51% attack
A 51% attack is a scenario where one party holds the majority of network hashrate and could theoretically reverse transactions or censor blocks.
With >50% of hashrate, an attacker can reverse recent transactions (double-spend) and censor transactions. What they cannot: steal coins from other addresses, change transaction rules, or rewrite old history. For Bitcoin in 2026 the attack is economically irrational: physical hardware + energy cost tens of billions USD and the attack destroys its own investment through price crash.
Example
Smaller PoW networks (Bitcoin Gold, Ethereum Classic) have been 51%-attacked in the past, losing millions. Bitcoin itself has never been at current hashrate scale.
Frequently asked questions
Could a government do a 51% attack?
In theory. In practice: requires >50% of global ASIC production plus >50% of global energy capacity for mining. Even for China/US not realistic without years of prep.
Related terms
Further reading
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