Fee Sniping

Fee sniping is a mining strategy in which a miner deliberately re-mines one or more of the most recent blocks in order to seize their transaction fees, instead of building honestly on top of them. The attacker attempts to orphan the current tip, reclaiming both the fees it contained and any lucrative new transactions that have arrived in the mempool since. It is a theft of revenue rather than of coins: no signatures are forged and no consensus rule is broken, which is exactly what makes it worth studying, because it is a behavior the protocol permits and only incentives restrain.

The name captures the posture: rather than hunting for new blocks on the open frontier, the sniper aims backward at a block already found, treating a rich tip as a bounty rather than a foundation. It belongs to a family of incentive-driven deviations from honest mining, alongside selfish mining and withholding strategies, that researchers study less because they are common today and more because they map where Bitcoin's incentive design is thinnest, the places where following the rules and maximizing revenue could stop being the same thing.

The economic trigger

Re-mining the tip is statistically harder than extending it, since the honest network keeps working on the next block while the sniper races to replace the last one, so under normal conditions the gamble does not pay. The calculus changes as the block subsidy shrinks through successive halvings and fees come to dominate block revenue. Fee income is lumpy where subsidy is constant: one block may capture a burst of high-fee activity while the next finds a depleted mempool. When the fees sitting in the block just found greatly exceed the fees available for the next block, a miner with meaningful hashrate may rationally prefer a one-block reorganization race over honest extension. Research on this incentive family suggests that in a fee-dominated future, revenue volatility itself becomes a consensus risk, since every unusually rich block paints a target on its own back.

How wallets defend

The standard countermeasure is anti-fee-sniping via nLockTime. Wallets such as Bitcoin Core and Electrum set each new transaction's locktime to the current block height, making it valid only from the next block forward. A sniper re-mining yesterday's tip cannot pull these freshly broadcast transactions back into the replacement block, because consensus forbids including them at that height. The defense works collectively: the more wallets adopt it, the less fresh fee-paying material a sniper can add to a re-mined block, and the more the attack decays into a pure race with no bonus prize. It also has the elegant side effect of improving privacy uniformity, since height-based locktimes stop being a wallet fingerprint once everyone sets them.

Why it matters for the network's future

Fee sniping sits inside the broader question of what secures Bitcoin when the subsidy approaches zero, alongside concerns like double-spending and the incentive edge cases of the longest-chain rule. The honest-mining assumption embedded in Bitcoin's security model is that extending the best chain is always the most profitable move; fee sniping marks one boundary of that assumption. For everyday users the practical guidance is old and boring: transactions gain security with confirmation depth, and one confirmation on a fee-heavy block is worth a touch less certainty than one confirmation on a quiet one. For miners and pool operators, resisting the snipe even when it briefly pays is part of the unwritten contract that keeps the whole revenue stream credible. Like most attacks in this literature, its importance today is preventive: name the incentive, patch the wallets, and the profitable moment never arrives.