Bitcoin Double Spending: A Rare but Interesting Vulnerability

There is a rare vulnerability in the Bitcoin network that allows certain specific transactions to be duplicated. This situation is possible because the coinbase transaction (, which miners use to receive block rewards, does not have inputs but instead directly generates new coins. Therefore, two different coinbase transactions could potentially be identical, including sending the same amount to the same address. Since these transaction data are the same, their transaction IDs )TXID( will also be the same.

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Two known sets of duplicate transactions occurred between November 14 and 15, 2010, spanning approximately 16 hours. Each of these duplicate transactions involved 50 BTC, totaling 200 BTC. To date, these Bitcoins have not been used. Theoretically, a person with the private key can spend these coins, but only 100 BTC are actually available, while the other 100 BTC will be unusable.

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Duplicate transactions can cause confusion for wallets and block explorers, and may also be used to attack exchanges, among other issues. To address this problem, the BIP30 soft fork was implemented in 2012, prohibiting the use of transactions with duplicate TXIDs. In 2013, BIP34 required that coinbase transactions must include the block height, further reducing the likelihood of duplicate transactions.

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However, the first byte of the scriptSig in some coinbase transactions before BIP34 happens to match the future block height and could theoretically still be replicated. The next block that may appear as a duplicate is 1,983,702, expected to be produced around January 2046. However, to exploit this vulnerability, miners would need to incur a huge cost, far exceeding any potential gains.

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Although this vulnerability has existed for a long time, it is not considered a major security threat to Bitcoin due to the difficulty of exploitation and high costs. However, considering the time span involved and the uniqueness of the issue, it remains an interesting technical detail. Developers are considering completely fixing this issue through methods such as soft forks, with possible solutions including enforcing SegWit commitments.

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