Front-running awareness in Blockchain / Solidity - Mini Project: Build & Apply

Front-running Awareness in Blockchain Transactions

📖 Scenario: You are working with blockchain transaction data. Front-running is when someone sees a transaction before it is confirmed and tries to act first to gain an advantage. We want to detect possible front-running by checking if a transaction with a higher gas price was submitted shortly after another transaction.

🎯 Goal: Build a simple program that stores a list of blockchain transactions, sets a time threshold, finds transactions that might be front-run based on gas price and time, and prints those suspicious transactions.

📋 What You'll Learn

Create a list of transactions with exact details

Add a time threshold variable

Use a loop to find transactions that might be front-run

Print the suspicious transactions

💡 Why This Matters

🌍 Real World

Detecting front-running helps protect users from unfair transaction ordering in blockchain networks.

💼 Career

Blockchain developers and security analysts use such techniques to monitor and improve transaction fairness.

1

Create the transaction list

Create a list called transactions with these exact dictionaries: {'tx_id': 'tx1', 'gas_price': 50, 'timestamp': 1000}, {'tx_id': 'tx2', 'gas_price': 70, 'timestamp': 1005}, {'tx_id': 'tx3', 'gas_price': 40, 'timestamp': 1010}, {'tx_id': 'tx4', 'gas_price': 80, 'timestamp': 1012}.

Blockchain / Solidity

# Create the list of transactions with exact entries
# Your code here

Hint

Use a list of dictionaries with keys 'tx_id', 'gas_price', and 'timestamp'.

2

Set the time threshold

Create a variable called time_threshold and set it to 7.

Blockchain / Solidity

transactions = [
    {'tx_id': 'tx1', 'gas_price': 50, 'timestamp': 1000},
    {'tx_id': 'tx2', 'gas_price': 70, 'timestamp': 1005},
    {'tx_id': 'tx3', 'gas_price': 40, 'timestamp': 1010},
    {'tx_id': 'tx4', 'gas_price': 80, 'timestamp': 1012}
]
# Set the time threshold variable
# Your code here

Hint

Just create a variable with the exact name and value.

3

Find possible front-running transactions

Create an empty list called front_running. Use a for loop with variables i and j to compare each pair of transactions in transactions. If transaction j has a higher gas_price than transaction i and the difference in timestamp is less than time_threshold, append a tuple (transactions[i]['tx_id'], transactions[j]['tx_id']) to front_running.

Blockchain / Solidity

transactions = [
    {'tx_id': 'tx1', 'gas_price': 50, 'timestamp': 1000},
    {'tx_id': 'tx2', 'gas_price': 70, 'timestamp': 1005},
    {'tx_id': 'tx3', 'gas_price': 40, 'timestamp': 1010},
    {'tx_id': 'tx4', 'gas_price': 80, 'timestamp': 1012}
]
time_threshold = 7
# Find front-running pairs and store in front_running
# Your code here

Hint

Use nested loops to compare each transaction pair and check conditions.

4

Print suspicious transactions

Write a print statement to display the front_running list.

Blockchain / Solidity

transactions = [
    {'tx_id': 'tx1', 'gas_price': 50, 'timestamp': 1000},
    {'tx_id': 'tx2', 'gas_price': 70, 'timestamp': 1005},
    {'tx_id': 'tx3', 'gas_price': 40, 'timestamp': 1010},
    {'tx_id': 'tx4', 'gas_price': 80, 'timestamp': 1012}
]
time_threshold = 7
front_running = []
for i in range(len(transactions)):
    for j in range(i + 1, len(transactions)):
        if (transactions[j]['gas_price'] > transactions[i]['gas_price'] and
            transactions[j]['timestamp'] - transactions[i]['timestamp'] < time_threshold):
            front_running.append((transactions[i]['tx_id'], transactions[j]['tx_id']))
# Print the front_running list
# Your code here

Hint

Just print the list variable front_running.

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