Imported from conjure-3301/skills (web3-audit/SKILL.md). Install upstream with npx skills add conjure-3301/skills --skill web3-audit. Copyright stays with the author.

WEB3 SMART CONTRACT AUDIT

10 bug classes. Pre-dive kill signals. Foundry PoC template. Real paid examples.

PRE-DIVE KILL SIGNALS (check BEFORE any code review)

ZKsync lesson: $322M TVL + OZ audit + 750K LOC + 5 sessions = 0 findings. Large well-audited bridges are extremely hard.

1. TVL < $500K → max payout capped too low for effort
2. 2+ top-tier audits (Halborn, ToB, Cyfrin, OpenZeppelin) on simple protocol → bugs already found
3. Protocol < 500 lines, single A→B→C flow → minimal attack surface
4. Formula: max_realistic_payout = min(10% × TVL, program_cap) — if < $10K, skip

Soft kill: OZ/ToB/Cyfrin audit on current version + codebase > 500K LOC → expect 40+ hours for maybe 1 finding. Only proceed if bounty floor > $50K AND you have protocol-specific expertise.

Target scoring (go if >= 6/10):

• TVL > $10M: +2
• Immunefi program with Critical >= $50K: +2
• No top-tier audit on current version: +2
• < 30 days since deploy: +1
• Protocol you've hunted before: +1
• Source code + natspec comments: +1
• Upgradeable proxies: +1

THE ONE RULE

"Read ALL sibling functions. If vote() has a modifier, check poke(), reset(), harvest(). The missing modifier on the sibling IS the bug."

This single rule explains 19% of all Critical findings.

1. ACCOUNTING STATE DESYNCHRONIZATION

#1 Critical bug class — 28% of all Criticals on Immunefi.

What It Is

Two state variables supposed to stay in sync. One code path updates A but forgets B. Later code reads both and makes decisions based on stale B.

Real Value = A - B
If A updated but B isn't → Real Value appears larger → phantom value

Root Cause Patterns

Variant 1: Phantom Yield (Yeet protocol — 35 duplicate reports)

function startUnstake(uint256 amount) external {
    totalSupply -= amount;  // decremented BEFORE transfer
    // aToken.balanceOf(this) still reflects old value
    // yieldAmount = aToken.balanceOf - totalSupply = phantom yield
}

Variant 2: Fast Path Skips State Update (Alchemix V3)

function claimRedemption(uint256 tokenId) external {
    if (transmuter.balance >= amount) {
        transmuter.transfer(user, amount);
        _burn(tokenId);
        return;  // EARLY RETURN — cumulativeEarmarked, _redemptionWeight, totalDebt never updated
    }
    // Slow path: updates all state vars correctly
    alchemist.redeem(...);
}

Variant 3: Update Happens in Wrong Order (Alchemix)

function deposit(uint256 amount) external {
    _shares = (amount * totalShares) / totalAssets;  // calculated BEFORE deposit
    totalAssets += amount;   // assets added AFTER shares calculated → wrong rate
}

Grep Patterns

# Find all accounting variables
grep -rn "totalSupply\|totalShares\|totalAssets\|totalDebt\|cumulativeReward\|rewardPerShare" contracts/

# Find all early returns in claim/redeem functions
grep -rn "\breturn\b" contracts/ -B3 | grep -B3 "if\b"

# For each early return: which state updates in normal path are skipped?

2. ACCESS CONTROL

#2 Critical — 19% of Criticals. $953M lost in 2024 alone.

Variant 1: Missing Modifier on Sibling Function

function vote(uint256 tokenId) external onlyNewEpoch(tokenId) {  // guarded
function reset(uint256 tokenId) external onlyNewEpoch(tokenId) { // guarded
function poke(uint256 tokenId) external {                         // NO GUARD → infinite FLUX inflation
}

Variant 2: Wrong Check (Existence vs Ownership)

function split(uint256 tokenId, uint256 amount) external {
    _requireOwned(tokenId);  // checks if token EXISTS, not if caller OWNS it
    _burn(tokenId);
    _mint(msg.sender, amount);  // attacker steals tokens they don't own
}

Variant 3: Silent Modifier (if vs require)

// VULNERABLE — non-admin silently gets through:
modifier onlyAdmin() {
    if (msg.sender == admin) {
        _;  // body only executes for admin, but non-admin doesn't revert
    }
}
// CORRECT: require(msg.sender == admin, "Not admin"); _;

