Spammers are increasingly making use of URL-shortening services to get their messages through, according to MessageLabs.
The security firm said in its July Intelligence Report that the services are being used in record numbers by botnet operators as a way to evade anti-spam filters.
MessageLabs found that during 14 days in June URL-shortening services accounted for more than 0.5 per cent of all spam.
"The average volume of spam containing shortened URLs has increased and MessageLabs Intelligence is seeing more days where shortened URLs are included in significant volumes of spam," the company said in the report.
"This indicates that the adoption of shortened URLs in spam is becoming a sustained tactic rather than an occasional use tactic."
Among the heaviest users of the tactic has been the Storm botnet. MessageLabs found that the botnet contributed more than 11 per cent of all URL-shortened spam messages.
Overall, MessageLabs reported that spam levels were down slightly from last month, accounting for 88.9 per cent of messages. Malware-laden messages and phishing attacks were also down slightly from June levels.
The company estimated that one in every 306.1 emails in June contained malware, while one out of every 557.5 emails was a phishing attempt.
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