What happened
On November 17, 2020, an attacker used flash-loan liquidity and a malicious token contract to reenter OUSD VaultCore's mint path. A gas-saving refactor had removed a validation check and the mint flow lacked reentrancy protection. Reentrant calls inflated the rebasing OUSD supply; the attacker then redeemed the inflated OUSD for collateral, draining the vault.
Case & protocol details
Attack Timeline
VaultCore accepted an attacker-controlled token during minting. Its transferFrom callback re-entered the mint flow while the vault's rebasing supply calculation was incomplete. Missing asset validation and reentrancy protection allowed OUSD supply to be inflated beyond the underlying collateral, which the attacker redeemed for vault assets.
Flash loans supplied the temporary capital required for the transaction. Origin later compensated eligible users for their deposited value; that was protocol compensation rather than recovery of attacker funds.
Evidence & learning
Attack pattern
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