Variant 4: Uninitialized Proxy

function initialize(address _owner) public {  // MISSING: initializer modifier
    owner = _owner;  // anyone can call → become owner
}
// Fix: constructor() { _disableInitializers(); }

Grep Patterns

# Find sibling function families — do ALL have the same modifier set?
grep -rn "function vote\|function poke\|function reset\|function update\|function claim\|function harvest" contracts/ -A2

# Ownership check: existence vs ownership?
grep -rn "_requireOwned\|ownerOf\|_isApprovedOrOwner\|_checkAuthorized" contracts/ -B5

# Silent modifiers
grep -rn "modifier\b" contracts/ -A8 | grep -B3 "if (" | grep -v "require\|revert"

# Uninitialized initializer
grep -rn "function initialize\b" contracts/ -A3
grep -rn "_disableInitializers()" contracts/

Real Paid Examples

3. INCOMPLETE CODE PATH

#3 Critical — 17% of Criticals.

The Function Family Comparison Test

1. List all state changes in function A (deposit/place/create)
2. List all state changes in function B (withdraw/update/cancel)
3. For each state change in A: does B have the corresponding reverse?
4. For each token transfer in A: does B have the corresponding refund?
If A does X but B doesn't do the reverse of X → BUG.

Variant 1: Update Function Missing Refund (ThunderNFT)

function place_order(OrderInput calldata order) external {
    token.safeTransferFrom(msg.sender, address(this), order.price);  // takes tokens
    orders[orderId] = order;
}
function update_order(OrderInput calldata updatedOrder) external {
    // BUG: NO REFUND for sell orders when price decreases → tokens permanently stuck
    orders[orderId] = updatedOrder;
}

Variant 2: Partial Fill Token Stuck (Plume)

function swapForETH(uint256 amountIn) external {
    token.safeTransferFrom(msg.sender, address(this), amountIn);
    uint256 filled = dex.swap(amountIn);  // partial fill possible
    _refundExcessEth(amountIn - filled);  // BUG: refunds ETH only, not ERC20
}

Variant 3: mint() Bypasses Check That deposit() Has (MetaPool)

function deposit(uint256 assets, address receiver) public override {
    shares = _deposit(assets, receiver);  // includes receipt validation
}
function mint(uint256 shares, address receiver) public override {
    assets = convertToAssets(shares);
    _mint(receiver, shares);  // MISSING: _deposit() validation → mints without receiving assets
}

Grep Patterns

grep -rn "function place_\|function create_\|function add_\|function open_" contracts/ -A5
grep -rn "function update_\|function modify_\|function cancel_" contracts/ -A5
grep -rn "safeApprove\b" contracts/    # safeApprove without zero-reset before
grep -rn "delete\b" contracts/ -B5 -A5  # delete before operation completes
grep -rn "function deposit\|function mint\|function withdraw\|function redeem" contracts/ -A10

4. OFF-BY-ONE & BOUNDARY CONDITIONS

#4 High — 22% of Highs. Single character change. Massive impact.

Root Cause

// VeChain Stargate — post-exit reward drain:
function _claimableDelegationPeriods(address delegator) internal view returns (uint256) {
    if (endPeriod > nextClaimablePeriod) {  // BUG: should be >=
        return 0;  // exited users get nothing
    }
    return nextClaimablePeriod - lastClaimedPeriod;  // rewards for period AFTER exit
}

Mental Test for Every Comparison

For every if (A > B): "What happens when A == B?" Is that correct?

6 Boundary Locations to Check

1. Period/Epoch boundaries: > vs >= at period end
2. Time-based locks: does block.timestamp == deadline lock or unlock?
3. Loop break conditions: break with > vs >=
4. Array index boundaries: i <= array.length (should be i < array.length)
5. Amount/balance boundaries: >= amount allows exact full withdrawal?
6. Rounding/precision: can any input produce 0 output that should be non-zero?

Grep Patterns

# Boundaries in comparisons
grep -rn "Period\|Epoch\|Round\|Deadline\|period\|epoch\|deadline" contracts/ -A3 | grep "[<>][^=]"

# Loop breaks
grep -rn "\bbreak\b" contracts/ -B10

# Off-by-one in array access
grep -rn "\.length\s*-\s*1\|i\s*<=\s*.*\.length\b" contracts/

5. ORACLE / PRICE MANIPULATION

12% of all reports. Largest individual payouts. $117M Mango, $70M Curve.

