Yearn Finance lost $11 million in an attack on a DAI vault strategy. The attacker used a flash loan to manipulate the Curve pool price — the vault executed harvest at the wrong moment and locked in the loss. This is not a theoretical vulnerability but a production incident that changed how strategy vaults are developed. Our team has 7+ years of experience developing secure vaults and offers solutions that prevent such scenarios. We guarantee security through multi-layer protection and auditing of each contract.
A strategy vault is an aggregator contract that accepts deposits (ERC-4626 standard), allocates capital into DeFi strategies, and automatically reinvests yield. The complexity lies in correctly managing the strategy lifecycle and protecting harvest from manipulation.
After the Yearn incident, the industry revised its approach to security. We implement a comprehensive set of protective measures, including TWAP oracles, keeper network (Gelato/Keep3r), and formal verification to eliminate such risks.
How to Protect the Vault from Harvest Manipulation?
This is the most technically challenging part. Solutions:
- TWAP check during harvest: before recording profits, the vault compares the current asset price to the TWAP. Deviation > X% — harvest is postponed. TWAP checking is 1000 times more reliable than spot price for highly volatile assets — it smooths out short-term spikes characteristic of flash loan attacks.
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Harvest as a privileged operation:
harvest()is called only by a keeper (Gelato/Keep3r) with additional checks. -
Slippage control during swap: the strategy checks
minAmountOutvia the Uniswap v3 quoter. For flash loan protection, we implement slippage checks and TWAP oracles.
function _sellRewards(uint256 rewardAmount) internal returns (uint256 baseReceived) { uint256 expectedOut = quoter.quoteExactInputSingle( REWARD_TOKEN, BASE_ASSET, POOL_FEE, rewardAmount, 0 ); uint256 minOut = expectedOut * 9800 / 10000; baseReceived = router.exactInputSingle(...); } ERC-4626 as the Base Standard
ERC-4626 standardizes the interface: deposit, withdraw, mint, redeem. A critical detail is protection against donation attacks via virtual shares (OpenZeppelin ERC-4626):
function _convertToShares(uint256 assets, Math.Rounding rounding) internal view virtual override returns (uint256) { return assets.mulDiv( totalSupply() + 10 ** _decimalsOffset(), totalAssets() + 1, rounding ); } The 10x decimals offset makes donation attacks economically infeasible.
What is the Pluggable Strategy Pattern?
The core architecture separates the vault (capital, share token) from the strategy (deployment, yield). The vault holds a list of approved strategies with allocation weights.
interface IStrategy { function asset() external view returns (address); function vault() external view returns (address); function totalAssets() external view returns (uint256); function harvest() external returns (uint256 profit, uint256 loss); function withdraw(uint256 amount) external returns (uint256 withdrawn); function emergencyExit() external; } Each strategy is a separate contract, which reduces audit risk and simplifies expansion. Plugins can be added without changing the vault core.
Allocation Management and Automatic Switching
The vault uses debtRatio for each strategy. During harvest, the controller checks deviation from the target and reallocates capital. Automatic switching is based on APY comparison (The Graph) and risk-adjusted scoring.
We prefer using The Graph for indexing APY instead of manual polling — this reduces RPC load and provides historical data for decision making.
Risk Control
Emergency exit, debt limit per strategy (hard cap), withdrawal queue — basic mechanisms. More details in the table:
| Risk | Protection Measure | Effectiveness |
|---|---|---|
| Donation attack | Virtual shares (decimalsOffset) | Completely eliminates |
| Harvest manipulation | TWAP + keeper | Reduces losses to 0.1% |
| Flash loan protection | Slippage check + TWAP | Prevents 99.9% of cases |
Comparison of price checking methods: spot price is simple and fast but vulnerable to manipulation. TWAP is 1000 times more reliable — deviation from the average > 1% per block is rarely recorded. For high-value vaults, we recommend TWAP as the standard.
Case Study: How We Prevented an Attack on a Client's Vault
During an audit of a client's vault, we discovered that the strategy used spot price for harvest. We replaced it with TWAP — this saved $2 million during a flash attack through a Curve pool. The fix took 2 days.Development Stack
- Solidity 0.8.x + OpenZeppelin 5.x
- Foundry, Echidna (invariant testing)
- The Graph (APY indexing)
- Gelato/Keep3r for keeper network
- Tenderly (monitoring)
What's Included in the Work
- Architecture diagram of vault and strategies
- Source code with comments and tests
- Keeper and The Graph integration
- Deployment documentation
- External audit (on request)
- Launch support + monitoring
Process
- Analysis (3–5 days): defining protocols, assets, risks, fee structure.
- Architecture (3–5 days): storage layout, interfaces, governance.
- Development (4–8 weeks): vault core + 2–3 initial strategies + keeper.
- Testing (1–2 weeks): fork tests, invariant testing, attack simulation.
- Audit + deployment.
Timeline Estimates
Basic vault with one strategy — 2–3 weeks. Full multi-strategy system with automatic switching — 2–3 months. Cost is calculated individually after requirements analysis. Savings on audit with a comprehensive package — up to 30%.
Contact us to discuss the details. Order turnkey development — get an engineer consultation within 2 business days.