Last night, Polymarket entered a maintenance window, paused trading, cleared the order book, and then officially launched CLOB V2.
According to the official disclosure, this upgrade includes a new contract, a new order book, a new collateral token Polymarket USD (PUSD), and a new version of the CLOB Client SDK. For users, these changes such as PUSD, SDK, and order structure may not be immediately noticeable.
What is really worth paying attention to from the very beginning is the Ghost Fills issue that has long plagued Polymarket, also known as the "Ghost Order" problem.
V2 did address this issue. The previously easily exploitable nonce mechanism has been removed, and the order structure and cancellation method have also changed.
However, this does not mean that ghost orders have completely disappeared because Polymarket's core trading model still involves off-chain matching and on-chain settlement. As long as there is a time difference between these two steps, similar issues are difficult to completely eliminate.
Order Shows Trade, Why Does It Eventually Fail?
The so-called ghost order is simply an order that appears to have been successfully matched off-chain, but ultimately did not settle on-chain.
Polymarket adopts off-chain order book matching, then returns to on-chain settlement mode. The advantage of this design is clear: faster transaction speed, lower cost, and more suitable for prediction markets with short cycles and high frequency such as 5-minute markets.
However, the problem lies in this time difference. Just because the off-chain order book shows a trade has been executed does not guarantee a successful on-chain settlement.
In some short-cycle markets, users may see that their orders have been executed off-chain and think they have already bought or sold in the intended direction. But when the transaction is actually submitted on-chain, the settlement fails. A transaction that seemed completed a moment ago is then retracted by the system.
For users, the most frustrating part of this experience is not just the failure itself but the uncertainty. Thinking that a buy or sell order has been completed, only to find out that it hasn't; by the time they replace the order, the price may have changed, and the trading opportunity may have been missed.
The Issue with the Old Version Was the Low Cost of Cancelling Orders
In V1, the most vulnerable path for front-running orders was through incrementNonce. Nonce can be understood as the state identifier in an order. Originally designed to help manage orders within the system, in the old version, attackers could call incrementNonce to invalidate orders with an old nonce on-chain during settlement.
This gave attackers a time delta to act. Attackers could first match orders off-chain, showing the system that the "trade has happened"; then, before settlement on-chain, update the nonce, causing these orders to ultimately fail to execute. The result was that a transaction that appeared to be completed ultimately did not settle on-chain.
The key issue was that this operation had an extremely low cost but could impact a set of orders. Attackers only needed to pay a very low gas cost to make orders that should have been executed fail during settlement. While the frontend displayed a seemingly completed order that later failed, the actual result was transaction unreliability, potentially causing users to miss out on their intended execution price and trading opportunity.
The front-running order issue is not merely a display error on the frontend or an occasional on-chain failure; it directly affects user trust in the transaction outcome.
V2 Patched the Issue, but Did Not Completely Eradicate It
The most critical change in V2 this time was the removal of the original global nonce design. This means that the method of affecting a set of old orders at once through incrementNonce in the past has been blocked. Additionally, V2 simplified the order structure, and cancellations now involve more granular single-order hashes. Compared to the old version, the scope of cancellation impact has been significantly reduced, making it difficult for attackers to disrupt a large number of orders through a single low-cost operation.
This was a substantial fix for the front-running order issue. In the past, the problem was that the cost of attack was low, the impact was widespread, and the reproducibility threshold was low.
After V2, the most vulnerable path to exploitation has been eliminated. If attackers want to continue creating similar problems, they need to incur higher costs and rely more on specific system responses. Furthermore, mechanisms such as pauseUser introduce delays, reducing the potential for certain state changes to be instantly abused within the matching and settlement window.
Overall, the direction of V2 is clear: first address the most vulnerable points that attackers can exploit, and then reduce the profit margin for such attacks.
However, this does not mean the front-running issue is completely resolved. This is because Polymarket still has not changed its basic model of off-chain matching and on-chain settlement.
As long as orders are not matched and settled in the same environment, there will always be a state gap between off-chain and on-chain. Changes in balances, authorization issues, order status changes, cancellation actions, or contract execution failures can all prevent an off-chain matched order from being eventually settled on-chain.
In other words, V2 addresses the most obvious and easily exploitable attack vector in the old version, rather than the underlying conditions that give rise to front-running.
Other Updates, More Like Safety Nets for the Trading System
In addition to front-running, V2 also introduces updates such as PUSD, SDK, and 1271 signatures:
· PUSD is a new collateralized stablecoin where Polymarket migrates from USDC.e to Polymarket USD backed 1:1 by USDC, making it almost imperceptible to regular users but providing a more standardized approach to underlying asset handling;
· The new version of the CLOB-Client SDK is mainly aimed at market makers, bots, and system integrators. After V2, relevant users need to upgrade their clients and resign orders with the new order structure;
· Support for 1271 signatures means smart contract wallets, multi-sig accounts, institutional accounts, and more sophisticated bot wallets can more smoothly connect to Polymarket.
Overall, Polymarket is not simply fixing a bug but transforming itself from a prediction market application into a more trading platform-like infrastructure. As more market makers, API users, and automated traders join, the ability for orders to reliably execute, settle, and clear becomes more important than whether the market is entertaining enough.
V2 Is Not the End but the Beginning of Ongoing Patches
With the launch of V2, Polymarket has at least blocked the most obvious front-running attack vector. The previous method of low-cost cancellations that could affect orders in bulk is now challenging to replicate as before. For a rapidly growing trading platform, this is a necessary step.
However, the root cause behind front-running will not completely disappear with a single version upgrade. As long as Polymarket continues to use the off-chain matching and on-chain settlement model, the system will need to continuously address the discrepancies between off-chain states and on-chain outcomes. V2 is more like the first step—taking care of the most obvious and easily exploitable issue first, and then continuing to enhance matching, settlement, monitoring, and risk management capabilities through subsequent updates.
The essence of the prediction market is uncertainty in itself. If even the order is filled with uncertainty, users are no longer facing just market risk, but system risk.
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