The amount of spam carrying embedded images designed to evade filters has doubled over the past three months, security experts have warned.
Messaging security firm CipherTrust warned that at peak times, the devious image-based spam accounted for more than 30% of all spam messages.
The increase has come as spammers have developed new tools that can generate completely randomised images and deploy them in spam at speeds of up to one million an hour.
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Overall, malicious message volumes have been rising sharply, with a 20% spike in global mail volumes last month, CipherTrust said.
The security firm also warned that the number of “zombies” – compromised PCs used to pump out spam, viruses and phishing frauds – was continuing to increase, with the number of newly created zombies shooting up by 20% in May alone, the last major spike in numbers.
CipherTrust chief technology officer Paul Judge said: "Over the last four years, the people responsible for spam have continued to use aggressive, and frankly ingenious, techniques to make their messages blend with legitimate e-mail.
“It is a high-stakes, high-profit business, and they continue to invest heavily in attempts to get messages delivered to users in the face of increasingly effective anti-spam systems. The growing use of zombies and image-based spammers are examples of these initiatives.”
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