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There have been several reports this week detailing how security firm Symantec took down a large portion of a bitcoin mining botnet called ZeroAccess. What few, if any, mention is that the bitcoin mining part of the botnet hasn’t been functional for almost six months, because the developers deliberately killed it. The question is, why?

ZeroAccess is a piece of malware that joins an infected computer to a large network of similarly compromised machines. They can then be controlled by a central administrator, commonly called a botherder, who then gets the machines to do his bidding.

Most botnets follow predictable criminal practices, using victims’ computers to send spam, or simply harvesting sensitive information on the infected machines, so that cybercriminals can use them to steal money. Others are used for click fraud, in which machines are made to click on profitable online links.

ZeroAccess isn’t a new botnet – Symantec first saw it in the summer of 2011, according to Vikram Thakur, a research with Symantec Security Response. The next major revision emerged a year later, with minor revisions found in between.

But something significant happened in April this year, he said, going on to explain:

“ZeroAccess deprecated the bitcoin mining module back in April 2013. The botnet harnessed the hashing power of all those bots until April 2013 and then pushed out an update which effectively removed the mining module. No mining has happened on the ZeroAccess network since then.”

Why would botherders kill a software module which was causing a lot of machines to happily churn out bitcoins?

Many technically astute people reading this will jump to the obvious conclusion, which is that CPU mining is pointless, given the high difficulty caused by the rapidly increasing hash rate on the network. This in turn is being caused by a flood of ASIC mining hardware which is pushing GPUs out of the picture, let alone computationally anaemic CPUs.

Symantec even does the math, taking a relatively old test computer as an example. It used a 2Gb, 3.4GHz Dell OptiPlex GX620 Pentium D machine to see how well the malware might cause it to mine. It used 136.25 Watts per hour to mine at 1.5Mh/sec. Put that next to the machines that KnC Miner just started shipping and it’s like watching a Reliant Robin next to a Ducati.