In just the past two weeks, three cryptocurrency networks have been hijacked by miners trying to enrich themselves.

A wave of selfishness: Motherboard reports that Verge, Monacoin, and Bitcoin Gold have all recently fallen victim to the dreaded “51 percent attack,” a nightmare scenario in which a miner or mining group obtains a majority of the network’s power to approve new transactions. An estimated $20 million was lost as a result.

A useful reminder: Just because a cryptocurrency works like Bitcoin, which is enormously difficult to tamper with, doesn’t mean its blockchain is “secure.” Smaller, newer networks that haven’t been battle-tested—of which there has been a proliferation—are particularly risky, and self-interested miners are capitalizing. “We are now beginning to see miners act more strategically,” Cornell professor Emin Gün Sirer told Motherboard, whereas in the past they “lacked technical sophistication” and had fewer potential targets (see “How secure is a blockchain really?”).