Grin, a privacy-focused cryptocurrency built on the Mimblewimble protocol, has just suffered a 51% attack on its blockchain.

According to a Nov. 7 tweet from crypto mining group 2Miners, an unknown group accumulated 57.4% of the total hash power of the Grin (GRIN) network on Saturday evening. 2Miners only had control of 19.1% of GRIN’s hash power, while sparkpool miners came in third at 18.9%.

Data from GrinScan shows the attackers were able to reorganize at least one forked block, at 23:17 UTC on Nov. 7. The unknown mining group has also increased its control since Saturday, commanding 58.1% of GRIN’s hash rate at the time of publication. 2Miners’ has risen to 24.6%, while sparkpool miners’ has dropped to 11.3%.

When a group of miners controls more than half of a network’s mining power, it can affect the organization of blocks, potentially causing volatility in the token price and disrupting confidence in the blockchain. However, a mining group having control of 51% or more of a network’s hash rate does not necessarily mean it has ill intentions — sparkpool has had up to 60% of the network’s hash power before and reportedly did not interrupt the production of new GRIN blocks.

The GRIN token remains relatively unchanged on the news, falling just 1.3% over a 24 hour period. 

The Ethereum Classic (ETC) blockchain suffered at least three 51% attacks in August, with attackers causing the reorganization of thousands of blocks. Unlike the ETC blockchain — which would cost attackers more than $7,000/hr to control more than half of the blockchain’s hash power — sustaining a 51% on the GRIN network only costs $75 at the time of publication, according to online resource Crypto51.