A 51% attack occurs when one entity secures control of more than 50% of a network’s mining power or staked tokens. Holding this majority control allows them to alter transaction records and undermine the integrity of the blockchain. It’s not a conventional hack. Rather, it’s a complete takeover of the consensus mechanism itself. Blockchain is frequently described as immutable, signifying that past transactions cannot be altered by anyone. However, this security hinges on a fundamental principle: trust in the majority. When an entity holds the majority of the network, it possesses the power to alter the past.
A 51% attack stands out as one of the most daunting threats in the world of cryptocurrency. Launching an attack of this magnitude now comes with a staggering price tag that surpasses billions of dollars. However, smaller blockchains continue to face vulnerabilities, ensuring that this threat is very real for the wider crypto ecosystem.
How a 51% Attack Works: The Mechanics
Blockchains work on a simple principle: the longest or heaviest chain is the genuine one. This “longest chain rule” allows nodes to agree on which version of transaction history is correct. When most miners or validators are honest, this approach works flawlessly. However, if someone has power over the majority, they can violate this rule.
The attacker starts by mining blocks secretly, resulting in a shadow version of the blockchain. They do not transmit these blocks to the network instantly. Instead, they wait until their private chain is longer than the public one. When it gets longer, they release it all at once, compelling the network to accept their version as fact.
This approach permits what is known as a double-spend attack. The attacker uses the public chain to send bitcoins to an exchange or merchant. When they get items or withdraw various currencies, they publish their private blockchain. The network switches to the attacker’s version, which erases the original transaction. The attacker now gets both their original coins and the items they purchased.
What an Attacker CAN Do:
With majority control of a blockchain network, an attacker gains several dangerous capabilities:
- Reverse their own transactions: They can double-spend coins by erasing confirmed payments after receiving goods or services.
- Halt new confirmations: They can prevent other users’ transactions from being added to the blockchain.
- Block miner rewards: They can invalidate blocks mined by honest participants, denying them their rightful earnings.
What an Attacker CANNOT Do:
Despite controlling the network, there are important limitations to what an attacker can accomplish:
- Steal from wallets: They cannot access your private keys or move funds you control.
- Create new coins: They cannot mint cryptocurrency beyond the protocol’s defined supply limits.
- Alter old history: They cannot realistically change deeply buried blocks from months or years ago.
The Astronomical Cost of Attack
To successfully attack Bitcoin, one would need to control the most powerful computing network globally. Research conducted by Professor Campbell Harvey in October 2025 estimates that a week-long attack would incur costs ranging from $6 billion to $8 billion. This entails acquiring specialised ASIC miners and the requisite electricity for their continuous operation. Nation-states would find it challenging to rationalise such an expenditure for ambiguous benefits.
Ethereum transitioned to a Proof-of-Stake consensus mechanism, resulting in increased risks of potential attacks compared to Bitcoin. Hackers would require control of 51% of the total staked ETH, which was estimated to exceed $44.8 billion in mid-2025. In contrast to mining equipment, these tokens would experience an immediate depreciation in value upon the public disclosure of the attack. The economic incentives significantly deter any logical endeavour.
Smaller networks encounter a contrasting issue. Attackers have the capability to lease mining power from platforms such as NiceHash for a cost of several thousand dollars per hour. Low-hashrate coins are vulnerable due to their limited security budget. This cost disparity elucidates why prominent chains infrequently experience attacks, whereas lesser-known cryptocurrencies remain consistently susceptible.
Real-World Incidents & The "Close Calls"
Between 2019 and 2020, Ethereum Classic saw many 51% assaults, resulting in millions of dollars in double-spent transactions. Similar attacks targeted Bitcoin Gold, resulting in a loss of nearly $18 million in a single 2018 incident. These chains share a common feature: they have relatively low mining power compared to their parent blockchains, making them economically viable targets.
Blockchain | Year of Attack | Estimated Loss | Current Status |
Ethereum Classic | 2019-2020 | $5.6 million+ | Still operational, increased confirmation requirements |
Bitcoin Gold | 2018, 2020 | $18 million+ | Operational, implemented enhanced security measures |
Vertcoin | 2018, 2019 | ~$100,000 | Operational with algorithm changes |
Grin | 2020 | Unknown | Operational, incident raised awareness |
In August 2025, the Qubic mining pool momentarily held about half of Monero’s hash rate. While no attack took place, the episode spurred heated debate regarding mining centralisation. The community responded rapidly, encouraging miners to move pools. This close call illustrated how privacy-conscious networks must constantly evaluate concentration hazards.