Bug A: Missing Staleness Check (most common)

// VULNERABLE:
(, int256 price,,,) = priceFeed.latestRoundData();
return uint256(price);  // If Chainlink node goes down, stale price returned indefinitely

// CORRECT:
(, int256 price,, uint256 updatedAt,) = priceFeed.latestRoundData();
require(block.timestamp - updatedAt <= MAX_PRICE_AGE, "Stale price");
require(price > 0, "Invalid price");

Bug B: Missing Confidence Interval (Pyth)

// VULNERABLE:
PythStructs.Price memory p = pyth.getPriceUnsafe(priceFeed);
return p.price;  // ignores p.conf (confidence interval)

// CORRECT:
require(p.conf * 10 <= uint64(p.price), "Price too uncertain");
// conf > 10% of price = untrustworthy

Bug C: TWAP Too Short (flash loan manipulatable)

// VULNERABLE: 60-second TWAP
uint32[] memory secondsAgos = new uint32[](2);
secondsAgos[0] = 60; secondsAgos[1] = 0;
// Flash loan can shift price for entire 60s window

// CORRECT: 1800s minimum TWAP (30 min)

Bug D: Single-Source Oracle

// VULNERABLE: only Uniswap spot price
uint price = getUniswapSpotPrice(token);  // flash loan manipulatable

// CORRECT: Chainlink primary, Uniswap TWAP as fallback, require close agreement

Grep Patterns

# Missing staleness check
grep -rn "latestRoundData" contracts/ -A5 | grep -v "updatedAt\|timestamp"

# Pyth price usage — confidence interval checked?
grep -rn "getPriceUnsafe\|getPrice\b" contracts/ -A8 | grep -v "conf\|confidence"

# TWAP windows — short TWAP flag
grep -rn "secondsAgo\|TWAP\|cardinality" contracts/ -A5

6. ERC4626 VAULT ATTACKS

Exchange Rate Manipulation (near-empty vault)

// VULNERABLE — first depositor attack:
// 1. Attacker deposits 1 wei → gets 1 share
// 2. Attacker donates large amount directly (transfer, not deposit)
// 3. Exchange rate: 1 share = (1 + donation) assets
// 4. Victim deposits → rounds down to 0 shares → free donation to attacker

// CORRECT: virtual shares (OpenZeppelin v4.9+)
function _decimalsOffset() internal view virtual override returns (uint8) {
    return 9;  // add 1e9 virtual shares + assets to prevent manipulation
}

ERC4626 Transfer (moves shares but not stake/lock records)

// VULNERABLE: shares transferred, but lock records stay with original owner
// → shares stuck, can't redeem → permanent freeze (Belong pattern)
function transfer(address to, uint256 amount) external override {
    _transfer(msg.sender, to, amount);  // moves shares
    // MISSING: transfer lock record from msg.sender to `to`
}

Grep Patterns

grep -rn "function transfer\|function transferFrom" contracts/ -A15
grep -rn "function deposit\|function mint\|function withdraw\|function redeem" contracts/ -A10

7. REENTRANCY

2016–present. CEI pattern prevents it. Still found in DeFi.

Variants

• Single-function: attacker re-enters same function before state updated
• Cross-function: re-enters a sibling function with stale state
• Cross-contract: re-enters via a callback to another protocol
• Read-only: re-enters a view function that returns stale data used by attacker

Root Cause Pattern

// VULNERABLE (effects after interaction):
function withdraw(uint256 amount) external {
    require(balances[msg.sender] >= amount);
    (bool success,) = msg.sender.call{value: amount}("");  // INTERACTION first
    require(success);
    balances[msg.sender] -= amount;  // EFFECT after → reentrancy window
}

// CORRECT (CEI — Checks, Effects, Interactions):
function withdraw(uint256 amount) external {
    require(balances[msg.sender] >= amount);  // CHECK
    balances[msg.sender] -= amount;            // EFFECT
    (bool success,) = msg.sender.call{value: amount}("");  // INTERACTION last
    require(success);
}

Grep Patterns

# External calls before state updates
grep -rn "\.call{value\|safeTransfer\|transfer(" contracts/ -B10 | grep -v "require\|revert"

# Missing nonReentrant modifier on critical functions
grep -rn "function withdraw\|function redeem\|function claim" contracts/ -A2 | grep -v "nonReentrant"

