On August 15, 2010, a software flaw let an invalid transaction record 184,467,440,737.09551616 BTC—often rounded to 184.4 billion—in block 74638. The incident briefly appeared on one branch of Bitcoin’s history, but the chain containing it was overtaken by the valid chain. The excess bitcoins did not remain in Bitcoin’s accepted history. This was an exploited transaction-validation bug, not a conventional wallet theft.
What happened in block 74638?
A transaction included in Bitcoin block 74638 recorded a total output of 184,467,440,737.09551616 BTC. According to the Bitcoin Wiki’s incident record, two addresses received approximately 92.2 billion BTC each, and the block solver received a further 0.01 BTC.
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That amount describes what the invalid transaction recorded on the affected chain branch. It was not a lasting spendable balance in the Bitcoin history that prevailed. The incident is often called Bitcoin’s “biggest hack,” but that label can mislead: the available account describes exploitation of a software validation flaw, not the theft of coins from ordinary users’ wallets.
How did the overflow bug work?
Bitcoin software checks transaction outputs before accepting a transaction into a block. In this case, the old validation code did not correctly handle a sum of output values so large that the arithmetic overflowed. The flawed check allowed the invalid transaction to pass.
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The problem was in the software enforcing transaction rules—not in a blockchain independently deciding to create valid coins, and not in private keys being exposed. The incident record and a supporting historical account of the emergency fix describe the failure as an output-value or integer overflow. See the Bitcoin Institute’s account of Bitcoin v0.3.10.
What happened, and when?
| Time or milestone | What happened |
|---|---|
| August 15, 2010, block 74638 | The overflowing transaction was included on one branch of Bitcoin’s history. |
| 17:05 UTC | The cutoff named in the warning: users were told not to trust transactions occurring after this time, at block 74638, until the issue was resolved. |
| 20:38:33 UTC | Satoshi Nakamoto sent the warning to the bitcoin mailing list. The archive preserves the message’s exact wording: “*** WARNING *** We are investigating a problem. DO NOT TRUST ANY TRANSACTIONS THAT HAPPENED AFTER 15.08.2010 17:05 UTC (block 74638) until the issue is resolved.” See the archived message. |
| Within five hours of discovery | A new client release added checks rejecting overflowed output values and any individual output above 21 million BTC. |
| Block height 74691 | The valid chain overtook the chain containing the invalid block. |
The warning’s timestamp and its 17:05 UTC cutoff are different times: the first is when the message was sent, while the second marks the transactions users were advised not to trust.
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Did Bitcoin’s 21 million limit get broken?
The incident showed that a software flaw could let a transaction violate Bitcoin’s intended value rules on a chain branch. But the invalid transaction did not survive in the accepted history described by the incident record. The valid chain rejected the overflowing transaction and later overtook the branch containing it. The episode was a serious implementation failure, not a permanent increase in Bitcoin’s supply.
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Nodes running the updated client applied the additional checks and rejected the invalid transaction. Some unpatched nodes continued following the branch that contained block 74638, but the valid chain overtook it at height 74691. The transaction and the bitcoins it purported to create therefore ceased to be part of the prevailing chain history described in the incident account. The recovery was a chain reorganization; the sources do not say that every node switched at the same instant.
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Why “hack” needs qualification
“Hack” captures that someone exploited a vulnerability, but the term can suggest a wallet compromise or a lasting theft. The documented event is more precisely described as a value-overflow incident: faulty transaction-output validation let an invalid transaction appear temporarily on one chain branch. The cited records do not establish the attacker’s identity, motive, or any lasting profit.
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