Double-Spend Attacks: Understanding Confirmation Risk in Crypto Transactions

Crypto exchanges and users often assume that once a blockchain transaction is sent, the funds have safely arrived. However, this assumption is not entirely accurate. In reality, what truly matters is not just whether a transaction has been broadcast, but whether it has been confirmed by a block, whether the number of confirmations is sufficient, and whether the underlying chain carries the risk of being reorganized.

This concept is particularly relevant for proof-of-work networks, where confirmation depth and cumulative proof of work are central to reorganization risk. A double-spend attack, as seen in the January 2019 Ethereum Classic (ETC) incident, occurs when an attacker attempts to spend the same digital asset twice. This can happen if a merchant or platform trusts a transaction that is 'not yet stably confirmed' too early.

The number of confirmations determines how difficult it is to roll back a transaction. As a general rule, exchanges do not credit users immediately at 0 to 1 confirmations due to the higher risk involved. In fact, different blockchains have varying security levels, with mainstream high-hash-power chains differing greatly from small-cap, low-hash-power chains in terms of attack costs.

To avoid double-spend attacks, it's essential to establish proper confirmation and risk control mechanisms. This includes verifying on-chain confirmation status, setting reasonable confirmation counts based on the chain's risk level, applying risk control monitoring to abnormal deposit behavior, and not trusting payment screenshots or single-page status.