A 51 percent attack allows the attacker to reorganize the blocks in blockchain and that is exactly what is happening with the Grin blockchain right now. As you see in above image, an unknown miner has taken over 57 percent of total hashrate.
This has given the attacker ability to reorganize blocks. Despite this, Grin developers on their keybase account are saying that hash rate has increased but there is no sign of attack.
As a precautionary measure, big mining pools, including 2Miners has stopped the payouts as new blocks could be rejected.
Experts say most PoW (proof of work) blockchains will suffer 51 percent attacks over the new few decades. Their halvenings will undermine their security more and more.
Official Grin’s twitter is suspended, but its developers have posted this message on their website,
“IMPORTANT NOTICE: The Grin network hashrate has increased considerably over a short period of time. Notably this coincides with the nicehash rate doubling in this time with well over 50% of the network hashrate currently outside of known pools. Considering these circumstances, it is wise to wait for extra confirmations on transactions for payment finality.”