A 51% attack occurs when one miner, validator, or coordinated group gains more than 50% of a blockchain network’s mining hash power or validation power. Once that threshold is crossed, the attackers can influence which transactions get confirmed, block legitimate payments, and even reverse transactions that were finalized during the period they controlled the network, creating the conditions for double-spending.
The term is also known as a majority attack. On Proof-of-Work networks, the deciding factor is hash rate. On Proof-of-Stake networks, it is the share of tokens accumulated and staked for validation. Blockchains rely on majority consensus to confirm transactions and add blocks, so control over the majority can turn that security model against the network itself.
How a majority attack works on-chain
Blockchains validate transactions through a consensus process, then package them into blocks that are added to the chain. Each block carries information tied to previous blocks, which makes older records difficult to modify once they have enough confirmations. That protection weakens if a single actor controls most of the network’s validating power.
For a 51% attack to succeed, attackers first need to obtain over half of a cryptocurrency network’s mining or validation power. They then introduce an altered version of the blockchain at a precise point in the network’s history. Because that alternative chain is backed by superior hash power or validation weight, the network may accept it as the valid chain. That gives attackers the ability to halt confirmations and reverse transactions that were finalized while they held control.
The source also draws an important limit. Transactions confirmed before the attack are far harder to rewrite, and the older they are, the harder they become to alter. A 51% attack is serious, but it does not mean every historical record can be changed at will.
PoW and PoS face the same risk through different routes
On PoW chains, attackers need to control more than 50% of mining power. On PoS chains, they would need to accumulate more than 50% of the network’s cryptocurrency and stake it. The mechanics differ, the weakness does not: if validation power becomes concentrated in one camp, the chain can lose the protections that decentralization is supposed to provide.
Such attacks are described as unlikely, not impossible. They are expensive and operationally difficult. Still, the source notes that when a cryptocurrency suffers a majority attack, the damage often extends beyond disrupted transactions and reaches the asset’s value and the community’s trust.
Notable cases: Bitcoin Gold, Ethereum Classic, and Verge
Bitcoin Gold (BTG) was hit by a majority attack in May 2018. Attackers gained control of a large share of the network’s hash rate and carried out double-spending while they were in control. Even after repeated efforts to raise exchange thresholds, the attackers ultimately escaped with $18 million worth of BTG. In 2020, BTG was hit again, leading to another double-spend worth $70,000.
Ethereum Classic (ETC) suffered three majority attacks in August 2020. The first caused a reorganization of 3,693 blocks. The second saw 4,000 blocks reorganized. In the third, more than 7,000 blocks were altered. The scale of those reorganizations showed how much influence the attackers had over network consensus during the incidents.
Verge also faced three 51% attacks in 2018. The first took place on April 4, 2018, when the attacker mined blocks using spoofed timestamps and accumulated 250,000 VXG. A second majority attack followed on May 22, 2018, allowing hackers to make off with $1.7 million worth of VXG. A third attack was reported just four days after the second.
Why Bitcoin is far harder to attack
The source describes a 51% attack on Bitcoin as extremely costly and difficult, to the point of being almost impossible to carry out successfully. The reason is simple: the network’s hash rate is so large that matching it would require enormous capital and infrastructure.
One example in the source is the Bitmain S19 XP Hyd ASIC miner, priced at more than $19,800 with hash power of 255 TH/s. It also lists FoundryUSA, F2Pool, and AntPool as the three largest Bitcoin mining pools by hash rate, holding 20.5%, 17.4%, and 15% of the network respectively. Together that amounts to 52.9% of Bitcoin’s total mining hash rate. To match that level of computing power, the estimated fixed cost would be about $8.1 billion.
That cost explains why mature, well-established cryptocurrencies are much less exposed to successful majority attacks. The source’s conclusion on prevention is straightforward: the only real way to stop a 51% attack is to ensure that no validator or validator group can control more than half of a network’s validation power.