Front-running awareness in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to detect if a transaction is front-run by checking if the current gas price is higher than the previous one.

Blockchain / Solidity

if current_gas_price [1] previous_gas_price:
    print("Possible front-running detected")

Drag options to blanks, or click blank then click option'

A>

B<=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' causes the check to miss front-running.

Using '==' only checks equality, not increase.

The code checks if the current gas price is greater than the previous gas price, which can indicate front-running.

2fill in blank

medium

Complete the code to add a delay before sending a transaction to reduce front-running risk.

Blockchain / Solidity

import time

time.[1](5)  # wait 5 seconds before sending transaction

Drag options to blanks, or click blank then click option'

Await

Bsleep

Cdelay

Dpause

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent functions like 'wait' or 'delay' causes errors.

Misspelling 'sleep' leads to runtime errors.

The time.sleep() function pauses the program for the given number of seconds.

3fill in blank

hard

Fix the error in the code that tries to check if a transaction nonce is less than the last confirmed nonce.

Blockchain / Solidity

if tx_nonce [1] last_confirmed_nonce:
    print("Transaction might be front-run")

Drag options to blanks, or click blank then click option'

A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' or '>' reverses the logic and misses front-running.

Using '<=' includes equal nonces which is not the intended check.

The code should check if the transaction nonce is less than the last confirmed nonce to detect replay or front-running.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps transaction hashes to their gas prices only if the gas price is higher than 100 gwei.

Blockchain / Solidity

tx_gas_prices = {tx_hash: tx[1] for tx_hash, tx in transactions.items() if tx['gas_price'] [2] 100}

Drag options to blanks, or click blank then click option'

A['gas_price']

B>

C<

D['nonce']

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong key like 'nonce' instead of 'gas_price'.

Using '<' instead of '>' reverses the filter logic.

The comprehension extracts the gas price from each transaction and includes only those with gas price greater than 100.

5fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps user addresses to their transaction counts only if the count is greater than 10.

Blockchain / Solidity

user_tx_counts = {user[1]: tx_count for user, tx_count in user_transactions.items() if tx_count [2] 10}

Drag options to blanks, or click blank then click option'

A.lower()

C>

D.upper()

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the user address unchanged may cause inconsistent keys.

Using '<' instead of '>' filters the wrong transaction counts.

The comprehension converts user addresses to uppercase, keeps the transaction count as is, and filters counts greater than 10.

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