BlackSpider Technologies has reported that a dual virus attack hit U.K. businesses with an estimated 650,000 trojan-laden emails, until antivirus vendors were able to issue a patch. Thousands of emails containing the trojan Downloader.Win32.Agent.adu were fired at the inboxes of many U.K. businesses. Lasting some three hours, the attack involved e-mails that used the same subject line and text in the body of the email, claiming to be a transaction to online retailer Amazon. In the first wave of the attack, the email was 5,564 bytes, in the second it was 5,712 bytes.

James Kay, chief technical officer, BlackSpider Technologies, said: "This was a particularly opportunist attack. Emails from the first wave were still being released by the hacker when the second wave struck. Antivirus vendors were probably not expecting a second and very similar wave to occur while the first attack was still happening. Not only was the first attack successful, it also effectively acted as a smokescreen and allowed the second strike to catch the antivirus community off guard, which is why it enjoyed a window of exposure of more than three hours."