Private Mempools Explained: Flashbots Protect, MEV Blocker & CoW Protocol

Published March 3, 2026 · Updated March 7, 2026 · By JaredFromSubway

Why the Public Mempool Exposes You to MEV Extraction

When you swap tokens on Uniswap, SushiSwap, or any on-chain DEX, your transaction enters the public mempool with all of its details exposed: the token pair, the amount, the slippage tolerance, and the contract being called. MEV searchers run sophisticated bots that monitor every pending transaction in real time, filtering for profitable targets.

A sandwich attack is the most common exploit. The bot front-runs your trade to move the price against you, lets your transaction execute at the worse price, then back-runs to capture the difference. According to data tracked by JaredFromSubway, sandwich bots extracted over $289 million in attack volume on Ethereum mainnet during 2025 alone. The root cause in every case is the same: the victim's transaction was visible in the public mempool before confirmation.

Beyond sandwich attacks, public mempool exposure enables mempool monitoring for front-running NFT mints, liquidation sniping, and generalized transaction reordering. If your transaction carries economic value, someone is watching for it. The only reliable solution is to stop broadcasting to the public mempool entirely.

Flashbots Protect: How It Works

Flashbots Protect is the most widely adopted private transaction service on Ethereum. It operates as a free RPC endpoint that you add to your wallet. When you submit a transaction through Flashbots Protect, it is sent directly to a network of block builders via the Flashbots relay — bypassing the public mempool entirely.

The key innovation is the MEV refund mechanism. Starting in late 2024, Flashbots introduced a system where up to 90% of the MEV that could have been extracted from your transaction is returned to you as a refund. This means that even if a searcher finds an arbitrage opportunity related to your trade, the majority of that value flows back to your wallet instead of being captured by the bot operator.

Adding Flashbots Protect to MetaMask:

1. Open MetaMask and click the network dropdown at the top.
2. Select "Add network" → "Add a network manually."
3. Enter the following details:
   • Network Name: Flashbots Protect
   • New RPC URL: https://rpc.flashbots.net
   • Chain ID: 1
   • Currency Symbol: ETH
   • Block Explorer: https://etherscan.io
4. Click "Save" and switch to the Flashbots Protect network.
5. All transactions you submit through this RPC will bypass the public mempool entirely.

JaredFromSubway recommends Flashbots Protect as the default RPC for any Ethereum user who interacts with DeFi. The setup takes under two minutes, costs nothing, and eliminates the most common attack vector. For a broader overview of protection strategies, see our complete MEV protection guide.

MEV Blocker by CoW Protocol

MEV Blocker is a private transaction service jointly developed by CoW Protocol, Beaver Build, and Agnostic Relay. Like Flashbots Protect, it routes your transactions away from the public mempool. The difference lies in its auction mechanism: MEV Blocker runs a competitive auction among searchers who bid for the right to back-run your transaction. Front-running and sandwich attacks are strictly prohibited, but benign back-running (arbitrage that does not harm the user) is allowed.

To use MEV Blocker, add https://rpc.mevblocker.io as a custom RPC in MetaMask using the same steps described above. MEV Blocker supports Ethereum mainnet (chain ID 1). The rebate from the back-running auction is sent directly to the transaction originator, giving users a share of the MEV that would otherwise be lost entirely.

As of early 2026, MEV Blocker has processed over 30 million transactions and returned more than $50 million in MEV rebates to users. It has become the default RPC for several major wallets including Safe (formerly Gnosis Safe) and is integrated into the CoW Swap interface.

CoW Protocol Batch Auctions: Off-Chain Order Matching

CoW Protocol takes a fundamentally different approach to MEV prevention. Instead of simply hiding your transaction from the mempool, it removes the on-chain swap entirely. When you place a trade on CoW Swap, you sign an off-chain intent (a gasless message) rather than submitting an on-chain transaction. This intent enters a batch auction where professional "solvers" compete to find the best execution path.

Solvers can match orders peer-to-peer (a "Coincidence of Wants" or CoW), route through on-chain AMMs, or use a combination of both. Because your order is never broadcast to the public mempool as a pending transaction, there is nothing for a sandwich bot to target. The batch auction settles all orders at a uniform clearing price, which eliminates front-running by design.

