An estimated 4.32 percent of all the Monero cryptocurrency currently in circulation has been mined by botnets and cyber-criminal operations, according to a study published earlier this month by academics in Spain and the UK.
The research was one of the biggest undertakings of its kind in recent years. Scientists analyzed around 4.4 million malware samples to identify one million malware strains that mined cryptocurrency on infected hosts.
The malware strains they analyzed spanned a period of a whopping twelve years, between 2007 and 2018.
The research team says it looked at IOCs (indicators of compromise) and used static and dynamic analysis techniques to extract information from malware strains, such as cryptocurrency addresses and mining pools that malware strains used in the past to collect and funnel money through.
Researchers used the data they collected to track down past payments from mining pools to the groups behind each wallet. Further, the also organized the malware strains in campaigns based on similarities and shared cryptocurrency addresses.
According to the study's findings, while some groups mined various cryptocurrencies in the late 2000s and early 2010s, Monero (XMR) is by far the most popular cryptocurrency among cyber-criminals in underground economies, at the current time.
"Although this depends on when criminals cash-out their earnings, we estimate that the total revenue accounts for nearly [$57 million]," researchers said in their paper.
Some criminal groups have been more efficient than others. Researchers say that the groups who rented malware and third-party server infrastructure on underground marketplaces were usually more successful than the vast majority of groups who built their own tools.
However, overall, regardless if they rented the malware or built it themselves, the most successful groups were the ones who used botnets to deploy their malware at scale.
The report mentions previously well-known Monero-mining campaigns such as Adylkuzz and Smominru, but researchers also said they uncovered new groups, with the biggest being one they called Freebuf and one called USA-138.
One group, in particular, made over $18 million worth of Monero, which would round up to roughly 1.45 percent of all Monero coins.
Some of these groups existed for small periods of time, but others updated their infrastructure and malware cosntantly, still being active to this day. Researchers say crooks usually modified their code when their XMR addresses got banned from certain mining pools, or when they needed to update the malware to apply mining protocol changes.
When researchers looked at what mining pool criminal groups preferred to handle mining operations and withdrawals to their private addresses, by far the number one choice was crypto-pool.fr, responsible for cashing out $47 million of the $57 million the researchers managed to track.
While researchers saw some criminal groups put efforts into mining other cryptocurrencies in past years, Monero is now the preferred cryptocurrencies for almost all crypto-mining operations.
The reason isn't hard to guess, as mining Bitcoin-based cryptocurrencies have now a higher mining difficulty and also need special hardware, which makes deploying malware mining these types of currencies on regular PCs both useless and unprofitable.
A last conclusion of this research paper, but not the least interesting, is that most of today's criminal mining operations rely on the open source tool named xmrig, around which most crooks have built their crypto-mining malware around.
While researchers saw more malware samples built around the Claymore mining software, most of the active malware campaigns they tracked used a xmrig-based malware strain.
This statistics can be explained by the somewhat experimental nature of Bitcoin-based crypto-mining malware strain in the late 2000s when malware authors toyed around with Bitcoin mining malware but never deployed it in the types of large scale campaigns we've seen Monero miners deployed in the past two years.