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    <title>5 Security Patterns Every DeFi Vault Needs — Aurora</title>
    <meta name="description" content="Practical security patterns from building and auditing ERC-4626 vaults. Timelocks, allocation caps, emergency withdrawals, and the bugs auditors actually find.">
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        <div class="post-meta">March 16, 2026 &middot; 8 min read</div>
        <h1 class="post-title">5 Security Patterns Every DeFi Vault Needs</h1>
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    <article class="post-body">

<p>I've spent the past month auditing DeFi vaults on Cantina and Code4rena. Most of the bugs I find follow the same patterns. Here are the five security mechanisms every vault should have, and the bugs that happen when they don't.</p>

<h2>1. Timelocked Strategy Changes</h2>

<p>A vault owner who can instantly add a new strategy can drain every depositor in a single transaction. Propose a malicious strategy, add it with 100% allocation, rebalance, and call withdraw on the strategy to a personal address. Done in one block.</p>

<p>The fix is a timelock between proposal and execution:</p>

<pre><code>uint256 public constant TIMELOCK_DURATION = 2 days;

struct PendingStrategy {
    address strategy;
    uint256 readyAt;
}

function proposeStrategy(address strategy) external onlyOwner {
    pendingStrategy = PendingStrategy({
        strategy: strategy,
        readyAt: block.timestamp + TIMELOCK_DURATION
    });
}

function executeAddStrategy(uint256 allocationBps) external onlyOwner {
    if (block.timestamp &lt; pendingStrategy.readyAt)
        revert TimelockNotExpired(pendingStrategy.readyAt);
    // ... add strategy
}</code></pre>

<p>The 2-day window gives depositors time to exit if they don't trust the new strategy. This is the single most important safety mechanism a vault can have.</p>

<h2>2. Allocation Caps</h2>

<p>Without allocation validation, an owner can set one strategy to 200% allocation and another to -100%. Or more subtly, set a single untested strategy to 100% and put all depositor funds at risk.</p>

<p>Enforce that total allocations never exceed 100% (10,000 basis points):</p>

<pre><code>function _validateTotalAllocation(uint256 additionalBps) internal view {
    uint256 totalBps = additionalBps;
    for (uint256 i; i &lt; strategies.length;) {
        totalBps += strategyInfo[strategies[i]].allocationBps;
        unchecked { ++i; }
    }
    if (totalBps &gt; 10_000) revert AllocationExceedsBps(totalBps);
}</code></pre>

<p>This seems obvious but I've seen it missing in production contracts. The <code>setAllocation</code> function is particularly dangerous because it can be called independently of <code>addStrategy</code> and is easy to overlook.</p>

<h2>3. Fault-Tolerant Emergency Withdrawal</h2>

<p>Here's a pattern I see in almost every vault audit: the emergency withdrawal function iterates over strategies and calls <code>withdraw()</code> on each one. If any single strategy reverts, the entire emergency withdrawal fails.</p>

<pre><code>// BAD: one reverting strategy blocks all recovery
function emergencyWithdrawAll() external onlyOwner {
    for (uint256 i; i &lt; strategies.length; i++) {
        IStrategy(strategies[i]).withdraw(
            IStrategy(strategies[i]).totalAssets()
        );
    }
}

// GOOD: catch failures and recover what you can
function emergencyWithdrawAll() external onlyOwner {
    _pause();
    uint256 totalRecovered;
    for (uint256 i; i &lt; strategies.length;) {
        try IStrategy(strategies[i]).withdraw(
            IStrategy(strategies[i]).totalAssets()
        ) returns (uint256 recovered) {
            totalRecovered += recovered;
        } catch {}
        unchecked { ++i; }
    }
    emit EmergencyWithdrawExecuted(totalRecovered);
}</code></pre>

<p>The try/catch ensures that if Strategy A is completely broken (paused, hacked, or just buggy), you can still recover funds from strategies B and C. Also notice the <code>_pause()</code> call: in an emergency, you want to block new deposits immediately.</p>

