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Ethereum's blockchain has split in two from a bug in a previous version of the chain's main node software. The software, known as Geth, makes up roughly 75% of Ethereum nodes and as much as 73% of Geth nodes have yet to update to the new version to fix the bug.

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This means that around 50% of Ethereum nodes are running a split-off chain with out-of-date and bugged software that could allow double-spends. 

Not only that but other chains that are compatible with the Etheruem virtual machine, like Polygon or the Binance Smart Chain, could be exposed to the issue as well. 

Fortunately, this seems to have had little impact so far as most of the miners had already switched to updated versions of Ethereum software. It seems that it is mostly non-mining nodes that have yet to make the switch to the new version of the software.

The Ethereum Foundation security lead, Martin Swende, said "A consensus bug hit #ethereum mainnet today, exploiting the consensus-bug that was fixed in geth v1.10.8. Fortunately, most miners were already updated, and the correct chain is also the longest (canon).”

In other words, the "correct" (updated software chain) is winning, but those who are making transactions on the out-of-date and bugged chain may have their transactions effectively reversed when they update to the correct chain. 

This means it is possible that individuals, or even some exchanges, that have yet to make the update and are making transactions are relying on a small amount of out-of-date miners with very low hashing power. This low hashing power could put them at risk of a 51% attack and double spends. 

The only obvious solution to this is that all nodes update to the new software client though some may be apprehensive if they don't want to undo transactions they've made. Ideally, very few transactions have been made on the forked chain and most are willing to simply update their nodes. 

This story is developing.