Fenny Attack

A Fenny attack is a specialized variant of a double-spend attack where an attacker pre-mines a block containing a transaction that sends cryptocurrency back to themselves, then strategically releases this block to the network to invalidate previously confirmed transactions. Named after early blockchain security research, this attack exploits the time gap between transaction confirmation and final settlement, allowing an attacker with significant mining power to rewrite recent transaction history. The attacker essentially creates an alternative chain that overtakes the honest chain, reversing payments while claiming the funds themselves. Example: If an attacker controlled 51% of Bitcoin's hash rate, they could send Bitcoin to an exchange, wait for confirmations, then release a pre-mined chain that reverses the transaction while keeping the original funds, achieving a fraudulent double-spend. Why it matters for crypto security: Fenny attacks demonstrate why blockchain security depends on distributed consensus and adequate confirmation times. Understanding this attack model informs mining pool design and exchange settlement policies for high-value transactions.

Category: wallets security

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