Front-running awareness in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Front-running awareness

User sends transaction

↓

Transaction enters mempool

↓

Miner/Validator sees transaction

↓

Miner can reorder transactions

↓

Front-runner inserts own transaction before yours

↓

Your transaction executes after front-runner

↓

Front-runner profits from your transaction

This flow shows how a transaction sent by a user can be seen and reordered by miners or validators to insert a front-running transaction before it.

Execution Sample

Blockchain / Solidity

mempool = []
user_tx = {'type': 'buy', 'amount': 10}
front_runner_tx = {'type': 'buy', 'amount': 20}
mempool.append(user_tx)
mempool.insert(0, front_runner_tx)
# Simulate execution by clearing mempool
mempool.clear()

This code simulates a user transaction entering the mempool, then a front-runner inserts their transaction before it, and finally transactions execute in order.

Execution Table

Step	Action	Mempool State	Explanation
1	User sends transaction	[{'type': 'buy', 'amount': 10}]	User's transaction added to mempool
2	Front-runner inserts transaction at front	[{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]	Front-runner's transaction placed before user's
3	Execute transactions in order	[]	Transactions executed: front-runner first, user second
4	End	[]	No more transactions to process

💡 All transactions executed; front-runner's transaction ran before user's

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
mempool	[]	[{'type': 'buy', 'amount': 10}]	[{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]	[]	[]

Key Moments - 3 Insights

Why does the front-runner's transaction appear before the user's in the mempool?

Because miners or validators can reorder transactions in the mempool to place their own transactions first, as shown in step 2 of the execution_table.

Does the user know their transaction will be executed second?

No, the user sends the transaction without control over ordering; front-runners exploit this by inserting transactions before, as seen in the mempool state changes.

What happens if the front-runner does not insert a transaction?

Then the user's transaction executes first, as the mempool remains unchanged after step 1, shown in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the order of transactions in the mempool?

A []

B [{'type': 'buy', 'amount': 10}, {'type': 'buy', 'amount': 20}]

C [{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]

D [{'type': 'sell', 'amount': 5}]

💡 Hint

Check the 'Mempool State' column at step 2 in the execution_table

At which step does the mempool become empty?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for '[]' in the 'Mempool State' column in the execution_table

If the front-runner did not insert their transaction, what would the mempool look like after step 2?

A [{'type': 'buy', 'amount': 10}]

B []

C [{'type': 'buy', 'amount': 20}]

D [{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]

💡 Hint

Refer to the mempool state after step 1 and imagine no insertion at step 2

Concept Snapshot

Front-running happens when miners reorder transactions in the mempool.
User sends transaction -> enters mempool.
Miner inserts own transaction before user's.
Transactions execute in new order.
User's transaction runs after front-runner's.
This can cause unfair profit for front-runners.

Full Transcript

Front-running awareness means understanding how transactions sent by users can be seen by miners or validators before they are confirmed. These miners can reorder transactions in the mempool, placing their own transactions ahead of users' transactions. This lets front-runners profit unfairly by acting first. The example code shows a user transaction entering the mempool, then a front-runner inserts their transaction at the front. The execution table traces each step: user transaction added, front-runner inserts before it, then transactions execute in order. The variable tracker shows how the mempool changes over time. Key moments clarify why the front-runner's transaction appears first and that users cannot control this ordering. The quiz tests understanding of mempool order and execution steps. Remember, front-running exploits transaction ordering in blockchain systems.

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