A mammoth army of infected computers is being assembled, but it's unclear yet what purpose they will be put to.
Wave after wave of malicious email attachments has been sent out since August, and with average success rates for such mailings, millions of machines could be compromised, says Internet security firm Commtouch.
Once infected, the computers can be loaded with additional malware that can perform a range of activities, including spamming, participating in DDoS attacks, stealing bank credentials and compromising email and social-network accounts, according to an upcoming Commtouch blog post.
But what this botnet will do remains a mystery. "The purpose of this vast computing force is still not clear," the blog says.
Since a record peak of 25 billion malicious attachments to emails being sent on a single day in mid-August, email-attached malware has peaked five times since, each spike smaller than the one before, says Commtouch. The company predicted this pattern in August just after the highest peak.
Each peak represents a surge in a particular scam used to dupe victims into opening the attack attachments. The first wave consisted mainly of phony notices from UPS or FedEx that a package has been misrouted. The second, called the Map of Love, is a PDF that purports to be a map of interesting destinations worldwide. The third is a false notice of an altered charge for a hotel room, the blog post says.
User forums indicate that the malware campaigns worked, with many users opening the attachments. While it doesn't have estimates of the number of machines compromised, Commtouch says that such campaigns have linear success, so the more attachments sent, the more opened.
If the purpose of the assembled botnet is to send spam, it hasn't had an impact on overall spam traffic, which has actually been trending a bit downward, Commtouch says.