A company called Qubic has been waging the 51% attack by offering economic rewards for miners who join the Qubic mining pool. They claim to be "stress testing" Monero, though many in the Monero community have condemned Qubic for what they see as a malicious attack on the network or a marketing stunt.
Though Qubic has claimed to have achieved 51% of the Monero hashrate, these claims have been disputed. However, they do appear to be very close if not there already, and there have been multiple chain reorganizations — including a 6-block reorganization — suggesting that Qubic has established significant control over Monero mining.