Exchanges have learnt from these occurrences, delisting currencies after multiple reorganisations. When a blockchain has repeated reorganisations, trading becomes unstable and dangerous for platforms. The possibility of delisting puts market pressure on initiatives to improve their security. For many smaller coins, this entails either integrating mining with larger chains or switching consensus algorithms entirely.
Why You (Usually) Don't Need to Worry
Game theory protects big blockchains better than technology alone. If an attacker spent $10 billion to compromise Bitcoin, the price would plummet immediately. Their investment would be worthless before they could make a meaningful profit. This creates a strong deterrent to attempting such an economically illogical attack.
Proof-of-Stake networks offer an additional defence known as “social slashing.” If validators seek to assault the network, the community can work together to erase their staked tokens. This is not automatic, but historical precedence indicates that communities will act to safeguard their chain. The attacker risks losing billions of dollars with no guarantees of victory or escape.
Transaction confirmations offer you personal security against reorganisation threats. Bitcoin users often wait six confirmations before declaring a transaction final. Each confirmation makes undoing the transaction significantly more difficult and expensive. For smaller, riskier blockchains, waiting 50 or more confirmations gives a comparable level of security against potential assaults.
How to Protect Your Assets
Protecting yourself from 51% attack risks requires understanding how different networks and platforms manage security:
Protection Strategy | How It Works | Best For |
Wait for multiple confirmations | Each confirmation makes reversal exponentially harder | All users, especially merchants |
Use established networks | Higher security budgets deter attacks | Long-term holders |
Monitor network hashrate | Sudden drops signal increased risk | Active traders |
Avoid low-cap PoW coins | Small networks are economically vulnerable | Conservative investors |
Enable exchange notifications | Alert you to abnormal network activity | All users |
For Exchange Users:
- Cryptocurrency exchanges set confirmation requirements based on each blockchain’s security profile.
- Bitcoin deposits typically require 3 confirmations, while smaller altcoins may demand 50 or more.
- Never consider a transaction complete until it meets the recipient’s confirmation threshold.
For Portfolio Management:
- Avoid holding significant value in cryptocurrencies with extremely low hash rates or validator counts.
- These “ghost chains” often have daily security budgets under $10,000, making attacks trivially cheap.
- Prioritise battle-tested networks like Bitcoin and Ethereum for your primary holdings.
- Lesser-known chains might offer higher returns but come with proportionally higher security risks.
Conclusion
A 51% attack is a theoretical vulnerability that is economically impossible on major blockchains. For profit-seeking attackers, attempting to breach Bitcoin or Ethereum is unreasonable due to the billions necessary. However, smaller networks are always at risk, resulting in a two-tier security scenario. The distinction is important because your security measures should match the networks you utilise. Major chains provide institutional-grade safety by sheer economic scale, whereas smaller enterprises necessitate personal supervision and lengthier confirmation times.
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Frequently Asked Questions
Can a 51% Attack Steal the Bitcoin Sitting in My Hardware Wallet?
No. An attacker cannot “hack” your private keys or move your funds. They can only manipulate their own transaction history (to double-spend) or prevent new transactions from being confirmed. Your existing balance is safe.
Is Bitcoin More Secure than Ethereum Against A 51% Attack in 2025?
Actually, current 2025 data suggests Ethereum is more expensive to attack. While a Bitcoin attack costs ~$6-$8 billion in hardware and electricity, an Ethereum attack requires owning 51% of all staked ETH, a figure exceeding $44 billion. Both, however, are considered “economically impossible” to successfully compromise.
What is A "Chain Reorg" And is it the Same as A 51% Attack?
A chain reorganisation (reorg) is the result of a 51% attack. It happens when the network realizes a different version of the blockchain is “longer” or “heavier” and switches to it, effectively deleting the transactions on the previous version.
Can Government Entities or Nation-states Launch A 51% Attack?
Theoretically, yes. While it’s not profitable for a hacker, a nation-state might do it for sabotage. However, the global distribution of ASICs and the sheer power requirements make it logistically nearly impossible to hide such an operation in 2025.
How Can I Protect Myself from the Risks of a 51% Attack?
The best way is to wait for multiple confirmations before considering a transaction “final.” For major coins like BTC, 6 confirmations are standard. For smaller, higher-risk altcoins, wait for at least 30–50 confirmations before shipping goods or services.