On May 14, 2024, the DeFi world woke up to news that Sonne Finance, a lending protocol on the Optimism network, had been drained of approximately $20 million through a sophisticated smart contract exploit. For newcomers to decentralized finance, headlines about multi-million dollar hacks can be alarming and confusing. What exactly happened? Could it happen to other protocols? And most importantly, how can you protect yourself? This guide breaks down the Sonne Finance exploit in plain language and provides actionable steps for anyone looking to participate in DeFi lending safely.
The Basics
Decentralized lending protocols are platforms where you can deposit your cryptocurrency and earn interest, or borrow cryptocurrency by providing collateral. Think of them as banks without the bank — no branch offices, no account managers, just smart contracts running on a blockchain. Sonne Finance was one such platform, operating on Optimism, a Layer-2 network built on top of Ethereum.
Like many DeFi lending protocols, Sonne Finance was built using code originally developed by Compound Finance, a pioneer in decentralized lending. This practice of reusing proven code — called “forking” — is common in DeFi because it saves development time and relies on code that has been battle-tested. However, as the Sonne Finance exploit demonstrated, forking also means inheriting any vulnerabilities in the original code.
With Bitcoin priced at around $61,550 and Ethereum at $2,880 at the time of the exploit, the total value locked in DeFi protocols represented billions of dollars — making them attractive targets for attackers seeking to exploit any weakness.
Why It Matters
The Sonne Finance exploit matters because it highlights a fundamental truth about DeFi: smart contract risk is real, and it affects everyone from individual retail users to large institutional players. When a protocol is exploited, users who have deposited their funds can lose everything — there is no FDIC insurance, no customer service hotline, no government safety net.
The attack on Sonne Finance used a technique called a “donation attack,” which manipulates the way lending protocols calculate how much collateral a user has deposited. By exploiting a known vulnerability in the Compound v2 codebase and timing the attack to coincide with a governance upgrade, the attacker was able to borrow far more than they should have been allowed, draining the protocol’s liquidity pools.
What makes this particularly relevant for beginners is that Sonne Finance was not an obscure, unaudited protocol. It had undergone review and was operating on a major Layer-2 network. The exploit demonstrates that even relatively established platforms carry significant smart contract risk.
Getting Started Guide
If you want to participate in DeFi lending while managing your risk, follow these fundamental steps. First, never invest more than you can afford to lose. This sounds obvious, but the high yields offered by DeFi protocols can tempt even cautious investors into overexposure. A good rule of thumb is to limit your DeFi allocation to a small percentage of your total crypto portfolio.
Second, research the protocols you use. Before depositing funds, check whether the protocol has been audited by reputable security firms. Look for audit reports from companies like Trail of Bits, OpenZeppelin, or Consensys Diligence. While audits do not guarantee safety — audited protocols have been exploited before — they indicate that the code has undergone professional review.
Third, understand the codebase heritage. If a protocol is a fork of another project, research whether the original project has had any known vulnerabilities and whether the fork has addressed them. The Sonne Finance exploit specifically targeted a vulnerability in Compound v2 forks that had been documented but not adequately patched.
Fourth, use hardware wallets for storing your assets and only connect hot wallets with limited funds to DeFi protocols. This way, even if a protocol you have interacted with is compromised, your main holdings remain secure.
Common Pitfalls
New DeFi users frequently make several mistakes that increase their risk exposure. The most common is chasing the highest yields without understanding why those yields are high. In DeFi, higher yields almost always correspond to higher risk. A protocol offering 20% annual returns on stablecoin deposits is either taking significant risks with your funds or offering unsustainable incentives that will eventually collapse.
Another common mistake is failing to revoke token approvals after interacting with protocols. When you deposit funds into a DeFi protocol, you grant it permission to spend your tokens. If the protocol is later exploited, attackers may be able to drain not just the deposited funds but any tokens in your wallet that you have approved. Tools like Revoke.cash allow you to review and remove these approvals.
Finally, many users fall victim to phishing attacks in the aftermath of exploits. Scammers impersonate protocol teams or recovery services, creating fake websites that ask users to connect their wallets. Always verify information through official channels and never connect your wallet to unverified websites.
Next Steps
Understanding DeFi lending risks is the first step toward participating safely. Start small, learn the mechanics of lending and borrowing, and gradually increase your exposure as you build confidence and knowledge. Follow reputable DeFi security researchers, join protocol community channels, and stay informed about developments in the space. The Sonne Finance exploit is a reminder that DeFi remains an emerging technology with significant risks, but with proper education and caution, it is possible to participate while managing those risks effectively.
Disclaimer: This article is for informational purposes only and does not constitute financial advice. Always conduct your own research before making investment decisions.
Sonne forked Compound and nobody audited the modifications. 20M gone for what, saving 2 weeks of dev time on a proper review
the fork-not-build culture in DeFi is the actual problem. Compound code works, your modifications to it probably dont
sonne forked compound finance code and the $20m exploit came from a modification they added on top. forking audited code is safe until you touch the parts you dont understand
this is the 4th compound fork on optimism that got hit. users need to stop treating lending protocols on l2 as if they inherit compounds security
Diluc K. nailed it, compound forks on L2 keep getting hit because devs treat the base code as finished when the real risk is in every line they add on top
this is actually a solid explainer for someone who just heard about DeFi lending after seeing the Sonne headline. the banks without the bank analogy is pretty spot on
agree it’s well written but i wish beginner guides would put the how to check if a protocol is forked section way earlier. people skim and miss the important stuff
beginner guides burying the fork check section is a feature not a bug. protocols dont want you looking too closely at their code lineage
audit_skip_ protocols burying the fork check section is not a coincidence. if users actually checked code lineage half of TVL would evacuate overnight
EthelR totally agree. the fork detection should be step 2, not buried at the bottom. Sonne was a Compound v2 fork and that vulnerability came from the original code
fork_audit_ sonne was a compound v2 fork and the bug came from the original code. checking lineage should be step 1 for any DeFi deposit
compound_gene_ checking code lineage is step 1 but nobody does it. Sonne was a compound v2 fork and the bug came from a module they modified on top. forking isnt copying
Compound v2 fork detection should be step 1 for any DeFi user. how many millions have been lost because people dont check the code origin
banks without the bank analogy works until the smart contract gets exploited and there is zero recourse. $20m drained and lenders just… lost their money
Rina G. exactly. FDIC exists for a reason. DeFi lending has zero fallback and $20M disappears in one tx. the tech is cool but the risk framing matters
the banks without the bank analogy is clean but it undersells the risk. when a bank fails there is FDIC insurance. when a DeFi protocol fails there is a tweet thread
forking Compound code without understanding the timelock module is how you lose $20M. the OG contract was battle tested, the fork wasnt
$20M drained from a compound fork on optimism and people still deposit without reading the audit report. the information is right there
the article mentions reusing proven code from Compound but skips over the fact that Sonne modified the collateral module. fork risk isnt in the copy, its in the patch
Greta V. the modified collateral module is where Sonne went wrong. Compound v2 was fine, the patch on top was the exploit vector
4th compound fork on Optimism drained and people still deposit without checking code lineage. greed beats reading the audit report every single time
sonne was a compound v2 fork and the vulnerability was in the delegation logic. same code that saved compound from similar attacks but sonne never applied the patch
this guide is useful but burying the fork check section at the bottom means the people who need it most will never scroll that far. put it in the first paragraph
20M gone and people still ape into compound forks on optimism without reading the audit. greed beats reading every time