H13 — nLockTime anti-fee-sniping

Severity: Info · Weight: 5 · Network: No

What it detects

The transaction's nLockTime field is set to 0. Most well-behaved wallets (Bitcoin Core, Sparrow, etc.) set nLockTime to the current block height as an anti-fee-sniping measure.

Why it matters

nLockTime = 0 is a wallet fingerprint. It reveals that the transaction was constructed by software that does not implement anti-fee-sniping — narrowing down which wallet or library was used. Blending in with the majority of transactions (which do set nLockTime) improves privacy.

Anti-fee-sniping protects against miners reorganising the chain to steal fees from recent blocks. Setting nLockTime to the current tip height makes the transaction invalid in any earlier block, removing the incentive to reorg.

What to do

Use a wallet that sets nLockTime to the current block height. Bitcoin Core, Sparrow Wallet, and most modern wallets do this by default.

Implementation

Fires when tx.locktime == 0. Low-weight (5) informational finding — reduces the score slightly but is rarely the dominant issue.