Sandwich Attack

Overview

The sandwich attack detector identifies swap functions in DEX and aggregator contracts that lack minimum output amount validation and deadline checks. A sandwich attack works by frontrunning the victim’s swap to move the price, letting the victim swap at a worse rate, then backrunning to reverse the price movement and pocket the difference.

Why This Is an Issue

Sandwich attacks extract over $500M annually from Ethereum users. Any swap function that accepts a user’s trade at whatever price the pool offers — without enforcing a minimum acceptable output — is vulnerable. The attacker needs only to observe the pending transaction in the mempool and submit bracketing transactions.

How to Resolve

// Before: Vulnerable — no slippage or deadline protection
function swap(address tokenIn, uint256 amountIn) external returns (uint256) {
    uint256 amountOut = pool.swap(tokenIn, amountIn);
    IERC20(tokenOut).transfer(msg.sender, amountOut);
    return amountOut;
}

// After: Fixed — enforce minimum output and deadline
function swap(
    address tokenIn,
    uint256 amountIn,
    uint256 minAmountOut,
    uint256 deadline
) external returns (uint256) {
    require(block.timestamp <= deadline, "Transaction expired");
    uint256 amountOut = pool.swap(tokenIn, amountIn);
    require(amountOut >= minAmountOut, "Insufficient output");
    IERC20(tokenOut).transfer(msg.sender, amountOut);
    return amountOut;
}

Detection Methodology

1. Swap pattern identification: Identifies functions that perform token-for-token exchanges based on call patterns (transfer in, external call, transfer out).
2. Minimum output check absence: Flags swap functions without a require(amountOut >= minAmount) pattern.
3. Deadline check absence: Flags swap functions without block.timestamp comparison.
4. Confidence scoring: Functions with neither check receive highest confidence; those with one of the two receive medium.

Limitations

False positives: Internal swap helpers called by outer functions that enforce slippage may be flagged. False negatives: Custom slippage mechanisms (e.g., oracle-based price floors) may not be recognized.

Related Detectors

• Slippage Validation — detects missing slippage checks broadly
• MEV Vulnerabilities — detects general MEV patterns
• Front-Running — detects frontrunning vulnerabilities

References

• Ethereum.org — MEV
• Uniswap V2 Router — Deadline and MinAmount