# Storage slot for reentrancy guard
grep -rn "nonReentrant\|ReentrancyGuard\|_notEntered" contracts/

8. FLASH LOAN ATTACKS

Oracle Manipulation via Flash Loan

// Attack flow:
// 1. Borrow $100M from Aave flash loan
// 2. Dump token in Uniswap pool → crash spot price
// 3. Protocol reads Uniswap spot → undercollateralized loans accepted
// 4. Borrow max against cheap collateral
// 5. Repay flash loan, keep profits

Price Oracle Sanity Checks (what to look for)

grep -rn "getReserves\|getAmountsOut\|slot0\b" contracts/ -A5
# spot price from reserves = manipulatable with flash loan
# slot0 = Uniswap V3 spot price = manipulatable

9. SIGNATURE REPLAY

Missing Nonce

// VULNERABLE:
function permit(address owner, address spender, uint256 value,
                uint256 deadline, uint8 v, bytes32 r, bytes32 s) external {
    bytes32 hash = keccak256(abi.encodePacked(owner, spender, value, deadline));
    // MISSING: nonce not included → same signature usable multiple times
    require(ecrecover(hash, v, r, s) == owner);
}

Missing Chain ID

// VULNERABLE: signature valid on mainnet AND testnet AND all forks
bytes32 hash = keccak256(abi.encodePacked(params));
// MISSING: block.chainid not in hash → works on any chain

Grep Patterns

grep -rn "ecrecover\|ECDSA\.recover" contracts/ -B20
# Check: does the signed hash include nonce + chainId + contract address?

grep -rn "nonce\|_nonces\|nonces\[" contracts/

10. PROXY / UPGRADE ISSUES

Storage Collision

// Implementation and proxy share storage layout
// Proxy slot 0: _owner
// Implementation slot 0: _initialized
// → writing to _initialized overwrites _owner

Uninitialized Implementation

// If implementation can be initialized directly → anyone becomes owner of implementation
// Attack: call initialize() on implementation contract → call upgradeTo() → replace logic

delegatecall to User-Controlled Address

function execute(address target, bytes calldata data) external onlyOwner {
    target.delegatecall(data);  // target is validated, but what if owner is compromised?
}

Grep Patterns

# UUPS initialization protection
grep -rn "function initialize\b\|_disableInitializers\|initializer" contracts/

# Delegate call
grep -rn "delegatecall\b" contracts/ -B3 -A5

# Storage layout — proxy uses EIP-1967 slots?
grep -rn "0x360894\|EIP1967\|_IMPLEMENTATION_SLOT" contracts/

FOUNDRY POC TEMPLATE

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "forge-std/Test.sol";
import "../src/VulnerableContract.sol";

contract ExploitTest is Test {
    VulnerableContract target;
    address attacker = makeAddr("attacker");
    address victim = makeAddr("victim");

    function setUp() public {
        // Fork mainnet at specific block
        vm.createSelectFork("mainnet", BLOCK_NUMBER);

        // Deploy or load target
        target = VulnerableContract(TARGET_ADDRESS);

        // Fund accounts
        deal(address(token), attacker, INITIAL_BALANCE);
        deal(address(token), victim, VICTIM_BALANCE);
    }

    function test_exploit() public {
        console.log("Attacker balance before:", token.balanceOf(attacker));

        vm.startPrank(attacker);

        // Step 1: Setup conditions
        // Step 2: Execute exploit
        // Step 3: Verify impact

        vm.stopPrank();

        console.log("Attacker balance after:", token.balanceOf(attacker));
        assertGt(token.balanceOf(attacker), INITIAL_BALANCE, "Exploit failed");
    }
}

Key Foundry Cheatcodes

vm.prank(address)           // next call from address
vm.startPrank(address)      // all calls from address until stopPrank()
vm.deal(address, amount)    // set ETH balance
deal(token, address, amount) // set ERC20 balance
vm.warp(timestamp)          // set block.timestamp
vm.roll(blockNumber)        // set block.number
vm.createSelectFork("mainnet", blockNumber)  // fork mainnet
vm.expectRevert(bytes)      // next call should revert
vm.label(address, "name")   // label for trace output
vm.assume(condition)        // fuzz: discard inputs where false

Running Tests

# Run specific test
forge test --match-test test_exploit -vvvv

# Run with fork
forge test --match-test test_exploit -vvvv --fork-url $MAINNET_RPC

# Gas report
forge test --gas-report

# Coverage
forge coverage --report summary