CoW Protocol also provides surplus: if a solver executes your trade at a better price than your specified limit, you keep the difference. This is the opposite of MEV extraction — value flows to the user rather than away from them. JaredFromSubway considers CoW Swap one of the strongest MEV protection mechanisms currently available, particularly for large trades where sandwich attack profits would be highest.

Comparison: Flashbots Protect vs MEV Blocker vs Blink vs Merkle

The Trade-Off: Can Private Transactions Get Worse Settlement?

Private mempools are not without drawbacks. When you send a transaction through the public mempool, every validator and builder on the network can include it. This maximizes the competition to include your transaction quickly. With a private relay, your transaction is only visible to the builders connected to that relay. If none of those builders win the block auction for the next slot, your transaction waits.

In practice, this means private transactions can experience slightly longer inclusion times — typically one to three extra blocks (12 to 36 seconds). In volatile markets, that delay can result in a worse execution price than you would have gotten with immediate public inclusion, even after accounting for the MEV you avoided.

For most users and most trades, the protection benefit far outweighs the marginal delay. But for time-sensitive transactions — such as liquidation repayments or arbitrage plays — the latency of private relays can be costly. JaredFromSubway advises using private RPCs as your default but understanding that speed-critical transactions may warrant a different approach.

How Much MEV Do Private Mempools Actually Prevent?

The adoption of private mempools has grown dramatically since 2023. Research from Flashbots estimates that roughly 30% of Ethereum mainnet transactions now bypass the public mempool through private relays. This has measurably reduced sandwich attack volume: while total DeFi trading volume increased by 40% between 2024 and 2025, sandwich attack profits grew by only 12%, suggesting that private transaction adoption is outpacing MEV growth.

However, the absolute numbers remain staggering. Even with 30% of transactions going private, MEV bots still extracted hundreds of millions in value from the remaining 70% of public mempool users. Until private RPCs become the default wallet setting — not an opt-in feature — the public mempool will continue to be a hunting ground for MEV bots.

What JaredFromSubway Sees: Private vs Public Mempool Trends

From the perspective of an active MEV operation, the shift toward private transactions is unmistakable. JaredFromSubway's monitoring systems show that the percentage of high-value DEX swaps routed through private RPCs has increased from roughly 8% in early 2024 to over 25% by early 2026. Sophisticated DeFi users — whales, DAOs, and protocol treasuries — have largely adopted private transaction channels.

But the public mempool is far from empty. Retail users, particularly those trading new token launches, meme coins, and high-tax tokens, still overwhelmingly use the default public RPC. These traders also tend to set high slippage tolerances (5% to 49%), making them extremely profitable sandwich targets. JaredFromSubway continues to process thousands of profitable opportunities per day from public mempool transactions.

The pattern is clear: informed users protect themselves, while uninformed users bear a disproportionate share of MEV extraction. This is why JaredFromSubway publishes educational content like this guide and our MEV protection guide — closing the knowledge gap is the single most effective way to reduce harmful MEV across the ecosystem.

The Future: Encrypted Mempools, SUAVE & Threshold Encryption

Private RPCs are a practical solution for today, but the Ethereum community is building more fundamental fixes. Several research directions could reshape MEV dynamics entirely over the coming years.

SUAVE (Single Unified Auction for Value Expression) is Flashbots' ambitious project to create a decentralized, chain-agnostic platform for MEV management. Rather than relying on centralized relays, SUAVE envisions a separate blockchain that serves as a decentralized sequencer and block builder. Users, searchers, and builders would express their preferences through encrypted bids, and the SUAVE chain would determine the optimal block construction without revealing individual transaction details.

Threshold encryption is another promising approach. Transactions would be encrypted before entering the mempool and only decrypted after a committee of validators reaches consensus on their ordering. This prevents any single party from viewing transaction contents before inclusion, eliminating front-running at the protocol level. Projects like Shutter Network are actively building threshold-encrypted mempool solutions for Ethereum and its rollups.

Encrypted mempools on Layer 2s are closer to deployment. Several rollup teams are exploring commit-reveal schemes where transactions are committed in encrypted form, ordered by the sequencer, and only decrypted after ordering is finalized. This would provide MEV protection without requiring users to trust a private relay operator.

JaredFromSubway views these developments with pragmatic realism. Protocol-level MEV prevention would fundamentally change the economics of MEV extraction. Until these systems ship and prove themselves in production, however, private RPCs remain the most effective tool available to users today.

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