How to Audit DeFi Lending Protocols for Donation Attack Vulnerabilities After the Sonne Finance Exploit

The $20 million exploit of Sonne Finance on May 14, 2024, exposed a recurring vulnerability pattern in Compound v2-forked lending protocols known as the donation attack. For experienced DeFi users and developers, this incident provides a critical case study in how to identify and assess similar vulnerabilities before they are exploited. This advanced tutorial walks through the technical mechanics of the donation attack, the specific conditions that enabled the Sonne Finance exploit, and a systematic methodology for auditing lending protocols for this class of vulnerability. With Bitcoin at $66,267 and Ethereum at $3,037, the broader DeFi ecosystem holds tens of billions in total value locked, making security literacy an essential skill for serious participants.

The Objective

This tutorial aims to equip you with the knowledge to independently assess whether a lending protocol is vulnerable to donation attacks. You will learn to identify the specific code patterns that create this vulnerability, understand the market conditions that enable exploitation, and develop a framework for evaluating the security of any Compound v2 fork. By the end of this guide, you should be able to review a lending protocol’s smart contracts and governance configuration to determine whether similar attack vectors exist.

Prerequisites

This tutorial assumes familiarity with Solidity smart contracts, Ethereum Virtual Machine mechanics, and the basic architecture of Compound v2-style lending protocols. You should understand how collateral factors work, how lending pools calculate borrow capacity, and how governance proposals interact with protocol parameters. Access to Etherscan or the block explorer for the relevant chain is required to inspect contract code and transaction logs. Familiarity with Foundry or Hardhat for local contract interaction is helpful but not strictly necessary.

Step-by-Step Walkthrough

The donation attack exploits a fundamental interaction between how lending protocols value collateral and how total supply calculations are performed. In a Compound v2 fork, each lending market maintains an internal accounting system where user balances are represented as shares of a total supply. When a user donates tokens directly to the contract without going through the deposit function, the total supply increases but no user shares are minted. This means the value of each existing share increases, effectively inflating the apparent value of collateral held by the attacker.

Step one is to examine the market initialization and configuration process. In Sonne Finance’s case, the vulnerability was exposed during the integration of new VELO markets. Review the governance proposal that adds new markets and pay particular attention to how collateral factors are set during the initial configuration. If a market is added with a high collateral factor before sufficient liquidity exists, the conditions for a donation attack are present.

Step two is to analyze the timelock configuration. Sonne Finance used a two-day timelock on governance transactions, which meant that the critical transactions for market setup were visible and predictable. The attacker monitored the mempool and positioned their transactions to execute at the precise moment the timelock expired. When auditing a protocol, check whether governance-critical transactions can be front-run during timelock execution windows.

Step three is to inspect the collateral factor update mechanism. The Sonne Finance attacker specifically targeted the transaction that increased collateral factors for the new VELO markets. Review the smart contract code that handles collateral factor updates and determine whether there are any safeguards against sudden, large increases. Look for rate limiting, gradual adjustment mechanisms, or sanity checks that prevent collateral factors from being set to dangerous levels.

Step four is to simulate the attack locally. Using Foundry, you can fork the Optimism network at the block just before the exploit and reproduce the attack transaction by transaction. The exploit involved four initial setup transactions followed by the collateral factor increase and then the final borrowing exploit. Simulating these transactions reveals exactly how the donation attack inflates collateral values and enables over-borrowing.

Step five is to evaluate the protocol’s emergency response capabilities. Sonne Finance detected the breach within 25 minutes and paused markets, and the Security Alliance’s Seal911 contributors managed to salvage $6.5 million by adding minimal VELO to the compromised markets. When assessing a protocol, verify that it has functioning pause mechanisms, that the pause authority is appropriately decentralized, and that response procedures are documented and tested.

Troubleshooting

If you encounter difficulties identifying the vulnerability in a specific protocol, start by comparing the contract code against the original Compound v2 implementation. Focus specifically on any modifications to the exchange rate calculation, the total supply accounting, or the collateral factor logic. Forks often introduce subtle changes that create new attack surfaces. If the protocol uses a non-standard token as collateral, pay special attention to how the protocol handles tokens with transfer fees, rebasing mechanics, or other non-standard behaviors, as these can introduce additional vectors.

Another common challenge is determining whether a timelock configuration is genuinely secure. Some protocols use timelocks that can be bypassed under certain conditions, or that have guardian roles with emergency override capabilities. Trace through the complete execution path from governance proposal to on-chain effect to identify any shortcuts that an attacker might exploit.

Mastering the Skill

To develop expertise in DeFi security auditing, practice by reviewing real exploits retrospectively. The Sonne Finance exploit is an excellent starting point because the attack transactions are publicly visible on Optimism Etherscan. Reproduce the attack in a local environment, then attempt to identify the vulnerability before reading the analysis. Gradually work through other major DeFi exploits to build a library of attack patterns. Contribute to public audit reports and participate in bug bounty programs to sharpen your skills against real-world targets. The $20 million lost in the Sonne Finance exploit underscores that protocol security is not merely an academic exercise but a critical competency for anyone participating in the DeFi ecosystem.

Disclaimer: The information presented in this article is for educational and informational purposes only and does not constitute financial or security advice. Always conduct your own research and consult with qualified security professionals before interacting with DeFi protocols.