Front-running awareness in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Detecting front-running in a transaction log

Given a list of blockchain transactions with timestamps and prices, what is the output of the code that detects if a transaction was front-run?

Blockchain / Solidity

transactions = [
    {'tx_id': 'a1', 'timestamp': 100, 'price': 10},
    {'tx_id': 'b2', 'timestamp': 101, 'price': 12},
    {'tx_id': 'c3', 'timestamp': 102, 'price': 11}
]

front_run_detected = False
for i in range(1, len(transactions)):
    if transactions[i]['price'] < transactions[i-1]['price']:
        front_run_detected = True
        break
print(front_run_detected)

A True

B False

C SyntaxError

D IndexError

💡 Hint

Look for a price drop after a higher price transaction.

The code checks if any transaction has a lower price than the previous one, indicating a possible front-run. Here, price drops from 12 to 11, so it prints True.

🧠 Conceptual

intermediate

Understanding front-running impact on transaction ordering

Which statement best describes the impact of front-running on blockchain transaction ordering?

A Front-running removes transactions from the blockchain permanently.

B Front-running delays all transactions equally to maintain fairness.

C Front-running changes the order of transactions to benefit the attacker financially.

D Front-running encrypts transactions to hide their content.

💡 Hint

Think about how attackers gain advantage by reordering.

Front-running involves reordering transactions so the attacker profits by acting before others.

🔧 Debug

advanced

Identify the error causing incorrect front-running detection

Why does the following code fail to detect front-running correctly?

Blockchain / Solidity

txs = [{'id': 'x1', 'price': 5}, {'id': 'x2', 'price': 7}, {'id': 'x3', 'price': 6}]

front_run = False
for i in range(len(txs)):
    if txs[i]['price'] < txs[i-1]['price']:
        front_run = True
        break
print(front_run)

A The list txs is empty, so the loop never runs.

B The price comparison uses '<' instead of '>' causing logic error.

C The variable front_run is never updated inside the loop.

D The loop starts at 0, so txs[i-1] is invalid on first iteration causing wrong comparison.

💡 Hint

Check the index used in the loop and how it accesses previous elements.

The loop starts at 0, so txs[i-1] accesses txs[-1] (last element) on first iteration, causing incorrect logic.

📝 Syntax

advanced

Identify the syntax error in front-running detection code

Which option contains the syntax error preventing the code from running?

Blockchain / Solidity

def detect_front_run(txs):
    for i in range(1, len(txs))
        if txs[i]['price'] < txs[i-1]['price']:
            return True
    return False

A Missing colon ':' after for loop declaration.

B Missing parentheses in function call range len(txs).

C Incorrect indentation of return statements.

D Using single quotes instead of double quotes in dictionary keys.

💡 Hint

Look carefully at the for loop syntax.

Python requires a colon ':' at the end of for loop declarations.

🚀 Application

expert

Calculate number of front-run opportunities in transaction batch

Given a batch of transactions with prices, how many front-run opportunities exist where a transaction is immediately followed by a lower price?

Blockchain / Solidity

transactions = [
    {'tx_id': 't1', 'price': 20},
    {'tx_id': 't2', 'price': 25},
    {'tx_id': 't3', 'price': 22},
    {'tx_id': 't4', 'price': 30},
    {'tx_id': 't5', 'price': 28}
]

count = 0
for i in range(1, len(transactions)):
    if transactions[i]['price'] < transactions[i-1]['price']:
        count += 1
print(count)

A 3

B 2

C 1

D 0

💡 Hint

Count how many times price drops compared to previous transaction.

Price drops occur from 25 to 22 and from 30 to 28, so count is 2.

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