Incident Overview
On March 6, 2025, 1inch’s Fusion v1 Settlement Contract suffered a $2.6 million exploit, losing approximately $1.2 million in USDC and 638 ETH. The root cause was a re-entrancy vulnerability in the fillOrderInteraction() function, which allowed attackers to repeatedly gain approvals for asset transfers.
The exploited function, fillOrderInteraction(), blindly accepted user-supplied input (takingAmount) and returned that value as the approved transfer amount. This function is part of the 1inch Aggregation Router (0x11111112…960582), which called fillOrderTo(). Critically, both functions accepted arbitrary calldata, opening the door to a re-entrancy attack.
This re-entrancy flaw enabled multiple fraudulent approvals, allowing the attacker to withdraw funds far beyond legitimate order amounts.
Incident Report
What the Attacker Needed to Succeed
Understanding the prerequisites for this type of attack helps auditors identify protocols that are most at risk and helps developers build better defenses.
What Auditors Should Check
If you're auditing a protocol with similar architecture to 1Inch, these are the critical security checks that could have prevented this incident (March 2025).
- Verify all logic paths related to Reentrancy are guarded by proper access controls and input validation - see the Reentrancy attack class for patterns
- Check that all state-changing functions follow the Checks-Effects-Interactions (CEI) pattern to prevent reentrancy and logic ordering bugs
- Review privileged functions (owner, admin, governance) for potential abuse vectors - centralization risks should be documented and bounded with timelocks or multi-sigs
Master these auditing techniques with hands-on labs and real exploit scenarios in the Smart Contract Hacking course.
Free TrialSecurity Audit History
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Related Attack Classes
The technique used in this hack maps to these vulnerability classes in our security curriculum:
Sources & References
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