What is a 51% Attack and Why It Still Threatens Blockchains

Blockchain is often seen as a synonym for transparency and security: a decentralised network where no one can “fiddle” with the data for their benefit. But in practice, the resilience of any network depends on how evenly power is distributed within it. The most well-known vulnerability of such systems is the 51% attack, which can destroy trust in a cryptocurrency in just a few hours.

How a 51% Attack Works

A 51% attack occurs when an entity gains control of more than half of the computing power (hashrate) of a Proof-of-Work (PoW) network. With such an advantage, an attacker can:

• Reverse their transactions (double spending);

• Create alternative chains

• Block or censor other users’ transactions

• Slow down block creation and destabilize the network.

This threat was described as early as Bitcoin’s whitepaper, yet it remains relevant to this day. The risk is especially high for smaller PoW networks with low hashrate, where a relatively small mining farm or rented cloud power can be enough to seize control.

Real-World Cases

51% attacks have already occurred on major projects:

• Ethereum Classic (2019): losses of over $1M;

• Bitcoin Gold (2018): around $18M in losses;

• ZenCash (2018): several successful double spends and reputational damage.

Each of these incidents resulted in investor outflows and a decline in coin prices.

Why This Matters for Developers and Investors

For IT audiences, 51% attacks are not just a financial threat. They highlight architectural weaknesses in systems that seem “resilient by definition.” It’s important to remember:

• Decentralisation ≠ security by default.

• In projects with low liquidity and a small number of validators, it’s easier for attackers to compromise the network.

• Rented computing power and botnets make attacks cheaper.

• Blockchain is not “magic” — it’s just as vulnerable to social and economic attacks as to technical ones.

Furthermore, PoW is becoming less relevant — more and more projects are switching to Proof of Stake (PoS) or hybrid consensus mechanisms, where the cost of an attack is expressed in staked tokens, and the attacker risks not only equipment but also their own funds.

How Blockchain Communities Respond

Some of the protective measures gaining popularity include:

• Increasing the number of independent validators and miners (to combat mining pool centralisation);

• switching to PoS or Delegated Proof of Stake;

• raising transaction confirmation thresholds on exchanges for “suspicious” networks;

• Implementing monitoring and alerts for abnormal network behaviour (a growing use case for DevOps and analytics teams).

What You Can Do

If you’re developing a dApp or integrating with a blockchain:

• Check the network’s decentralisation level and hashrate trends.

• Monitor signs of centralisation (such as the share of the largest pool);

• Don’t rely on minimal confirmations for high-value transactions.

When investing, select networks with active communities and robust architectures.

Conclusion

The 51% attack is not an “outdated theory” but a real risk that still occurs in practice. It reminds us that the security of decentralised systems is not just about cryptography — it’s about economics and thoughtful design. The more active developers and the community are in monitoring the network’s health, the lower the chances of attackers succeeding.