In early January, the Ethereum Classic (ETC) network reportedly suffered a 51% attack, sparking renewed debate about blockchain security and the real cost of double spending. Major exchanges like Coinbase, Coincheck, and bitFlyer detected multiple double-spending transactions across several block heights and temporarily suspended ETC deposits and withdrawals.
According to data from blockchain security firm PeckShield, between January 5 and January 8, at least 15 suspicious double-spend transactions occurred on the ETC chain. These attacks led to the loss of approximately 219,500 ETC—valued at around $1.1 million at the time.
Crypto51.app, a platform that estimates the cost of executing 51% attacks on various blockchains, reported that renting enough hashing power to attack ETC would cost about $4,528 per hour. Some analysts suggested the total cost of the recent attack was around $20,000. In just four hours, the attacker allegedly double-spent over 54,200 ETC—worth roughly $270,000—yielding a return more than tenfold their investment.
But is this calculation accurate?
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Understanding 51% Attacks: The Basics
A 51% attack occurs when a single entity or group gains control of more than half of a blockchain’s total hashing power. With majority control, they can manipulate the blockchain by reversing transactions, preventing new ones from being confirmed, or double-spending coins.
The core security assumption of most proof-of-work (PoW) blockchains is that no single actor can economically justify acquiring such a large share of the network's computational power. For large networks like Bitcoin (BTC) or Ethereum (ETH), the cost is prohibitively high.
However, smaller chains like Ethereum Classic are inherently more vulnerable due to lower hash rate and market capitalization.
Why Smaller Chains Are at Greater Risk
Smaller PoW blockchains face disproportionate risks because their security is directly tied to their hash rate. With fewer miners contributing computational power, it becomes feasible—though still expensive—for malicious actors to rent or deploy enough hardware to overpower the network temporarily.
Ethereum Classic, as a continuation of the original Ethereum chain after the DAO fork, maintains a loyal community but operates with significantly less mining power than Ethereum or Bitcoin. This makes it an attractive target for attackers seeking high-impact results with relatively low investment.
Calculating the True Cost of a 51% Attack
While crypto51.app suggests an hourly attack cost of $4,528 for ETC, this figure only tells part of the story. A successful double-spend attack isn’t completed in a few hours—it requires sustained control over the network for days.
PeckShield observed rollback depths exceeding 100 blocks during the recent incident. Such deep reorganizations imply prolonged mining activity on a private fork before broadcasting it to overtake the honest chain. Additionally, failed attempts and preparatory phases aren’t visible on-chain but contribute to overall costs.
Assuming a conservative estimate of four full days of continuous hashing power rental:
- $4,528/hour × 24 hours × 4 days = **$434,688**
This revised figure dramatically reduces the supposed profit margin. Even if attackers recovered $270,000 in double-spent ETC, they would have operated at a significant loss unless they had access to subsidized or covert resources.
The Role of Hash Power Rental Markets
Platforms like NiceHash allow users to rent hashing power on-demand using GPU or ASIC miners connected globally. This commoditization of computational power lowers the barrier to launching short-term attacks—especially on smaller chains.
Crucially, ETC is one of the few major PoW chains where nearly 100% of its hash rate can be rented via NiceHash, according to Crypto51.app data. This unique vulnerability makes ETC particularly susceptible compared to larger networks.
For contrast:
- Bitcoin (BTC): Hourly attack cost ~$285,244
- Ethereum (ETH): Hourly attack cost ~$99,349
But here’s the key difference: you cannot rent sufficient hash power to attack BTC or ETH because NiceHash doesn’t offer anywhere near enough capacity for those massive networks.
To launch a 51% attack on Bitcoin today would require building your own mining infrastructure.
Let’s explore what that would entail.
Why Attacking Bitcoin Is Nearly Impossible
Suppose we attempt to replicate the same strategy on Bitcoin. As of early January, Bitcoin’s network hash rate stood at approximately 40.3 exahashes per second (EH/s).
Using Antminer S9 units—a now-outdated but illustrative model—with a hash rate of 14 terahashes per second (TH/s) and priced at around $400 each:
- Number of miners needed: 40,300,000 TH/s ÷ 14 TH/s ≈ 2.88 million units
- Hardware cost: 2.88 million × $400 = **$1.15 billion**
Add in electricity and operational expenses:
- Power consumption per S9: 1.35 kW
- Estimated electricity cost (at $0.03/kWh): ~$19.6 million over seven days
- Total estimated cost: Over $1.17 billion
This astronomical figure underscores why large-scale blockchains remain secure against economically rational attackers. The return on investment simply doesn’t justify the risk or capital outlay.
Key Takeaways for Investors and Users
While ETC has experienced multiple 51% attacks in recent years, including this latest incident, such events highlight broader concerns about long-term viability and security assumptions for smaller forks of major blockchains.
PeckShield warns users to exercise caution when holding or transacting with smaller PoW-based cryptocurrencies post-fork. Only chains with substantial hash rate decentralization and economic incentives for honest mining can resist coordinated attacks.
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Frequently Asked Questions (FAQ)
Q: What is a 51% attack?
A: A 51% attack happens when a single entity controls more than half of a blockchain’s mining hash rate, allowing them to manipulate transaction history, including reversing transactions and enabling double spending.
Q: Can double spending happen on Bitcoin or Ethereum?
A: Theoretically yes, but practically no. The cost of acquiring sufficient hash power to execute such an attack on Bitcoin or Ethereum is so high—exceeding billions of dollars—that it’s considered economically unfeasible.
Q: How do exchanges protect against double spending?
A: Exchanges typically require multiple confirmations (often 12–60 blocks) before crediting deposits. Some also monitor for chain reorganizations and delay withdrawals if suspicious activity is detected.
Q: Is Ethereum Classic still safe to use?
A: While functional, ETC carries higher risk due to repeated security incidents. Users should consider transaction finality less certain and avoid large unconfirmed transfers.
Q: Can renting hash power prevent detection?
A: Not entirely. Security firms and exchanges actively monitor network behavior for unusual hash rate spikes or deep reorganizations, which often signal ongoing attacks.
Q: Are proof-of-stake blockchains immune to 51% attacks?
A: They face different risks—called “nothing-at-stake” problems—but are generally more resistant because attackers must own a majority of staked tokens, which devalues upon malicious behavior.
Despite sensational headlines suggesting that a $20,000 budget could compromise ETC, the reality involves far greater complexity and cost. While smaller chains remain vulnerable due to accessible hash power markets, true profitability for attackers is questionable without hidden advantages.
Ultimately, blockchain security relies not just on technology—but on economic disincentives that make attacks irrational.
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