In the previous security lesson, we focused on Sybil attacks. I believe everyone now has a basic understanding of hacker attack methods in blockchain. Today, we continue digging deeper into the course “Blockchain Security Risks — Hacker Series,” and today’s theme is: 51% Attack & Eclipse Attack.

These two types of attacks are often discussed together, but they are not the same at all. A 51% attack targets the entire chain — it is a consensus-layer security challenge; An eclipse attack targets individual nodes — it is a network-layer precision manipulation.One is “frontal battlefield,” the other is “flanking infiltration”; One requires massive cost, the other requires technical sophistication.

This lesson will start from first principles, explaining how these two attacks operate, what threats they bring to the industry, why they are becoming increasingly important, and how we should truly understand on-chain security.

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Why Should We Understand 51% Attacks and Eclipse Attacks?

As AI-driven quantitative trading expands, Layer 2 ecosystems explode, and cross-chain assets grow, on-chain value is no longer the “small-scale experiment” it was years ago. Between 2024–2025, the total on-chain asset scale of the crypto market exceeded $3.7 trillion.

This means:

• Any chain whose consensus layer is compromised could see billions disappear.
  
  

• Any protocol whose node layer is attacked could be precisely manipulated or have prices influenced.
  
  

• Any weakness in a cross-chain system becomes the fastest cash-out channel for attackers.
  
  

In other words: Understanding 51% attacks and eclipse attacks = understanding the real risk boundaries of blockchains.

In a world dominated by DeFi, AI, cross-chain bridges, and restaking competition, security is never “just a technical topic.” It is a discussion of systemic market risk.

Basic Concept: What Is a 51% Attack?

A 51% attack refers to a situation where an entity controls more than half (51%) of a blockchain’s hash power or staking power, enabling it to “rewrite history” for malicious purposes.

It applies to both PoW and PoS, just in different ways:

• PoW: Controlling >51% hashrate allows control over block production order.
  
  

• PoS: Controlling sufficient stake influences consensus voting.
  
  

What can’t an attacker do?

• ❌ They cannot steal coins from wallets.
  
  

• ❌ They cannot change your wallet balance.
  
  

• ❌ They cannot break private keys.
  
  

But what can they do?

• ✔ Perform double spending
  
  

• ✔ Prevent certain transactions from being included
  
  

• ✔ Manipulate transaction ordering (MEV)
  
  

• ✔ Create short-term network chaos
  
  

• ✔ Trick exchanges into accepting deposits from an invalidated chain
  
  

In real life, attackers often use chain reorganization to perform double-spending, impacting exchanges and cross-chain bridges.

Basic Concept: What Is an Eclipse Attack?

An eclipse attack does not target the entire blockchain. Instead, it isolates a specific node by forcing it to connect only to attacker-controlled fake peers.

This is a network-layer attack, not a consensus-layer attack.

Targets can include:

• Validator nodes
  
  

• Miners
  
  

• Wallet infrastructure
  
  

• Exchange nodes
  
  

• Oracle nodes
  
  

• MEV algorithmic nodes
  
  

The attacker’s goals often include:

• ✔ Preventing the node from seeing the real chain
  
  

• ✔ Assisting a 51% attack
  
  

• ✔ Influencing validator voting
  
  

• ✔ Manipulating MEV and transaction ordering
  
  

• ✔ Manipulating oracle prices
  
  

• ✔ Blocking transactions from entering the mempool
  
  

Eclipse attacks are terrifying because: low cost, precision targeting, high efficiency — and no massive hashrate is required.

51% Attack: The Underlying Mechanism of “Forcibly Rewriting History”

To understand a 51% attack, we must understand a core principle:

Blockchains are not maintained by all nodes — they follow the “longest chain rule.”

Whoever produces the longest valid chain becomes the source of truth.

Thus, if you control 51% of the network’s total hashrate or staking power, your chain will always be the longest.

1. PoW Logic

Because the side with the most hashrate produces valid blocks the fastest.

If attackers gain >51% hashrate, they can secretly build a shadow chain, and once it becomes longer than the real chain, they broadcast it:

• All nodes switch to the longer chain
  
  

• The attacker cancels previous transactions
  
  

• Double spending becomes possible
  
  

2. PoS Logic

The principles are similar, but instead of hashrate, attackers rely on:

• ✔ Large stake
  
  

• ✔ Validator distribution
  
  

• ✔ Voting weight
  
  

• ✔ Exploiting slashing/latency weaknesses
  
  

3. Attack methods include:

• Preventing other validators from voting
  
  

• Submitting malicious forks
  
  

• Conducting short-term censorship
  
  

• Rewriting transactions within certain epochs
  
  

PoS chains have more complex 51% attack risks: Attackers do not always need 51%—just a majority of active validators.

How a 51% Attack Works (Step-by-Step Breakdown)

Step 1: Gain control of hashrate or stake
Attackers prepare:

• massive GPU/ASIC power
  
  

• large staking capital
  
  

• network/node dominance
  
  

For small-cap blockchains, this is surprisingly feasible.

Step 2: Build a shadow chain
Attackers mine blocks but do not broadcast them.

