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On 18 September 2018, a critical double spend inflation exploit was patched with the release of Bitcoin Core 0.16.3. However, the developers warned that a Bitcoin blockchain split was possible, and to wait up to 200 confirmations (about 1.5 days) when accepting payments just in case a split occurred. The fork did not occur on the Bitcoin blockchain but the Bitcoin testnet experienced a blockchain split due to this exploit, with its state being critical during 27 September 2018.
This exploit allowed miners to double spend a transaction, with two consequences. First, any nodes that receive a double spent transaction would freeze up and crash. Second, the double spent transaction would actually add to Bitcoin’s coin supply beyond the 21 million coins fixed limit, causing inflation. This is an unacceptable situation, since one of Bitcoin’s biggest selling points is its fixed supply, so developers rushed out a fix within a day and released Bitcoin Core 0.16.3.
However, the fix only works if miners and full nodes upgrade to Bitcoin Core 0.16.3; older versions are still susceptible. Therefore, if a miner using an older version of Bitcoin Core uses the exploit to double spend Bitcoins, the block mined by that miner would be accepted by wallets with older versions of Bitcoin Core. Wallets that are using Bitcoin Core 0.16.3 would simply reject the block.
This would cause a chain split, with the Bitcoin blockchain on wallets with Bitcoin Core 0.16.3 being different than the blockchain being used by older versions of Bitcoin Core. This could still happen at any time as long as there are a significant amount of full nodes using older versions of Bitcoin Core and unfortunately, 69% of all Bitcoin full nodes are still using older versions as of this writing on 27 September. The percentage of full nodes using Bitcoin Core 0.16.3 has only risen from 25% to 31% in the past four days.
This Bitcoin testnet blockchain split is proof that it could happen to Bitcoin, since the testnet uses the same software and rules as Bitcoin. Fortunately, testnet Bitcoins do not have any value, so no money has been lost from this blockchain split. It should also be noted, however, that testnet is far less secure than the Bitcoin mainnet, with far less hashpower securing testnet – it is far easier to perform a double spend attack on testnet.
Testnet admitted to ICU after inflation bug forked the network a few hours ago. Recitation in progress, vitals stabilizing ❤️📈
— Conner Fromknecht ⚡️ (@bitconner) September 27, 2018
Developers depend on the testnet to perform testing with Bitcoin-related software, so it was important to keep the proper testnet blockchain alive. Conner Fromknecht, a Lightning Network developer, single-handedly kept the proper testnet blockchain running only using his laptop and a mobile hotspot.
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