At least 23% of Ethereum blocks are complying with U.S. sanctions due to the use of a service called Flashbots.

Ethereum researcher Toni Wahrstätter has published research showing that maximum extractable value (MEV) operator Flashbots is censoring all transactions originating from Tornado Cash. Out of all 19,436 Ethereum blocks generated on Ethereum after The Merge using Flashbots' relay software (for MEV Boost), not a single block contained a transaction connected to Tornado Cash, according to Wahrstätter.

“It's concerning to see how quickly the potential for censorship has grown unchecked since the Merge, and its potential to get much worse as more validators opt-in to mev-boost, unless awareness is raised,” said Labrys, a web3 development company that built a site tracking compliant Ethereum blocks, on Twitter.

Flashbots appears to have been censoring transactions by default on its offering as it aims to be compliant with regulatory requirements. This is in line with comments made by Flashbots Strategy Lead Hasu in August that it planned on filtering out sanctioned transactions.

While Flashbots alone accounts for 23% of Ethereum blocks, it’s possible the actual percentage of compliant Ethereum blocks may be much higher. “This data currently only shows validators that use MEV-Boost but those that don’t can also be OFAC compliant through their own internal systems,” said The Block researcher Steven Zheng.

The data shows that Flashbots is the only MEV relay participating in transaction censorship. Meanwhile, MEV relay operators like Blocknative, Eden Network, Bloxroute, and Manifold are still producing blocks that have Tornado Cash transactions in them, according to Wahrstätter. This contrasts claims by Blocknative, Eden Network and BloXroute Regulated that they are complying with sanctions.

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