Fee Sniping

Fee sniping is a theoretical mining strategy where a miner who has just found a block decides to not build on top of it, but instead try to re-mine the same block height to capture the high-fee transactions for themselves. It's a niche attack that becomes more relevant as the block subsidy decreases and transaction fees become the dominant share of miner revenue.

How it would work, hypothetically:

1. Block N is mined with collected fees of, say, 0.5 BTC.
2. A different miner sees this and computes: "If I re-mine block N with the same transaction set, I can collect those 0.5 BTC in fees. That's worth more than the expected reward of building block N+1 on top of the existing block."
3. They try to mine an alternative block N. If they succeed before anyone mines N+1, the network may reorg to their chain.

Why this is rare and risky:

• They need significant hash rate to have a meaningful chance of out-racing the existing block. A small miner has near-zero probability of success.
• Failure costs. If their re-mining attempt fails (someone else mines N+1 first), they've lost the time they could have spent mining N+1. Opportunity cost.
• Reputation costs. Visible fee-sniping behavior signals to other miners that this operator is unreliable and may make them less willing to coordinate.

Defensive measures already in place:

• nLockTime set to current height. Modern wallets set transaction locktimes to the current block height. This makes those transactions invalid for any earlier block, so a fee sniper can't include them in a re-mined block at the same height - only in N or later.
• Fast block propagation. With compact block relay (BIP-152) propagating new blocks in milliseconds, the window during which a competing block could be assembled is tiny.

Fee sniping is more of an open theoretical concern than a present-day problem. As Bitcoin approaches the fee-dominated era (post-2140), the math changes, and the strategy could become more attractive. The community is aware and watching. So far, no observed cases. See Miner Extractable Value (MEV) for the broader category of strategies fee sniping fits into.