What Is Front Running?

TL;DR: Front running is the practice of using advance knowledge of a pending transaction to place your own order first and profit from the price movement that follows. In traditional finance, it's illegal insider trading. In crypto and DeFi, it's a structural feature of public blockchains: validators and bots exploit the visibility of unconfirmed transactions in the mempool to extract value, a practice known as MEV (Maximal Extractable Value). Same concept, radically different enforcement environment.

Front Running in Traditional Finance

In traditional markets, front running occurs when a broker, trader, or anyone with advance knowledge of a large pending order uses that information to trade ahead of it for personal gain.

The mechanism is straightforward: a broker receives a large buy order from a client that will, when executed, push the price of an asset up. Before executing the client's order, the broker buys the same asset with their own account. The client's order then executes, driving the price up as expected, and the broker sells their position at a profit at the client's expense.

Front running in traditional finance is illegal in most jurisdictions. In the United States, it violates securities law under the Securities Exchange Act and constitutes a breach of fiduciary duty. Enforcement is handled by the SEC and FINRA, and documented cases have resulted in fines, trading bans, and criminal prosecution.

It's worth distinguishing front running from legal practices that can look similar at a distance. High-frequency trading (HFT) firms using superior technology to react faster to public market data are generally legal — there's no non-public information involved. Front running specifically requires trading on knowledge of a specific pending order that others can't see.

Front Running in Crypto and DeFi

In public blockchains, front running is structural rather than illegal, because everything that makes it possible in traditional markets (advance knowledge of pending transactions) is simply how the technology works.

When a user submits a transaction on Ethereum or another public blockchain, that transaction enters the mempool, which is a public queue of unconfirmed transactions visible to anyone. Before a block is produced, validators and specialized bots scan the mempool continuously, looking for transactions they can profit from by adjusting the ordering.

This is MEV — Maximal Extractable Value. Front running is one of the most common MEV strategies:

DEX front running. A bot detects a large pending swap on a decentralized exchange that will move the price of a token. It submits its own buy transaction with a higher gas fee to ensure it executes first, then sells immediately after the victim's transaction completes — capturing the price impact at the user's expense. This is also called a sandwich attack when combined with a sell transaction placed immediately after.
Liquidation front running. When a DeFi position becomes eligible for liquidation, multiple bots race to be the first to execute it and claim the liquidation bonus.
Arbitrage front running. Bots detect price discrepancies between DEXs before other arbitrageurs can act, inserting transactions to capture the spread.

Unlike traditional finance, this activity is generally not illegal on public blockchains — the mempool is public, and the rules of the network don't prohibit transaction reordering. Whether it's ethically acceptable is a different and actively debated question within the crypto community.

Front running on Solana operates somewhat differently because of Solana's architecture. The absence of a traditional mempool (Solana uses a different transaction forwarding model) reduces some MEV opportunities, but front running still occurs through validator-level manipulation and through infrastructure like Jito Labs, which provides MEV-aware block building tools and a separate transaction mempool that searchers can access.

How Front Running Attacks Work

The mechanics differ slightly between traditional and crypto contexts, but the core structure is the same:

In traditional finance:

A broker or insider gains knowledge of a large pending order (a client's buy order for 100,000 shares, for example).
The insider buys the asset in their own account before executing the client's order.
The client's large order executes, moving the price upward.
The insider sells their position at the higher price, pocketing the difference.
The client's order is filled at a worse average price than it would have been without front-running.

In DeFi (sandwich attack example):

A user submits a large swap on a DEX — say, buying $100,000 of Token A with USDC.
A MEV bot detects this transaction in the mempool.
The bot submits a buy transaction for Token A with a higher gas fee, ensuring it executes first (the "front" of the sandwich).
The user's transaction executes, driving the price of Token A up due to the large size.
The bot immediately submits a sell transaction, executing right after the user's swap (the "back" of the sandwich).
The bot profits from the price movement caused by the user's transaction. The user receives fewer tokens than expected.

The user in this example experiences it as slippage. They get fewer tokens than the quoted price without necessarily understanding that a bot extracted value from their trade.

How to Protect Against Front Running

In DeFi:

Slippage tolerance settings. Most DEX interfaces allow users to set a maximum acceptable slippage. Setting it low reduces the profit opportunity for sandwich bots, making the transaction less attractive to front run.
Private RPC endpoints and mempools. Services like Flashbots Protect (Ethereum) and Jito's private transaction channels (Solana) allow users to submit transactions directly to block builders without broadcasting to the public mempool, eliminating the visibility that makes front running possible.
Commit-reveal schemes. For protocols, implementing a two-phase transaction model where users commit to an action without revealing details until a later block prevents bots from seeing and exploiting the specifics in advance.
Frequent batch auctions. Some DEX designs (like CoW Protocol) batch trades together and execute them at a uniform clearing price rather than sequentially, eliminating the transaction ordering advantage that MEV bots exploit.
MEV-aware DEX design. Protocol-level design choices, including on-chain randomness for ordering and time-weighted average price (TWAP) oracles, reduce the attack surface available to front runners.

In traditional finance:

Dark pools. Large institutional orders are sometimes executed through private trading venues not visible to the public market, reducing the window for front running.
Regulatory oversight. Compliance programs, trade surveillance systems, and clear separation between client-facing and proprietary trading desks are the primary institutional defenses.

Conclusion

Front running is one of those concepts that looks completely different depending on the context — a criminal act in traditional markets, a structural reality in public blockchains. What they share is the exploitation of information asymmetry: knowing what's about to happen and acting on it before others can. In DeFi, where the information is public by design, the response has been engineering — private mempools, batch auctions, and slippage controls rather than regulation. Neither environment has fully solved the problem, but the crypto ecosystem's transparent nature at least makes the scale of value extraction measurable in a way traditional markets never were.