Security vulnerabilities and defences — Solidity & Smart Contract Development

8. Security vulnerabilities and defences

Advanced★ Must knowLesson 8 of 11

Vulnerability	What happens	Defence
Reentrancy	External call re-enters your function before state is updated (The DAO, 2016)	Checks-Effects-Interactions, `ReentrancyGuard` (`nonReentrant`), pull payments
Access control bugs	Missing `onlyOwner`/role checks, unprotected `initialize`	Explicit modifiers, tests for every privileged function, OZ AccessControl
`tx.origin` auth	Phishing contract calls yours on behalf of the user	Use `msg.sender`
Integer overflow	Pre-0.8 wrap-around (BEC token 2018)	Solidity ≥ 0.8 (checked), careful `unchecked`
Oracle manipulation	Spot price from one pool manipulated (often with flash loans)	Chainlink/TWAP, multiple sources, sanity bounds
Front-running / MEV	Attackers see pending txs and reorder	Commit-reveal, slippage limits, batch auctions
Denial of service	Unbounded loops, failing recipient blocks payouts	Pagination, pull payments, avoid loops over user arrays
Signature replay	Same signature reused (other chain/contract/after use)	Nonces, `chainid`, EIP-712 domain separator, deadlines
Unchecked return values	Ignoring `call` result or non-standard ERC-20s (USDT returns nothing)	Check `ok`; use SafeERC20
`delegatecall` / proxy storage collisions	Logic contract overwrites proxy storage	OZ upgradeable patterns, storage gaps / ERC-7201 namespaced storage, `_disableInitializers()`
Randomness	`block.timestamp`/`prevrandao` predictable/influenceable	Chainlink VRF
Precision loss	Divide before multiply	Multiply first, use fixed-point libraries
Private data	`private` vars readable from storage	Never store secrets on-chain; commit hashes

solidity

import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
import {SafeERC20, IERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

contract Vault is ReentrancyGuard {
    using SafeERC20 for IERC20;
    IERC20 public immutable token;
    mapping(address => uint256) public shares;
    constructor(IERC20 t) { token = t; }

    function withdraw(uint256 amount) external nonReentrant {
        require(shares[msg.sender] >= amount, "Too much");   // checks
        shares[msg.sender] -= amount;                         // effects
        token.safeTransfer(msg.sender, amount);               // interactions
    }
}