About a quarter of all spams come as an attachment, according to a study by security supplier Marshal.
While the sophistication of spamming remains low at the moment, if the spammers' social engineering skills improve, Britain could face a security crisis as harmful spam causes havoc, warned Ed Rowley, Marshal's head of technical sales.
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"When spam senders improve their social engineering skills and they know enough about their targets to write an enticing or convincing e-mail top line, they could net many more victims," he warned. "This is what is known as spear phishing. They know enough about you to attach a convincing looking document - like an invoice relevant to your business - and you are suckered in."
Marshal reported today that spam containing attached PDF, Excel, Text and Zip files now represents almost 25% of all spam, which has surged up from 2% last week. Meanwhile the previously dominant Image Spam has reached a 12-month low of just 6% of all spam.
"Spam senders have to constantly evolve their forms. It will not be long before they evolve to make their intros look more genuine," he warned.