Bet on a coin flip that’s already landed and you’ll win a lot. That’s roughly what 821 accounts were doing on Polymarket, and researchers put a number on it: $8.2 million pulled out of settlement windows they classified as likely manipulated.
Polymarket has now changed how its short-dated crypto contracts resolve. The single-price snapshot is gone, replaced by a time-weighted average price, or TWAP. It took months of trader complaints and an academic paper to get there.
The new math is specific. Five-minute markets will use a 30-second average, while 15-minute and four-hour markets will use a 60-second average, the platform explained. The data will be delivered through Chainlink Data Streams, it added.
The exploit was never sophisticated
Anyone who traded these markets could describe the play. Variance Lover, a pseudonymous onchain analyst, laid it out in a detailed post dated May 21, well before the study landed.
“By now, most people are aware that market manipulation has become a major problem on Polymarket’s 5-minute crypto markets. The mechanism is simple: accumulate a large position on Polymarket, then move the price on Binance during the settlement window to force the market to resolve in your favor.”
That’s it. Buy the outcome you want on the prediction market, then shove the underlying spot price the right way for a few seconds. The contract reads one number at one instant, and whoever controls that instant controls the payout.
What the researchers actually found
The authors, from Stanford University and Singapore Management University, examined roughly two months of five-minute bitcoin contracts. They looked for unusually large orders on Binance in the final seconds before settlement, and they found them, followed by rapid price reversals in bitcoin.
The reversal is the tell. A price that moves hard and then snaps back the moment money is no longer on the line isn’t a market discovering anything.
“The vulnerability is structural,” the researchers wrote. “An asset-price contract settles on a financial price, and that price can be moved by trading the underlying market itself.”
Here’s the line that should bother anyone who traded these markets: “A bet the market treated as near-certain was overturned one time in three,” the authors wrote.
The paper is careful about what it can’t say
It did not prove traders’ intent, and it did not directly establish that the spot-market orders were placed by the same people holding positions on Polymarket. That’s an honest limitation and worth stating plainly.
But the distribution of the damage is hard to wave away. Excluding market makers, 93% of the losses in windows classified as manipulated fell on retail traders.
Polymarket did not respond to an email requesting more information.
A developer said don’t worry, in May
The complaints predate the study by months. An Axis Robotics contributor who goes by Christine on X noted on May 11 that manipulation in Polymarket’s five-minute bitcoin market was getting worse, citing “precise reversals in the last few seconds.”
Josh Stevens, a Polymarket developer, replied: “we are looking into this a bit deeper – don’t worry.”
The fix arrived after the paper did.
Polymarket is paying to keep the books full
“To protect market integrity in our crypto up/down markets, we’re updating how these markets resolve,” Polymarket said in the X post detailing the changes. “To support liquidity through this transition, we’re adding $1M in liquidity rewards across all impacted markets through the month of August.”
That second sentence is the interesting one. Changing settlement mechanics mid-flight tends to scare off the people quoting prices, and $1 million is what Polymarket thinks it costs to keep them at the table through August.
Kalshi says it’s immune. Someone on X says otherwise
IcoBeast.eth, a Kalshi developer, wrote on X that “this problem doesn’t exist on Kalshi fwiw.” Tomdnc replied: “it literally does happen on Kalshi. I have seen with my own eyes.”
A Kalshi spokesperson said the platform resolves its markets using a regulated CF Benchmarks price index and can more easily investigate suspicious activity because all its traders are identity-verified.
The spokesperson acknowledged that offshore markets can affect prices. But the company said its 60-second moving average, based on regulated exchanges, makes brief attempts to move a price “significantly harder and more expensive” than on platforms using instant snapshots. Kalshi added that arbitrageurs quickly correct artificial moves.
Note the number. Kalshi’s defense is a 60-second average, which is exactly what Polymarket just adopted for its 15-minute and four-hour markets. Its five-minute markets get half that.
The enforcement numbers are the real difference
Kalshi said it has conducted 150 to 250 material investigations per quarter and made about 40 to 50 referrals to the Commodity Futures Trading Commission so far this year. Those are company-wide figures and are not limited to its short-dated crypto markets.
Identity verification and a regulator to refer cases to is a different kind of deterrent than a longer averaging window. A TWAP raises the cost of an attack. It doesn’t identify who ran one.
What this changes if you trade these markets
A 30-second average on a five-minute contract means a manipulator now has to hold an artificial price for 30 seconds instead of hitting a single tick. That’s a much larger position, held much longer, against arbitrageurs who’ll trade against it. The economics get worse fast.
It doesn’t make the attack impossible. It makes it expensive, and expensive is the only defense a permissionless market really has.
If you’re trading five-minute crypto contracts on any venue, the number to ask about is the averaging window and where the price feed comes from. A snapshot from one exchange is a settlement price with a lever attached, and the researchers just showed what pulling it is worth: $8.2 million, split across 821 accounts, taken mostly from retail.
