Four-fifths of the spam clogging ISPs' mail servers and affecting businesses emanates from so-called zombie home PCs infected with spam Trojans, according to security firm Sandvine.
The company found that Trojans, typically installed surreptitiously by worms or spyware, exploit vulnerabilities to bypass normal email routing and drop spam messages directly into end user machines.
Many of the most well-publicised worm attacks in recent months were launched expressly to install spam Trojans on unsuspecting end users’ machines, waiting to be used at a later date as a spam delivery relay, Sandvine warned.
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