Spam volumes are at their highest volumes since mid-2010 and the reason isn't an advancement in spambot technology or a surge of new spammers: it's simply a new business strategy, according to researchers at Cisco Talos Labs.
Before 2016, the researchers say, the SpamCop Block List size "hovers somewhere under 200K IP addresses." More recently, the averages are "closer to 400K IP addresses, spiking to over 450K IPs in August."
The main culprit is the Necurs botnet, which brought back a mostly outdated tactic in 2016: high-volume spam in which bots launch a huge amount of spam in a very short amount of time. This method is easy for spam filters to detect and block, so it's mostly fallen into disuse, in favor of stealthier, more targeted, low-volume trickles of spam.
"What could possibly be the advantage for spammers to crank up the volume?" the researchers wrote in a blog posted Wednesday.
The Necurs operators, instead of aiming for persistence, have been aiming for speed. The window of opportunity before a spam filter kicks in may only be minutes or even seconds, so Necurs has transmitted as much email as possible. And for a short time they may land some malware successfully, the researchers say.
Earlier in the year, there were a several notable spikes in spam volumes. During that time, Necurs mostly sent Russian dating or "pump and dump" stock spam. However, in June the botnet shifted tactics and began to push malicious attachments, which mostly propagated the Dridex banking Trojan and Locky ransomware. Since then, high-volume spam campaigns have been a constant.
Craig Williams, senior technical leader and security outreach manager for Cisco Talos Labs, explains that the Necurs operators can afford to run such short-term campaigns "because they've moved towards payloads that are more profitable."
As they become more profitable, "they can have more overhead," he says.
Another contributing factor to Necurs' success is that they may have picked up some of the customers left high-and-dry after the Lurk takedown that also took out the Angler exploit kit.
Williams cautions that if this method works for Necurs, if it is economically viable, then there will be copycats; so we'd better get our spam protections ready.
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