Paweł Rejkowicz
min read
Last Update:
May 21, 2024

In the world of finance, ensuring a level playing field for all participants is paramount. However, the advent of blockchain technology and decentralized exchanges (DEXs) introduces a wrinkle to this principle – the possibility of front-running. Front-running, the practice of getting a transaction into the blockchain ahead of a known, pending transaction for profit, exists in traditional finance but poses an even greater threat within the transparent environment of decentralized finance (DeFi).  DeFi's inherent visibility of transactions amplifies the potential for manipulation to the detriment of regular market participants.

How Does Blockchain Front-Running Work?

To understand front-running, it's crucial to grasp the mechanics behind how transactions are processed on a blockchain. Miners or validators, depending on the blockchain's consensus mechanism, play the vital role of ordering and including transactions in new blocks. Before a transaction gets confirmed on the blockchain, it sits in a waiting area called the mempool. This pool of pending transactions is publicly visible, providing a valuable window of opportunity for those with the right tools.

Malicious actors actively monitor the mempool, hunting for potentially profitable transactions. If they spot, for example, a large buy order, they can place their buy order ahead of it by paying a higher gas fee (transaction cost). This strategic move drives up the price of the asset, allowing the front-runner to then sell their holdings immediately after the original large order executes, pocketing a profit from the price difference. Moreover, a significant challenge in mitigating front-running comes from the miners themselves, who have complete control over the transaction order in a block. 

This control not only allows them to prioritize transactions but also potentially to front-run any participant in the network, including other front-runners, if their interests are not aligned, complicating the dynamics of transaction ordering further

Types of Front-Running Attacks

Front-running takes on various forms, each exploiting market information gleaned from the mempool.

  • Sandwich Attacks: This is the bread-and-butter of front-running.  An attacker spots a sizable buy order in the mempool and quickly places a buy order of their own just before it, with a higher transaction fee to ensure priority. This artificially boosts the price, allowing them to then sell at the new, inflated price once the larger order triggers, pocketing a quick profit.

    A whale (large investor) is about to buy a significant amount of a less popular cryptocurrency token. Bots monitoring the mempool spot this and place a flurry of smaller "front-running" buy orders, driving the price up. Right after the whale's order executes, these bots sell for a profit.

  • Simple front-running: It occurs when an observer notices an unconfirmed transaction in the mempool that will likely impact the price of a cryptocurrency and acts on this information before the original transaction is processed. This type of front-running is straightforward and direct, without the complexities of other strategies like sandwich attacks or displacement.

    Example: Consider a scenario where a trader plans to buy a large amount of Ethereum, which is likely to push the price higher once executed. A front-runner sees this pending transaction in the mempool and quickly submits their own buy order for Ethereum with a higher transaction fee to ensure it gets processed first. As the front-runner anticipates, the price of Ethereum rises following the execution of the large trade. The front-runner then sells their Ethereum at a higher price, securing a profit-driven by the initial trader's impact on the market.

  • Back-running: The mirror image of front-running. Here, a large sell order is the trigger. The attacker places a sell order right after, hoping to ride the temporary price dip caused by the large sell-off. They then buy back at an even lower price, profiting from the spread.

    Example: Someone needs to liquidate a big position quickly. Front-runners alerted to the large sell order, place their sell orders to further depress the price. They then buy back the asset at the lowered price for a gain.

  • Displacement front-running: Aggressive and costly form of front-running where attackers identify a valuable transaction in the mempool, such as an arbitrage opportunity, which they cannot preempt directly. Instead, they flood the network with transactions that carry exceptionally high gas fees but are intended to fail. These transactions dominate the mempool, delaying or even displacing the original transaction. This not only allows the attacker to place their transaction first but also hampers overall network performance, making it difficult for others to use.

    Example: Imagine an arbitrage bot that detects a price discrepancy between two cryptocurrency exchanges, which could yield significant profits. A competing bot, recognizing the same opportunity but unable to get its transaction into the mempool ahead of the original bot, decides to clog the network. It sends multiple high-fee transactions designed to fail, congesting the network and delaying the original bot's arbitrage transaction. This displacement gives the competing bot the chance to execute its transaction first, capturing the arbitrage profit while temporarily reducing the network's usability for everyone else.

Risks and Negative Impacts of Front-Running

Front-running poses a significant threat to the integrity of decentralized markets and has far-reaching negative consequences. Regular traders bear the brunt of the damage, often unknowingly buying assets at artificially inflated prices or selling them at unfairly lowered ones. This manipulation not only results in immediate financial losses but also profoundly erodes trust in DeFi's promise of a level playing field. However, it's important to recognize that front-running is not a universal problem across all decentralized applications. The issue primarily stems from two sources: the design of the most popular decentralized apps (dApps), which often do not limit buy/sell orders to specific prices, and certain blockchain designs that inadvertently facilitate front-running. In such blockchains, even setting transaction limits does not guarantee security, as miners can manipulate the transaction queue—failing to execute your transaction while still charging fees, thus adding complexity to resolving these issues.

The fear of front-running can discourage participation, hindering the growth and adoption of decentralized platforms. Furthermore, large-scale front-running has the potential to exacerbate market volatility and contribute to systemic instability. This undermines the long-term health of the decentralized financial ecosystem and could even impact the perception of blockchain technology as a whole.

How to Protect Yourself from Front-Running

While eliminating front-running is an ongoing challenge, there are measures traders and platforms can take to minimize its impact:

Strategies for Traders

  • Slippage Tolerance: When placing orders, carefully adjust your slippage tolerance (the amount of price change you're willing to accept). A lower slippage tolerance from the other side can make your transaction fail and you still have to pay for it.
  • Splitting Orders: Break down large orders into smaller ones. This makes it harder for front-runners to spot and exploit your full trading intent.
  • Front-running Resistant DEXs: Choose decentralized exchanges that have implemented specific anti-front-running measures (these will be discussed below).
  • Monitoring Tools: Some services offer tools to monitor the mempool and alert you to potential front-running activity on your trades. 

Role of Platforms

Decentralized exchanges and blockchain projects have a crucial responsibility in mitigating front-running. One effective strategy is to implement transaction reordering techniques within a block that focus on fairness, such as prioritizing transactions based on the time of submission rather than solely on gas fees. Additionally, these platforms can create private transaction pools where traders can submit orders that remain obscured from the public mempool and are executed only after a specific time or condition is met. These measures are essential for maintaining integrity and fairness in the execution of orders on decentralized platforms.

Front-Running: An Ongoing Challenge

Front-running presents a complex challenge within the blockchain environment, raising both ethical dilemmas and technical hurdles. It's a constant battle between those seeking to exploit the system and those developing increasingly sophisticated countermeasures. On the blockchain, notions of justice and ethics are challenging to enforce, leaving market participants with practical considerations as their primary guide. 

The only consolation is that a frontrunner can only profit to the extent of your price impact. If the cost incurred from your price impact is less than the transaction fees, your trade is relatively safe—unless external factors come into play. However, if the impact cost exceeds your fees, it may be wise to explore other trading options. Staying informed about these risks is crucial for traders as they navigate decentralized markets. Ultimately, solutions will likely require a blend of individual awareness, responsible trading practices, and the ongoing evolution of blockchain technology itself.

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