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Bagle.aq arrives via an e-mail message with a spoofed sending address and no subject line. The only text in the message body is one or two words, either "price" or "new price".
Once activated, Bagle connects to a number of remote sites to download the actual viral code, spreading itself via its own SMTP mailing engine.
Bagle.aq appeared last week and circulated quickly, with some users reporting as many as 100 infected messages in an hour.
Anti-virus supplier McAfee raised its security threat rating to medium for W32/Bagle.aq@mm, also known as Bagle.aq, as the worm became more prolific.
Meanwhile, the first serious electronic security threat to target mobile phones has emerged this month.
The attack is based on a pirated copy of a game called Mosquitoes. The rogue game sends premium rate SMS messages in the background as the user plays the game.
Sal Viveros, McAfee's marketing manager for small and medium-sized businesses, said he had seen examples of the security threat on the internet targeting the Symbian smartphone operating system.
Viveros added that he expects copycat Trojan horse attacks to appear soon.