Soft Yearn Finance Hack

TOTAL LOST $281K
Low Other

Summarize with AI

ChatGPT Perplexity Claude Grok
Affected Chain 2020 Incident surface
Recovered - No recovery reported
All-Time Rank #1168 By amount stolen
Auditors 1 Prior security audit

Incident Overview

The protocol did not handle the rebase event correctly. It enabled a user to start Uniswap sell transaction immediately following the modification of wallet balances, but before any price change in the token was registered.

The rebase event functioned wrong. Because the YFI to SYFI conversion rate was incorrectly entered, an erroneous balance change was started. Together, these two errors permitted a transaction to be initiated that would basically remove the pool's aggregate cash.

The transaction, where the external address buys 2 SYFI using 0.5 ETH on the Uniswap:

https://etherscan.io/tx/0xed33e727…c4e59a

15 minutes later, the same external address sells 15,551 SYFI for 747 ETH after the positive rebase event:

https://etherscan.io/tx/0xbb45a3aa…64ea83

Incident Report

Protocol / Project Soft Yearn Finance
Date of Incident
Attack Technique Other
Classification Yield Aggregator
Primary Source View Post-Mortem #1 View Post-Mortem #2

Protocol Information

Protocol Type Yield Aggregator
Affected Token SYFI
Official Website syfi.finance/
Protocol Twitter/X @softyearnfi

Market Context at Time of Hack

Token Categories
DeFi DAO Ethereum Ecosystem Yield Farming Yield Aggregator Yearn Partnerships Three Arrows Capital Portfolio Polychain Capital Portfolio
Protocol Links
GitHub

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.

Technical Knowledge Deep understanding of other and Solidity and EVM internals
Capital Required Seed capital to cover gas and initial position setup
On-Chain Access Ability to interact with smart contracts and deploy a custom exploit contract
Protocol Analysis Identification of the exploitable vulnerability in Soft Yearn Finance's contract logic - root cause: yield aggregator
Execution Speed Precise transaction ordering and timing to exploit the vulnerability within a single atomic block
Obfuscation Plan A strategy to launder and move stolen funds - typically through mixers, cross-chain bridges, or decentralized DEX swaps to resist tracing

What Auditors Should Check

Could this have been caught in audit? Likely — with a thorough Other audit checklist and test coverage
Audited by Audit Report 1 — still lost $281K. Prior audits don't guarantee safety, especially after post-audit code changes.

If you're auditing a protocol with similar architecture to Soft Yearn Finance, these are the critical security checks that could have prevented this incident (September 2020).

  • Verify all logic paths related to Other are guarded by proper access controls and input validation
  • 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.

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Security Audit History

Sources & References

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