51% Attack
An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.
Explain Like I'm 12
An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.
Why It Matters
Security concepts help users and developers recognize common blockchain attack surfaces and defenses.
How It Works
The attacker acquires enough computational power or staked tokens to control the majority of block production. They then ignore the honest network nodes, building a secret fork that follows their own rules. Eventually, they publish this chain to the network, which, due to the protocol's longest-chain rule, is accepted by nodes as the legitimate version.
Real-World Example
The Ethereum Classic (ETC) network has suffered several 51% attacks where attackers reorganized blocks to perform double-spend operations on exchanges.
Advantages
- Identifies weak consensus mechanisms
- Encourages wider distribution of mining power
- Validates robustness of network security
Limitations
- Extremely expensive to execute on large networks
- Damages the reputation of the attacked blockchain
- Difficult to maintain over a long duration
Common Misconceptions
- People think 51% attacks can allow hackers to steal everyone's private keys. In reality, they cannot access individual wallets but can only reorder or block transactions.
- Some believe these attacks are common on major chains like Bitcoin, but the cost required makes such an attack virtually impossible.
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Related Terms
Phishing
Phishing is a social engineering attack where malicious actors impersonate legitimate platforms, services, or individuals to deceive users into disclosing sensitive information, such as private keys, seed phrases, or login credentials. In the crypto sector, phishing is highly sophisticated; attackers often create fake websites, send fraudulent emails, or use social media bots to trick users into signing malicious transactions that drain their wallets.
Private Key
A private key is a secret, mathematically generated string of characters that grants the owner complete control over an associated cryptocurrency address. It acts as a digital signature tool, allowing users to authorize transactions and prove ownership of funds. In a decentralized network, the private key is the ultimate proof of authority; whoever possesses the private key effectively owns the assets associated with the corresponding address. It is never meant to be shared with anyone.
Smart Contract Audit
A structured review of smart-contract code and system design intended to identify vulnerabilities, logic errors, and security risks.
Sybil Attack
A Sybil attack is a security threat in decentralized networks where an attacker creates a large number of pseudonymous identities to gain a disproportionate influence over the system. By controlling the majority of nodes or participating addresses, the attacker can disrupt network consensus, censor transactions, or manipulate voting processes in governance mechanisms, thereby subverting the decentralization and integrity of the blockchain.
Address Poisoning
A scam where attackers create lookalike address activity so victims may accidentally send funds to the wrong address.
Audit
A smart contract audit is a comprehensive, systematic examination of a blockchain project's source code by independent security experts. The goal is to identify vulnerabilities, logical errors, and potential security flaws that could lead to exploits or financial loss. Auditors review the code for compliance with best practices, test for edge cases, and analyze the interaction between different smart contracts to ensure the protocol functions as intended before it is deployed on a mainnet.