MEV Bots & Uniswap Arbitrage: Sandwich Attacks in DeFi

15/7/2025
6-minute read

Why your Uniswap trade sometimes gets a worse price than you expected? You might be up against a MEV bot running a sandwich attack. In 2025, automated trading bots are everywhere in DeFi, and understanding how they work is key to protecting your assets and even profiting from the new landscape.

This article breaks down:

What MEV bots are and how they operate
The mechanics of Uniswap arbitrage and sandwich attacks
Real-world bot performance and risks
How to protect yourself and optimize your trades

What Is a Uniswap Trade?

Uniswap is a decentralized exchange (DEX) on Ethereum that lets you swap tokens directly from your wallet—no middleman, no order book. Instead, Uniswap uses automated market makers (AMMs) and liquidity pools. When you trade on Uniswap, you:

Choose the tokens you want to swap (e.g., ETH for USDC)
Specify the amount
The protocol calculates the price based on pool reserves
You approve the transaction and pay a small fee
The swap happens instantly, and your wallet balance updates

Key features:

Anyone can provide liquidity and earn fees
Prices are set by a constant product formula (x * y = k)
Trades are public and visible in the Ethereum mempool before they’re confirmed

What Is a MEV Bot?

A MEV bot is an automated program that scans the Ethereum mempool for profitable opportunities. It looks for:

Large trades that will move prices
Arbitrage gaps between exchanges
Vulnerable transactions it can front-run or back-run

MEV bots use custom algorithms to:

Analyze pending transactions
Calculate the best way to profit (e.g., sandwich attack, arbitrage)
Submit their own transactions with higher gas fees to get priority
Repeat the process 24/7, often earning significant profits

Why do they matter?

MEV bots can impact regular traders by changing prices before their trades are confirmed
They’re a major force in DeFi, driving innovation—and controversy—around transaction ordering and fairness

Executive Summary

MEV bots exploit transaction ordering for profit, often at the expense of regular traders.
Sandwich attacks are the most common MEV strategy on Uniswap.
Open-source bots like Mev-Bot-Uniswap make these tactics accessible to anyone.
The bot’s developers claim daily gains of 8%+, but this is unverified and risks include slippage, failed transactions, and front-running.
Best practices: use slippage protection, monitor mempool, and consider bot strategies for your own trades.

MEV Bots: What, Why, and How

MEV (Miner/Maximal Extractable Value) refers to the profit that can be made by reordering, including, or excluding transactions in a block. On Ethereum, MEV bots scan the mempool for profitable opportunities, especially on decentralized exchanges like Uniswap.

How MEV Bots Work

Monitor pending transactions in the mempool
Identify arbitrage or sandwich attack opportunities
Submit their own transactions with higher gas fees to get priority
Profit from price movements caused by other traders

Uniswap Arbitrage and Sandwich Attacks Explained

Uniswap Arbitrage

Arbitrage bots exploit price differences between Uniswap and other exchanges. For example, if ETH/USDC is cheaper on Uniswap than Binance, a bot can buy on Uniswap and sell on Binance for a risk-free profit.

Sandwich Attacks

A sandwich attack involves three steps:

Front-run: Bot submits a buy order just before a victim’s large trade
Victim’s trade: The victim’s trade moves the price
Back-run: Bot sells immediately after, profiting from the price change

sequenceDiagram participant User participant Attacker participant DEX User->>DEX: Places large BUY order activate DEX Attacker->>DEX: Places BUY order (front-run) DEX-->>Attacker: Executes Attacker's BUY DEX-->>User: Executes User's BUY at higher price Attacker->>DEX: Places SELL order (back-run) deactivate DEX DEX-->>Attacker: Executes Attacker's SELL for profit

Real-World Example: Mev-Bot-Uniswap

Open-source bots like Mev-Bot-Uniswap claim to achieve daily gains of 8%+ in live trading, though these claims are unverified.

A Word of Caution: While open-source MEV bots can be a powerful tool, they also come with significant risks. You are trusting the bot’s code with your funds, and there is no guarantee that the bot will perform as advertised. Malicious code could be hidden within the bot, leading to a complete loss of your investment. Always exercise extreme caution and thoroughly vet any bot before deploying it with real funds.

These bots are beginner-friendly and automate the entire process:

Scan for large pending trades
Calculate optimal gas fees
Execute sandwich attacks
Track profits and losses

Performance Table

Bot Name	Platform	Daily Gain	Risk Level	Open Source
Mev-Bot-Uniswap	Ethereum	8.43%	High	Yes
Solana Trading Bot	Solana	5.2%	Medium	Yes
MarketVision	Multi	3.1%	Medium	Yes

Optimization and Protection Strategies

Set Slippage Limits: This is your first line of defense. By setting a low slippage tolerance (e.g., 0.5% or less) on your trades, you limit the maximum price change you are willing to accept. If a sandwich attack pushes the price beyond your limit, your trade will fail, protecting you from a bad deal.
Monitor the Mempool: While not practical for most users, monitoring the mempool for suspicious activity can provide insights into MEV activity. Services like Etherscan’s Mempool Explorer can help you visualize pending transactions.
Use Private Transaction Relays: Services like Flashbots Protect allow you to send your transactions directly to miners, bypassing the public mempool where MEV bots are lurking. This is one of the most effective ways to avoid sandwich attacks.
Test on Ethereum Testnets: Before deploying any trading strategy or bot with real funds, always test it thoroughly on a testnet like Sepolia or Goerli. This will help you understand its behavior and identify any potential issues without risking your capital.
Regularly Update Bot Code: If you are running your own bot, ensure you are using the latest version and that you are aware of any changes to the Uniswap protocol.
Understand the Risks of Front-running: Even with slippage protection, a front-running bot can still execute a trade before yours, impacting the price you get. The only way to completely avoid this is to use a private transaction relay.
Beware of Failed Transactions: When a trade fails due to slippage, you still have to pay the gas fees. This is a cost to consider when trading in highly volatile or MEV-heavy environments.

Migration and Future-Proofing

Uniswap v4 and future DEXs will change MEV dynamics—stay updated
Layer 2 solutions (Arbitrum, Optimism) offer new opportunities and risks
Regulatory changes may impact bot legality and profitability

Conclusion

MEV bots and sandwich attacks are now a core part of DeFi trading. By understanding how they work, you can protect your assets, optimize your trades, or even build your own bot. The landscape is evolving fast—stay informed, test carefully, and always manage your risk.

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