USM Protocol has suffered an exploit that resulted in the loss of approximately 70.83 ETH after an attacker manipulated the way its Ethereum-based redemption mechanism calculated payouts. Security analysis points to a pricing logic flaw that made fragmented redemptions more profitable than processing the same amount in a single transaction.
SolidityScan and SlowMist attributed the incident to a split-invariance vulnerability affecting the protocol’s defund() and ethFromDefund() functions. Instead of submitting one large redemption, the attacker divided the operation into 64 smaller calls. That fragmentation reportedly allowed the attacker to withdraw more ETH than the protocol’s pricing model intended.
USM (@usmfum) reported an exploit with ~70.83 ETH lost.
SlowMist attributes it to a split-invariance flaw in defund()/ethFromDefund(): splitting the same redemption into 64 smaller calls reportedly yielded more ETH than one large call.#DeFi #Security
— SolidityScan (@SolidityScan) August 11, 2026
Redemption logic failed under fragmented transactions
In a technical analysis published by SolidityScan, the security firm said USM’s redemption algorithm did not produce consistent results when a transaction was split into multiple smaller operations. The flaw meant that economically equivalent redemptions could return different ETH amounts depending on how they were structured.
The issue was linked to the protocol’s method of calculating compensation using an arithmetic average between current and estimated future prices. Integer rounding and state shrinkage factors also affected the calculation as each redemption changed the protocol’s internal state. Repeated smaller calls were therefore able to exploit cumulative pricing differences that would not appear in a single redemption.
This type of weakness is known as a split-invariance failure because dividing an operation changes its final economic outcome. In a correctly invariant mechanism, splitting a redemption should not materially increase the total amount returned. USM’s implementation instead allowed transaction fragmentation to alter the effective redemption price, creating the opening used in the exploit.
USM faces scrutiny over pricing safeguards
The incident adds to concerns around complex arithmetic and pricing mechanisms in decentralized finance, where relatively small calculation inconsistencies can become exploitable when repeated across many transactions. The supplied material also points to the Kelp DAO incident as another example involving flawed protocol logic, although the USM exploit specifically centered on redemption pricing rather than an identical attack mechanism.
Attention is now focused on the affected functions and the adjShrinkFactor state used within the pricing process. Security researchers are monitoring whether related calculations could expose additional weaknesses, particularly where state changes interact with repeated redemption requests.
Trading activity around the protocol has reportedly stalled following the exploit, while no detailed recovery plan or specific patch for the affected defund functions has yet been announced in the supplied information. The immediate technical priority is preventing the redemption mechanism from returning different economic outcomes when equivalent transactions are divided into smaller calls.
The broader impact will depend on how the protocol addresses the vulnerable pricing logic and whether additional weaknesses are identified during subsequent review. The exploit demonstrates how rounding behavior and state-dependent calculations can become material security risks when economic outcomes vary according to transaction structure.
