Front-running awareness in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is front-running in blockchain transactions?

Front-running is when someone sees a pending transaction and tries to get their own transaction processed first to benefit from the information, often by paying higher fees.

beginner

How does front-running affect decentralized exchanges (DEXs)?

Front-running can cause users to get worse prices because attackers place orders before them, exploiting the time delay in transaction processing.

intermediate

What is a common method to reduce front-running risk on blockchain?

Using techniques like transaction ordering protocols, commit-reveal schemes, or private transaction pools to hide transaction details until confirmed.

intermediate

Explain the role of gas fees in front-running.

Attackers pay higher gas fees to get miners to prioritize their transactions, allowing them to jump ahead of others in the transaction queue.

advanced

What is MEV (Miner Extractable Value) and how is it related to front-running?

MEV is the profit miners can make by reordering, including, or excluding transactions in a block, often exploiting front-running opportunities.

What does front-running in blockchain usually involve?

A Paying higher fees to get a transaction processed first

B Submitting a transaction before seeing others

C Copying a transaction after it is confirmed

D Ignoring transaction fees

Front-running involves paying higher fees to get a transaction processed before others.

Which of these helps reduce front-running risk?

A Increasing transaction size

B Broadcasting transactions publicly early

C Using low gas fees

D Commit-reveal schemes

Commit-reveal schemes hide transaction details until confirmation, reducing front-running.

What role do miners play in front-running?

A They ignore transaction order

B They only process transactions with zero fees

C They can reorder transactions for profit

D They prevent front-running automatically

Miners can reorder transactions to maximize their profit, enabling front-running.

What is MEV in blockchain?

A Miner Extractable Value

B Maximum Ethereum Value

C Minimum Exchange Volume

D Multi-Exchange Verification

MEV stands for Miner Extractable Value, related to profits from transaction ordering.

Why do attackers pay higher gas fees in front-running?

A To slow down their transactions

B To get their transactions processed faster

C To reduce transaction size

D To avoid transaction confirmation

Higher gas fees incentivize miners to process transactions faster, helping attackers front-run.

Describe what front-running is and why it is a problem in blockchain.

Think about how someone might try to get ahead in the transaction queue.

Explain at least two methods used to reduce front-running risks in blockchain systems.

Consider how hiding or delaying transaction details can help.

Practice

1. What does front-running mean in blockchain transactions?

easy

A. A user sending a transaction with insufficient gas fees

B. Someone sees your pending transaction and acts before it to gain advantage

C. A miner refusing to include your transaction in a block

D. A wallet automatically signing transactions without user approval

Solution

1. Step 1: Understand front-running concept

   Front-running happens when someone notices your transaction before it is confirmed and tries to act first to benefit.

2. Step 2: Identify correct description

   Someone sees your pending transaction and acts before it to gain advantage correctly describes this behavior, while others describe unrelated blockchain actions.

3. Final Answer:

   Someone sees your pending transaction and acts before it to gain advantage -> Option B

4. Quick Check:

   Front-running = Acting before your transaction [OK]

Hint: Front-running means acting before your transaction [OK]

Common Mistakes:

• Confusing front-running with low gas fees
• Thinking front-running is miner censorship
• Mixing front-running with wallet automation

2. Which of the following is a correct way to reduce front-running risk in smart contracts?

easy

A. Use commit-reveal scheme to hide transaction details until reveal phase

B. Broadcast transactions with zero gas fees to avoid attention

C. Send transactions only during network congestion to delay miners

D. Use public mempool to announce your transaction early

Solution

1. Step 1: Review front-running prevention methods

   Commit-reveal schemes hide transaction details until a later phase, preventing others from copying or front-running.

2. Step 2: Evaluate options

   Use commit-reveal scheme to hide transaction details until reveal phase correctly uses commit-reveal. Options B, C, and D either increase risk or are ineffective.

3. Final Answer:

   Use commit-reveal scheme to hide transaction details until reveal phase -> Option A

4. Quick Check:

   Commit-reveal = Prevent front-running [OK]

Hint: Commit-reveal hides info to stop front-running [OK]

Common Mistakes:

• Thinking zero gas fees hide transactions
• Believing network congestion prevents front-running
• Using public mempool to announce transactions early

3. Consider this simplified pseudocode for a transaction queue:

queue = [tx1, tx2, tx3]
miner_orders = [tx3, tx1, tx2]
for tx in miner_orders:
    process(tx)

Which transaction is most likely to be front-run if tx1 was broadcast first?

medium

A. tx3

B. tx2

C. None, order is random

D. tx1

Solution

1. Step 1: Analyze transaction broadcast and miner order

   tx1 was broadcast first, but miner processes tx3 before tx1.

2. Step 2: Identify front-run transaction

   Since tx3 is processed before tx1, tx3 front-runs tx1.

3. Final Answer:

   tx3 -> Option A

4. Quick Check:

   Miner order front-runs broadcast order = tx3 [OK]

Hint: Miner order can front-run broadcast order [OK]

Common Mistakes:

• Assuming broadcast order equals processing order
• Ignoring miner transaction reordering
• Thinking no front-running if all txs are in queue

4. This Solidity snippet aims to prevent front-running using a commit-reveal scheme:

mapping(address => bytes32) public commitments;

function commit(bytes32 hash) public {
    commitments[msg.sender] = hash;
}

function reveal(uint256 value, string memory secret) public {
    require(commitments[msg.sender] == keccak256(abi.encodePacked(value, secret)), "Invalid reveal");
    // process value
}

What is the bug that could allow front-running here?

medium

A. The reveal function does not check msg.sender ownership

B. The hash comparison uses incorrect encoding causing mismatch

C. The commit function overwrites previous commitments without expiration

D. The secret is revealed in commit phase exposing value

Solution

1. Step 1: Understand commit-reveal logic

   Commit stores a hash, reveal checks hash matches value and secret.

2. Step 2: Identify bug in commit function

   Commit overwrites previous commitments without expiration, allowing replay or front-running by reusing old commits.

3. Final Answer:

   The commit function overwrites previous commitments without expiration -> Option C

4. Quick Check:

   Overwriting commits = front-running risk [OK]

Hint: Commit must expire to prevent front-running [OK]

Common Mistakes:

• Thinking reveal lacks ownership check
• Assuming encoding is incorrect
• Believing secret is revealed too early

5. You want to design a decentralized exchange (DEX) smart contract to reduce front-running attacks on trades. Which combined approach is best?

hard

A. Use zero gas fees and randomize transaction order in contract

B. Broadcast all orders publicly immediately and rely on miners to order fairly

C. Only accept orders from trusted addresses to avoid front-running

D. Use commit-reveal for order submission and allow users to pay higher gas fees for priority

Solution

1. Step 1: Evaluate front-running prevention techniques

   Commit-reveal hides order details until reveal, reducing front-running. Paying higher gas fees can prioritize transactions.

2. Step 2: Assess options for DEX design

   Use commit-reveal for order submission and allow users to pay higher gas fees for priority combines commit-reveal and gas fee priority, effectively reducing front-running. Others either increase risk or are impractical.

3. Final Answer:

   Use commit-reveal for order submission and allow users to pay higher gas fees for priority -> Option D

4. Quick Check:

   Commit-reveal + gas fees = best front-running defense [OK]

Hint: Combine commit-reveal and gas fees to reduce front-running [OK]

Common Mistakes:

• Trusting miners to order fairly without protection
• Restricting orders to trusted addresses limits decentralization
• Using zero gas fees removes transaction priority