Credit Coop

The Challenge

The protocol enforces caps on how much can be minted against each collateral type to limit protocol exposure. The cap enforcement interacts with interest accrual, collateral price updates, and partial repayments. Rounding in these calculations could theoretically allow users to mint slightly more than the cap permits through carefully sequenced operations. Testing this manually would require enumerating an impractical number of operation sequences across multiple collateral types and user states.

Our Approach

The invariant test suite defined properties requiring that the total minted amount for each collateral type must never exceed the configured cap. Additional properties covered interest accrual correctness — ensuring accrued interest is always non-negative and monotonically increasing — and collateral accounting consistency. The fuzzer tested combinations of borrows, repayments, liquidations, and interest accrual across multiple users and collateral types, exploring sequences that would be impractical to enumerate by hand.

Findings

Results

The fuzzer discovered that sequences of small minting operations could exploit rounding in the cap check to incrementally exceed the intended limit. The vulnerability was in the interaction between the cap enforcement and interest accrual — as interest accrued, the effective utilization changed, and the rounding in the cap check did not account for this correctly. Credit Coop's CTO described the ROI as extremely high, noting that the cost of the engagement was negligible compared to the potential loss if the vulnerability had been exploited in production.