What is a fee sniping attack? In several places I came across a mention of this type of attack, however, it is not clear to me what it represents.

They also say that today it is not so common and profitable, but that it can become when the reward from the collected fees starts to be greater than the reward for the mined block.

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Fee sniping refers to the process of re-mining the last block in the chain to claim the fees of both the transactions mined in this block and the new transactions that came in since it was mined.

In an environment with low or no block subsidy and high fees it might be more profitable for a miner to re-mine the last block rather than to mine a new block on top.

Most wallets nowadays create transactions with the nLockTime field set to the next block. This is such as the transaction can only be mined in the next block: it's only available to a miner mining on top of the last block. In a low-subsidy and high-fee environment this means a fee sniping miner would have to give up on a large share of revenue, making this strategy much less attractive (if it ever was in the first place).

Side note: BIP326 suggests using the transaction input's nSequence field instead of nLockTime to achieve the same in Taproot transaction.

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