MEV / front-running detection

When this applies

• Swaps, mints, redemptions on AMMs / vaults
• NFT mints with random/reveal mechanics
• Auctions, Dutch auctions, English auctions
• Liquidations
• Trade routers
• DEX aggregator paths
• Any function with amountOutMin or slippage parameters

Detection patterns

Missing slippage parameter (HIGH)

function swap(address tokenIn, uint256 amountIn) external {
    router.swapExactTokensForTokens(amountIn, 0, path, msg.sender, block.timestamp);
    //                                       ^ amountOutMin = 0 → 100% sandwichable
}

Manipulable deadline (HIGH)

router.swap(..., block.timestamp);   // ← deadline = now means no deadline at all

Searcher can delay your tx indefinitely. Use a real deadline (block.timestamp + 15 min or user-supplied).

Permit + transferFrom front-running (MEDIUM)

Submitting permit() separately from the consuming tx lets an MEV bot front-run the permit and grief the user. Bundle in one tx.

Reveal-based randomness without commit-reveal (HIGH)

function mintRare() external {
    uint256 r = uint256(keccak256(abi.encode(block.timestamp, msg.sender)));
    if (r % 100 == 0) _mint(msg.sender, RARE_ID);   // ← searcher can simulate and skip
}

Searcher simulates → only sends tx if outcome is favorable. Use commit-reveal or Chainlink VRF.

block.prevrandao / block.difficulty for randomness (HIGH)

Proposer can manipulate post-merge prevrandao within bounded ranges; predictable enough for valuable mints.

NFT mint without per-tx limits + public sale (MEDIUM)

Bots will sweep entire allocation. Not a vuln per se but a UX/fairness issue worth flagging.

Just-in-time (JIT) liquidity exposure (LOW)

LPs in concentrated-liquidity pools can be JIT'd. Inform user, not a fix.

Auction first-price bidding (HIGH)

First-price sealed bids on a public mempool = bid front-run. Use commit-reveal.

Approve race (MEDIUM)

Classic ERC-20 approve(spender, X) → approve(spender, Y) front-run lets spender drain X + Y. See [[approval-issues]].

Severity rubric

Remediation patterns

• Slippage: surface as a user parameter, enforce a sane minimum.
• Deadline: surface as user parameter, never block.timestamp.
• Randomness: Chainlink VRF, RANDAO with epoch lookback, or commit-reveal.
• Bid auctions: commit-reveal (hash bid + nonce → reveal after close).
• For high-value flows: support private mempools (Flashbots Protect, MEV-Share, Berachain's BBS).
• Bundle permit + use in single tx.
• ERC-20 approval: prefer permit (EIP-2612) or increaseAllowance.

False-positive notes

• Atomic protocol-internal swaps with no user-controlled output token aren't front-runnable.
• Sequenced L2s (Arbitrum One, Base) currently have no public mempool ordering attack — flag risk as L2-conditional.
• Deadline checks that come from a trusted off-chain oracle/router (e.g. CoW Protocol) can be skipped.

Related

• [[oracle-manipulation]] — sandwich attacks are oracle moves
• [[approval-issues]]
• [[signature-replay]] — replayable signed messages are MEV gold