Step 3: Perform a real-chain transaction
Example: deposit 10,000 tokens to an exchange.

Step 4: Publish the longer shadow chain
The exchange’s confirmed deposit disappears as the chain reorganizes.

Step 5: Profit via double spend
Attackers:

• receive the exchange’s payout
  
  

• erase the original deposit
  
  

• profit without consequence
  
  

Eclipse Attack: Precision Isolation of a Node

An eclipse attack works like this: Make a node blind to the real network by feeding it only attacker-controlled data.

Common techniques:

1. Control all peer connections of the target

Nodes typically maintain fixed numbers of:

• Incoming peers
  
  

• Outgoing peers
  
  

Attackers:

• continuously connect
  
  

• fill all slots
  
  

• block real peers
  
  

Now the node receives only attacker data.

2. IP spoofing / fake nodes / zombie nodes

Attackers deploy:

• many fake nodes
  
  

• spoofed IP addresses
  
  

• protocol-modified malicious nodes
  
  

These create a “fake reality bubble” around the target.

3. Precision targeting PoS validators

Once a validator is eclipsed:

• ✔ it votes on fake blocks
  
  

• ✔ it finalizes the wrong chain
  
  

• ✔ it loses rewards
  
  

• ✔ it enables coordinated attacks
  
  

This is one of the most dangerous forms of consensus disruption.

Core Differences Between the Two Attacks

From the above table, we can clearly see:

• 51% attacks are “nuclear-level” attacks
  
  

• Eclipse attacks are sniper-level attacks
  
  

Especially in the PoS era, the two are often used together — and this is precisely why they are the two themes of today’s lesson.

The Crypto Market of 2025 Is Not the Market of Five Years Ago

• Layer 2 proliferation
  
  

• Restaking causing risk concentration
  
  

• Cross-chain bridges now hold massive value
  
  

• AI trading relies on oracles
  
  

• MEME markets create huge MEV
  
  

• LRTs and RWAs become new narratives
  
  

• On-chain stocks and bonds rising rapidly
  
  

Under such conditions, we must reassess both attacks:

1. Rising risk of 51% attacks

Reasons:

• increasing number of small PoW chains
  
  

• MEV incentives for chain manipulation
  
  

• validator centralization in some PoS chains
  
  

• compute power concentration due to AI
  
  

2. Eclipse attacks are now even more dangerous

Because the systems relying on node connectivity include:

• cross-chain bridges (most vulnerable)
  
  

• oracles (price manipulation = instant profit)
  
  

• MEV systems
  
  

• exchange nodes
  
  

• DeFi protocols
  
  

Eclipse attacks degrade a blockchain from “global ledger” to “isolated local ledger.”

Attackers can manipulate expectations, transactions, and prices within seconds.

Many Fear That 51% Attacks Steal Tokens — This Is Wrong

A 51% attack cannot:

• Not change balances
  
  

• Not break private keys
  
  

• Not alter smart contract code
  
  

But it can:

• revert transactions
  
  

• reorder transactions
  
  

• censor transactions
  
  

• create chain instability
  
  

Exchanges are the ones most afraid — because they are the final victims of double-spends.

Real-World Meaning of These Attacks(No historical examples listed, only principle-level analysis)

Targets most at risk:

• small-cap PoW chains
  
  

• PoS chains with validator concentration
  
  

• systems with weak routing security
  
  

• oracles relying on single nodes
  
  

• cross-chain bridges (most fragile component in the industry)
  
  

Attackers often use:

• MEV extraction
  
  

• off-chain shorting
  
  

• exchange deposits
  
  

• cross-chain withdrawals
  
  

• hedge models for guaranteed profit
  
  

Today’s blockchain is a highly financialized system. Attackers do not attack to “destroy the chain”— they attack to profit.

Defense Systems: How Blockchain Can Resist 51% & Eclipse Attacks

1. Defenses Against 51% Attacks

• Increase validator count
  
  

• Increase PoW hashrate cost
  
  

• Strengthen slashing rules
  
  

• Hybrid BFT + PoS consensus
  
  

• Strong randomness (VRF)
  
  

• Reduce mining pool centralization
  
  

2. Defenses Against Eclipse Attacks

• Increase peer randomness
  
  

• Increase peer counts
  
  

• Sybil-resistant peer selection
  
  

• Active suspicious-connection detection
  
  

• Multi-source mempool design
  
  

• Multi-path gossip protocol
  
  

• Minimum network quality requirement for validators
  
  

PoS chains especially need robust network-layer protection.

Conclusion: Blockchain Is Not Afraid of Attacks — It Is Afraid of Blind Confidence

“Decentralization guarantees absolute security” is an outdated myth.Blockchain security is not a static state. It is a dynamic equilibrium shaped by:

• hashrate
  
  

• stake distribution
  
  

• validator diversity
  
  

• network design
  
  

• consensus rules
  
  

• economic incentives
  
  

• community behavior
  
  

• asset scale
  
  

51% attacks and eclipse attacks are not “weaknesses” — they are reminders that:True decentralization is not the number of nodes — it is the system’s total resistance to attacks.As blockchain carries greater financial value, understanding security = understanding the lifeline of the crypto market.