Ethereum Classic outage sparks speculation of 51% attack

The post on hackmd.io detailed the mechanics: "There was about 3,000 block-insertions by a miner who was mining (either offline or there) total difficulty could have exceeded current network difficulty…All Ethereum Classic Parity/OpenEthereum nodes couldn't handle about 3,000 block reorg from the Core-Geth chain, so there was a chain split which made the network unstable"

The diagnosis confirmed the reorganization was accidental, not malicious. Developers urged miners to keep mining the chain as-is.

The initial alarm had some logic. The symptoms resembled a 51% attack, a threat that looms over all proof-of-work blockchains. If miners controlling more than 50% of network hash power coordinated, they could dominate new blocks, reverse prior transactions, and pocket the mining rewards. Because the system credits miners when blocks finalize and only cross-checks later through consensus, a hostile majority could exploit that gap. Double-spending becomes possible.

But the investigation uncovered no actual double-spending. ETC recovered.

Fear had precedent. January 2019 brought similar warnings of a potential 51% attack on ETC. Developers attributed it to a powerful ehash machine. Coinbase saw things differently. The exchange shut down ETC transactions after determining a 51% attack had actually occurred, complete with confirmed double-spending. Coinbase disclosed: "We have identified a total of 15 reorganizations, 12 of which contained double spends, totalling 219,500 ETC (~$1.1M)".