Euler Finance Exploit

On March 13, 2023, the Euler Finance lending protocol was exploited for roughly $197M. Nearly all of it was returned by the attacker over the following month, making this the largest recovered exploit on Ethereum at the time.

What happened

Euler exposed a donateToReserves function that allowed any holder of eTokens to donate them to the protocol's reserves. The function decremented the donor's balance but did not run a health check on the donor's resulting position.

By taking a flash loan, depositing as collateral, borrowing against it, and then donating a slice of their eToken balance, an attacker could push their own account underwater on purpose. They then liquidated themselves in the same transaction, exploiting Euler's discounted-liquidation reward to extract more value than they'd put in. Repeating in a loop drained the lending pools.

Aftermath

• The Euler team paused the contracts within hours and began on-chain negotiation with the attacker.
• The attacker returned all funds — roughly $197M — over the following weeks, citing apparent regret. The case remains unusual among large DeFi exploits.
• Euler published a detailed post-mortem and has since redeployed with re-audited contracts.

Why it matters

The bug was not in flash loans, oracles, or liquidation logic — those were all working correctly. The flaw was a single missing invariant: every function that modifies a user's balance must call the same health-check helper. Several lending protocols audited their codebases for analogous omissions in the weeks following.