What Happened in the Euler Finance Exploit?
In 2023, a missing check in a new function let an attacker take about $197M from audited Euler Finance with flash loans. Here's how it happened and was recovered.
In March 2023, lending protocol Euler Finance lost around $197 million in a flash loan attack. The protocol had been audited multiple times. A few weeks later, the attacker returned the funds. Both parts of the story carry important lessons.
What Euler Finance Was
Euler was a DeFi lending protocol on Ethereum, where users could deposit assets to earn interest or borrow against collateral. Like other lending protocols, it depended on rules ensuring every borrower's position stayed sufficiently collateralized.
What Went Wrong
The flaw came from a function added in a later update, which let users donate funds to the protocol's reserves. The function didn't check whether the donation left the user's own position dangerously undercollateralized. That missing check let the attacker deliberately put a position into a state that could be liquidated in their own favor, extracting far more than they'd put in.
How Flash Loans Amplified the Attack
The attacker used flash loans, which provide huge amounts of capital that must be repaid within the same transaction. They didn't need their own money to exploit the flaw at scale; they borrowed it, ran the exploit, repaid the loan, and kept the difference.
Why Audits Didn't Catch It
Euler had been reviewed by several auditing firms, including the update that introduced the vulnerable function. The missing check was subtle and depended on how the new function interacted with the rest of the protocol. Audits reduce risk but don't eliminate it, especially when new code changes how existing parts behave.
How the Funds Came Back
Euler offered a reward for returning the funds and warned of a large bounty for information leading to the attacker's identification if they didn't. After on-chain messages between the team and the attacker, including apologies from the attacker, the funds were returned over the following weeks, one of the largest white hat-style recoveries in DeFi history.
Lesson One: Updates Are a Risk Point
A protocol can be secure at launch and become vulnerable after a single change. New features deserve the same scrutiny as the original code, and ideally more.
Lesson Two: "Audited" Is a Starting Point
Multiple audits signaled a serious team, and still didn't prevent the exploit. Checking audit history is worthwhile, but it's one signal among several.
Lesson Three: Recovery Was the Exception
As with Poly Network, Euler's users were fortunate. Most DeFi exploit victims don't get their funds back, so the size of your deposit in any single protocol should reflect that possibility.
Check a protocol's audit history and recent changes before depositing significant funds. Even well-audited code can hide one missing check.