative" since it propagates by MSN Instant Messenger.

According to Lovet, the technique deployed by this worm "represents a major breakthrough in terms of social engineering methods" as users are likely to trust their instant messaging contacts and to open the files which they send.

The company also pointed to the Sober.K worm, which uses social engineering to entice users into opening the attachment by posing as an email from the FBI.

"As computer uses become more security conscious, malware writers are having to deploy ever-more far-reaching social engineering techniques to ensure the spread of their malicious code," Lovet warned.

"The advice remains the same: do not click on attachments unless you are sure they originate from a trusted source. The advent of the Bropia worm also illustrates that additional care must be taken when using instant messaging.

"Always make sure that the message is coming from the person who claims to have sent it, and make sure you are running up-to-date virus protection on your IT systems."