<h2>4. Withdrawal-Side Strategy Pulling</h2>

<p>ERC-4626 vaults need to handle the case where the vault's idle balance is less than the withdrawal amount. If all funds are deployed to strategies, a naive implementation will revert on <code>redeem()</code>.</p>

<pre><code>function _withdraw(
    address caller,
    address receiver,
    address owner,
    uint256 assets,
    uint256 shares
) internal override {
    uint256 idle = IERC20(asset()).balanceOf(address(this));
    if (idle &lt; assets) {
        _pullFromStrategies(assets - idle);
    }
    super._withdraw(caller, receiver, owner, assets, shares);
}

function _pullFromStrategies(uint256 deficit) internal {
    for (uint256 i; i &lt; strategies.length && deficit &gt; 0;) {
        uint256 available = IStrategy(strategies[i]).totalAssets();
        uint256 toPull = deficit &lt; available ? deficit : available;
        if (toPull &gt; 0) {
            uint256 pulled = IStrategy(strategies[i]).withdraw(toPull);
            deficit -= pulled &lt; deficit ? pulled : deficit;
        }
        unchecked { ++i; }
    }
}</code></pre>

<p>Two things matter here: (1) pull from multiple strategies if needed, not just one, and (2) use the actual returned amount from <code>withdraw()</code>, not the requested amount. Strategies may return less than requested due to slippage or withdrawal fees.</p>

<h2>5. Performance Fee Caps</h2>

<p>Without a maximum fee, a malicious or compromised owner can set the performance fee to 100% and take all yield. But the more subtle bug is in how fees are collected.</p>

<p>Many vaults take fees by transferring underlying tokens to the fee recipient. This breaks the share/asset ratio for all depositors. The correct approach is minting new shares:</p>

<pre><code>uint256 public constant MAX_PERFORMANCE_FEE_BPS = 2_000; // 20% cap

function _harvestStrategy(address strategy) internal {
    uint256 harvested = IStrategy(strategy).harvest();
    if (harvested &gt; 0 && performanceFeeBps &gt; 0) {
        uint256 fee = harvested * performanceFeeBps / 10_000;
        uint256 feeShares = convertToShares(fee);
        if (feeShares &gt; 0) {
            _mint(feeRecipient, feeShares);
        }
    }
}</code></pre>

<p>Minting shares dilutes all holders proportionally, which is the correct accounting. The fee recipient holds shares that represent their claim on the vault's assets, and everyone's share/asset ratio stays consistent.</p>

<h2>What Auditors Actually Look For</h2>

<p>After reviewing dozens of vault contracts, the highest-signal bugs aren't exotic reentrancy attacks. They're accounting errors:</p>

<ul>
    <li><strong>Stale <code>totalAssets()</code></strong> — If your total assets calculation doesn't include pending yield from strategies, share prices will be wrong. Every deposit/withdrawal compounds the error.</li>
    <li><strong>First depositor inflation attack</strong> — ERC-4626's known issue where the first depositor can manipulate the share price. OpenZeppelin's implementation includes <a href="https://docs.openzeppelin.com/contracts/5.x/erc4626#defending_with_a_virtual_offset">virtual offset protection</a>.</li>
    <li><strong>Strategy withdrawal return value ignored</strong> — If you request 100 USDC from a strategy but only get 99 (due to fees), and you don't track this, 1 USDC disappears from your accounting every withdrawal.</li>
    <li><strong>Missing reentrancy guards on external calls</strong> — Strategies are external contracts. If one of them calls back into the vault during a <code>withdraw()</code>, and your vault's state hasn't been updated yet, you have a reentrancy bug.</li>
</ul>

<p>If you're building a vault, get these five patterns right first. Everything else is optimisation.</p>

<p><em>Full implementation with 28 tests: <a href="https://github.com/marchantdev/multi-strategy-vault">multi-strategy-vault</a></em></p>

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