Hundred Finance Suffers $7.4 Million Flash Loan and Precision Loss Exploit → Security

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Briefing

On April 15, 2023, the Hundred Finance lending protocol on the Optimism blockchain was subjected to a sophisticated flash loan attack, resulting in a loss of approximately $7.4 million. This incident leveraged a critical vulnerability stemming from exchange rate manipulation within the hWBTC contract, exacerbated by a rounding error in the redeemUnderlying function and the presence of empty markets. The exploit allowed the attacker to disproportionately inflate the value of hTokens, enabling massive, unauthorized borrowing against minimal collateral. The total financial impact of this event stands at $7.4 million, underscoring the systemic risks associated with precision loss in DeFi protocols.

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Context

Prior to this incident, the DeFi ecosystem, particularly lending protocols forked from established codebases like Compound V2, faced inherent risks from complex smart contract interactions and potential for precision-related vulnerabilities. While Compound V2 itself had undergone audits, the re-deployment of its codebase in various forks often introduced new attack surfaces, especially in markets with low liquidity. The prevailing security posture often underestimated the impact of rounding errors when combined with flash loan capabilities and empty market conditions, leaving protocols susceptible to exchange rate manipulation.

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Analysis

The attack on Hundred Finance exploited a multi-faceted vulnerability within its hWBTC contract, a fork of Compound V2. The core mechanism involved the attacker utilizing a flash loan to acquire a substantial amount of Wrapped Bitcoin (WBTC). This WBTC was then strategically deposited into an empty hWBTC market, establishing the attacker as the sole hWBTC holder.

By subsequently “donating” a portion of the WBTC to the contract and exploiting a rounding error in the redeemUnderlying function, the attacker drastically manipulated the hWBTC exchange rate. This inflated rate allowed the attacker to borrow a disproportionately large amount of Ether with minimal hWBTC, effectively draining the protocol’s liquidity before repaying the initial flash loan.

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Parameters

Protocol Targeted → Hundred Finance
Attack Vector → Flash Loan, Exchange Rate Manipulation, Precision Loss
Financial Impact → $7.4 Million
Blockchain Affected → Optimism (Ethereum Layer-2)
Vulnerability Type → Rounding Error in redeemUnderlying function, Empty Market Exploitation
Date of Incident → April 15, 2023

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Outlook

Immediate mitigation for users involved with similar Compound V2 forks should include verifying protocol liquidity and the presence of robust oracle systems that prevent exchange rate manipulation. This incident highlights the critical need for comprehensive security audits that specifically address precision loss, especially in low-liquidity markets and across all contract functions, not just the primary logic. Furthermore, it reinforces the importance of continuous monitoring for anomalous exchange rate fluctuations and rapid response mechanisms to pause vulnerable markets. The broader implication is a heightened awareness for all DeFi protocols to implement circuit breakers and dynamic risk parameters that adapt to market conditions and prevent flash loan-enabled exploits.

The Hundred Finance exploit serves as a stark reminder that even well-audited codebases, when forked and deployed under specific market conditions, can harbor critical vulnerabilities, demanding continuous vigilance and adaptive security measures across the DeFi landscape.

Signal Acquired from → ImmuneBytes