BitcoinNews.com recently reported on a bug fixed in the new version of Bitcoin Core 0.16.3. But apparently, Bitcoin developers decided not to release full information about the bug at that point as they wanted to maintain secrecy until most of the mining hash rate can be upgraded to Bitcoin Core 0.16.3. The bug could have been exploited to inflate the supply of Bitcoin beyond the fixed limit of 21 million.
As first reported, the bug would allow the miner who finds a block to double spend an input within the block, which would cause all full Bitcoin nodes using Bitcoin Core 0.14.0 or later to freeze up and crash. With most of the full nodes still running on Bitcoin Core 0.14.0, a large-scale exploit of the bug could have easily caused a severe disruption of the entire Bitcoin network.
However, the bug was even worse than initially disclosed, and Bitcoin developers kept the full extent of the bug a secret. They released the information about how full nodes must upgrade to Bitcoin Core 0.16.3 to prevent them from crashing or being exploited, as they didn’t want people to know that this bug could be exploited to mint bitcoins until most Bitcoin miners were using Bitcoin Core 0.16.3. Someone actually figured out the full extent of the bug on a forum and posted it on 20 September 2018, but their post got deleted.
How this bug exactly works is that if a miner finds a block and double spends an output from a previous block, they could actually create new bitcoins. The bug was originally introduced in Bitcoin Core 0.14.0 since that version removes a pre-relay block check that ensures no double spending, but at that point, there was no danger of inflating the supply of bitcoins. In Bitcoin Core 0.15.0 and later, the code was subtly changed, creating the situation where a miner could inflate Bitcoin’s supply.
This could have had severe consequences if miners began to exploit this bug and print bitcoins. Aside from inflating coin supply which could cause Bitcoin’s price to go down, it would cause people to lose confidence in the cryptocurrency, since one of Bitcoin’s main selling points is that it can never be printed outside of a set amount of mining.
The developer team was informed about this bug by an anonymous source on 17 September, and by 18 September Bitcoin Core 0.16.3 was released and a public media campaign was launched to get people to upgrade. This shows the strength of Bitcoin’s developer team to quickly respond to critical bugs and fix them.
As of this writing, about 25% of Bitcoin full nodes use Bitcoin Core 0.16.3, and according to the developer team, these nodes include a majority of mining power. Theoretically, miners could decide to exploit this bug by using older versions of Bitcoin Core, but full nodes running Bitcoin Core 0.16.3 would reject these transactions. Therefore, if this bug is exploited at this point it would cause a Bitcoin chain split, but it is unlikely that the exploited buggy chain would gain any momentum.
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