Solidity Security

Reentrancy

CEI Pattern (Checks-Effects-Interactions) — NON-NEGOTIABLE

function withdraw(uint256 amount) external {
    // CHECKS
    require(balances[msg.sender] >= amount, InsufficientBalance());

    // EFFECTS (update state BEFORE external call)
    balances[msg.sender] -= amount;

    // INTERACTIONS (external call LAST)
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, TransferFailed());
}

Additional Protection

• Use ReentrancyGuard from OpenZeppelin for complex functions
• Cross-function reentrancy: guard shared state across multiple functions
• Read-only reentrancy: be aware of view functions reading stale state during callbacks

Access Control

Pattern	Use When
Ownable	Single admin, simple contracts
AccessControl (roles)	Multiple roles, granular permissions
Multi-sig (Safe)	High-value operations, mainnet
Timelock	Governance, delayed execution

• Never use tx.origin for authorization — use msg.sender
• Check msg.sender in modifiers, not function bodies (unless complex logic)
• Consider 2-step ownership transfer (Ownable2Step)

Integer Safety

• Solidity 0.8+ has built-in overflow/underflow protection
• Use unchecked blocks ONLY when overflow is mathematically impossible
• Document WHY each unchecked block is safe
• Use type(uint256).max instead of magic numbers
• Watch for division-before-multiplication precision loss

Front-Running / MEV

Attack	Mitigation
Sandwich attacks	Slippage protection, deadline params
Front-running	Commit-reveal schemes
MEV extraction	Use Flashbots, private mempools
Oracle manipulation	TWAP, multiple sources

Oracle Security

• Never use spot price from a single DEX — use TWAP
• Chainlink: check updatedAt freshness, answeredInRound >= roundId
• Handle oracle downtime gracefully (circuit breaker)
• Use multiple oracle sources with fallback logic

Delegate Call Risks

• Storage layout MUST match between proxy and implementation
• Never delegatecall to untrusted addresses
• Implementation contracts must call _disableInitializers() in constructor
• Watch for storage collision in proxy patterns

Flash Loan Vectors

• Assume any amount of tokens can be borrowed in a single transaction
• Don't rely on token balances for governance weight in the same block
• Use snapshot mechanisms for voting power
• Add time-weighted checks for critical operations

Storage Collision (Proxies)

• Use EIP-1967 storage slots for proxy state
• Never modify storage layout order in upgrades — only append
• Use /// @custom:storage-location for namespaced storage (EIP-7201)
• Run forge inspect to verify storage layout before upgrade

Detailed References

• Vulnerability Checklist — SWC registry mapping
• Gas Optimization — Storage packing, calldata, custom errors

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