What happened
On May 22, 2026 an attacker took about 13,700 USDC.e (about $13,700) from Fractal Protocol's USDF vault on Arbitrum. That was the vault's entire on-hand withdrawal buffer; Defimon put the vault's pre-hack TVL at about $97,000, most of it deployed elsewhere.
The vault prices its USDF receipt token with a stored tokenPrice that rises by a fixed daily APR, but the increase is only applied when _compute() runs, and each run catches up at most 30 days. At the time of the attack the price had not been updated for 78 days. Anyone could call compute(). The attacker deposited while the price was still partly stale, pushed it to its true value with two more compute() calls, and redeemed at the higher price in the same transaction.
Some alerts describe the flow as re-entrant. The DeFiHackLabs reproduction and the Crypto Training trace analysis show nested flash loans followed by an ordinary deposit, compute(), withdraw sequence, with no reentrancy into the vault.
Attacker: 0xe2acec13c6d1aaca584f827a22f2e4a02131e39a
Attack contract: 0x43514743caa5a7d4a8b07f5d25fb242391bbc8da
Attack tx: 0x20db78913a51c3b3aece860ea142c240f3f8fa3b5bbf533a3d1d48eed857e10f
How it happened
- The attack contract stacked flash loans of USDC.e from Aave V3, Balancer and six Uniswap V3 and Algebra pools, about 1.40 million USDC.e in total.
- It deposited about 1,403,260 USDC.e into the vault. The deposit's internal
_compute()advanced the price by only 30 of the 78 missing days, so USDF was minted at the stale price of about 1.2570 USDC.e. - It called
compute()twice, bringingtokenPriceup to its current value of about 1.2714 USDC.e, roughly 1.15% higher. - It withdrew in 15 steps, each capped by
getMaxWithdrawalAmount(), burning USDF at the higher price until the vault's buffer was empty. It got back about 14,778 USDC.e more than it had deposited. - It repaid all flash loans and fees and kept about 13,708 USDC.e.
Protocol details
Evidence
- report Defimon alert: Fractal Protocol - Loss ~$13.7K (2026-05-22) x.com
- transaction Arbiscan transaction 0x20db7891... arbiscan.io
- analysis DeFiLlama defillama.com
- analysis DeFiHackLabs FractalProtocol_exp.sol raw.githubusercontent.com
- analysis Fractal Protocol Exploit: Stale-Price / Retroactive-Yield Arbitrage via Permissionless compute() (Crypto Training) crypto.training
- analysis SlowMist Hacked: Fractal Protocol (2026-05-22) hacked.slowmist.io
Understand the attack patterns
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