Imagine submitting a swap expecting a fair price — and, within seconds, realizing you received fewer tokens than anticipated. In many cases, this isn’t bad luck or market volatility: it’s a sandwich attack in action.
This type of attack is very common on blockchains where pending transactions are publicly visible, allowing bots to exploit transaction order and timing.
Understanding how these attacks work, why they exist, and what can be done to reduce this risk is essential for anyone trading on DEXs.
What Are Sandwich Attacks?
Sandwich attacks are a form of transaction manipulation that affects users trading on decentralized exchanges (DEXs).
They occur when bots identify a pending transaction and insert operations before and after it to extract profit, causing the original trader to receive worse execution.
This type of attack is directly linked to MEV (Maximal Extractable Value) and is more common on blockchains where pending transactions are publicly visible.
How a Sandwich Attack Works
A sandwich attack happens when an attacker:
1. Identifies a transaction in the mempool
Bots continuously monitor the mempool for DEX trades, especially large swaps or those with high slippage tolerance. When they detect an opportunity, they act within milliseconds.
2. Sends a transaction before the victim (front-running)
The attacker submits a buy order for the same asset, paying a higher fee so the transaction is processed first. This pushes the price up before the victim’s swap executes.
3. The victim’s transaction executes
Because the price has already been moved by the front-running transaction, the victim ends up buying at a worse price or receiving fewer tokens than expected.
4. Sends a second transaction right after (back-running)
Finally, the attacker sells the tokens acquired in the first step, taking advantage of the inflated price caused by their own action and the victim’s trade.
As a result, the victim’s transaction is literally placed between two attacker transactions — one before and one after — which explains the term sandwich attack.
The attacker’s profit comes directly from the worse execution suffered by the user, even though the protocol itself is functioning correctly and without any technical flaws.
How Sandwich Attacks Work on DEXs
Sandwich attacks mainly target AMM-based DEXs, where prices are determined by liquidity pool formulas.
Step-by-step flow
- Mempool monitoring
Bots continuously scan the mempool for large swaps or trades with high slippage tolerance. - Front-run transaction
The attacker submits a buy order with higher gas fees, ensuring it is executed before the victim’s trade. - Victim trade executes
The user’s trade goes through at a worse price due to the price movement caused by the front-run. - Back-run transaction
The attacker immediately sells the acquired tokens at the inflated price, locking in profit.
Why Are Sandwich Attacks Possible?
Several structural factors make sandwich attacks viable:
- Public mempools expose pending transactions
- AMM pricing reacts instantly to large trades
- High slippage settings give bots room to exploit price movement
- Block builders and validators can reorder transactions
These conditions exist on most major smart-contract blockchains.
Risks and Impact on Traders on Sandwich Attacks
Sandwich attacks can cause:
- Higher execution costs
- Unexpected slippage
- Lower token output than estimated
- Reduced trust in decentralized trading
Although the protocol itself isn’t hacked, the user still suffers a direct financial loss.
How to Avoid or Reduce Sandwich Attacks
While it’s not possible to eliminate the risk entirely, users can reduce exposure:
1. Use lower slippage tolerance
Tighter slippage limits reduce the profit window for bots.
2. Avoid large trades in low-liquidity pools
Thin liquidity makes price manipulation easier.
3. Use MEV-protected RPCs
Private or protected RPC endpoints prevent transactions from being broadcast to the public mempool before execution.
4. Split large trades into smaller ones
This lowers price impact and makes the trade less attractive to bots.
Sandwich Attacks and MEV
Sandwich attacks are one of the most common MEV extraction strategies. They don’t require breaking cryptography or exploiting smart-contract bugs — they rely purely on transaction ordering and speed.
As MEV research advances, many wallets, RPC providers, and DEXs are implementing protections to limit this behavior.
Trading Safer With Klever Wallet
Sandwich attacks highlight an important reality of decentralized trading: transparency is powerful, but it also creates opportunities for exploitation when users are unprotected. Understanding how these attacks work is the first step, but choosing the right tools is what truly reduces risk.
For anyone interacting with DEXs regularly, using a wallet built with security, performance, and infrastructure awareness in mind is no longer optional..
