Invisible theft in DeFi: This is how MEV bots manipulate your transactions (and how to avoid it)
We explain how MEV bots work on blockchain networks. Discover what sandwich attacks are and how to use private RPCs to protect your purchases.
Invisible theft in DeFi: This is how MEV bots manipulate your transactions (and how to avoid it)
Operating in decentralized finance (DeFi) promises disintermediation and transparency. However, behind the screen, there is a high-frequency underworld where automatic algorithms wage a constant battle to squeeze cent by penny out of user transactions.
This phenomenon is known as MEV (Maximal Extractable Value), and if you've traded on Uniswap, PancakeSwap, or other decentralized exchanges, there's a good chance you've been the victim of a sandwich attack without even realizing it.
The mempool: The hunting ground of MEV bots
When you execute a swap in DeFi, your transaction is not processed instantly. It first goes to the mempool, a public waiting room where validators select transactions to package into the next block.
The problem is that the mempool is totally transparent. Specialized bots (known as searchers) constantly scan it looking for vulnerable transactions from average users who have set a high slippage (slippage).
Anatomía de un Ataque Sándwich (Sandwich Attack):
1. [El Bot detecta tu swap pendiente] ➔ Compras token X con un slippage alto.
2. [Frontrun del Bot] ➔ Compra token X primero pagando una comisión de gas altísima. El precio sube.
3. [Tu Swap se ejecuta] ➔ Compras el token X al precio inflado que dejó el bot.
4. [Backrun del Bot] ➔ Vende inmediatamente el token X al nuevo precio.
➔ El bot obtiene ganancias sin riesgo y tú recibes menos tokens de los esperados.
The hidden cost of MEV for users
The FSM is not a theoretical attack. Hundreds of millions of dollars are withdrawn annually from the Ethereum network. The most common attack vectors are:
- Sandwich Attacks: As illustrated above, the bot buys before and sells after you, leaving you "in the middle" and making your purchase more expensive to the maximum of your configured tolerance.
- Frontrunning: Get ahead of an arbitrage purchase or loan settlement by paying more gas for validators to process your transaction first.
How to protect yourself from FSM?
Fortunately, the Ethereum ecosystem has developed solid defenses available to any user:
1. Use Private RPCs (Flashbots Protect)
By default, your wallet sends transactions to public nodes (like Infura). You can change this by setting up a private RPC on your MetaMask's network. Services like Flashbots Protect or MEV-Blocker send your transaction directly to the associated block builders, keeping it hidden from the mempool bots until it has been confirmed on the blockchain.
URL de RPC recomendada para MetaMask:
https://rpc.flashbots.net
2. Set Slippage to Minimum
In the settings menu of your DEX (like Uniswap), reduce the slippage tolerance to a low value, such as 0.1% or 0.5%. If the bot does not have enough margin to raise your price without your transaction failing, it will abort the sandwich attack due to lack of profitability.
By understanding the inner workings of the mempool and applying these tools, you can shield your trades and avoid paying the “invisible tax” of MEV bots on your daily DeFi trades.
