---
name: web3-audit
description: Smart contract security audit — 10 DeFi bug classes (accounting desync, access control, incomplete path, off-by-one, oracle, ERC4626, reentrancy, flash loan, signature replay, proxy), pre-dive kill signals (TVL < $500K etc), Foundry PoC template, grep patterns for each class, and real Immunefi paid examples. Use for any Solidity/Rust contract audit or when deciding whether a DeFi target is worth hunting.
sources: public_research, community
---

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

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

### Grep Patterns
```bash
# 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

| Protocol | Payout | Bug |
|---|---|---|
| Wormhole | $10M | Uninitialized UUPS proxy → anyone calls initialize() |
| ZeroLend | n/a | split() uses existence check, not ownership check |
| Alchemix | n/a | poke() missing onlyNewEpoch → infinite FLUX inflation |
| Parity | $150M frozen | No access control on initWallet() in library |

---

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

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

### Grep Patterns
```bash
# 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)
```solidity
// 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)
```solidity
// 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
```bash
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
```solidity
// 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
```bash
# 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
```solidity
// 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)
```bash
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
```solidity
// 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
```solidity
// 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
```bash
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
```solidity
// Implementation and proxy share storage layout
// Proxy slot 0: _owner
// Implementation slot 0: _initialized
// → writing to _initialized overwrites _owner
```

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

### Grep Patterns
```bash
# 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

```solidity
// 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
```solidity
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
```bash
# 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
```

---

## Related Skills & Chains

- **`meme-coin-audit`** — When the target is a meme coin / SPL token rather than a DeFi protocol. Workflow primitive: pre-dive kill signals diverge — this skill's "TVL < $500K skip" doesn't apply to meme coins where the rug check (mint authority, freeze authority, LP lock) is the entire audit; route to `meme-coin-audit` instead.
- **`triage-validation`** — When a contract finding is ready to be filed on Immunefi. Workflow primitive: Immunefi has its own report format, but the impact-validated, chain-end-to-end discipline of `triage-validation` still applies; run the 7Q gate against the Foundry PoC before submitting.
- **`report-writing`** — When writing the Immunefi report body. Workflow primitive: `report-writing`'s Immunefi template (with Foundry PoC, root cause code snippet, quantified economic impact) is the body skeleton this skill's findings feed into.
- **`offensive-osint`** — When auditing a protocol's off-chain attack surface (frontend, admin API, RPC gateways). Workflow primitive: on-chain audit is this skill's job; any web2 component of the protocol (web-frontend, admin panel, indexer API) routes to `offensive-osint` for recon.
- **`bb-methodology`** — When deciding whether to dive at all. Workflow primitive: PART 0 of `bb-methodology` confirms engagement (web3 bug bounty / private audit / smart-contract review); this skill's pre-dive kill signals replace the standard scoring rubric for that engagement type.

---

## Operator Notes (Claude-BugHunter)

> Engagement-derived + 2026-specific additions to the vendored foundation.
> Wisdom from real authorized engagements + Phase 2 verification across
> this repo's 31+ skill-area live tests. The upstream content covers the WHAT;
> this layer covers the WHEN-IT-WORKS-vs-WHEN-IT-DOESN'T.

### Bug classes still paying in 2026

Flash-loan attacks remain top-paid on Immunefi (top 5 in 2024-2026 by bounty). The economic primitive — borrow $50M, manipulate price oracle, drain pool, repay — keeps reappearing because new protocols keep shipping with composability assumptions that don't hold under flash-loaned imbalance.

Reentrancy IS still paying because new protocols keep shipping with ERC-777 / hooks / callbacks. Don't assume the class is dead — the 2023-2025 paid corpus contains 40+ reentrancy bugs against post-Checks-Effects-Interactions codebases (cross-function reentrancy, read-only reentrancy via view functions called during state-mid-flight).

Oracle manipulation: still paid heavily but harder. Most projects use Chainlink price feeds now; the attack target is the SECONDARY oracle most projects also use (TWAP from a low-liquidity Uniswap V2 pair, the protocol's own internal oracle, a stale fallback path). Audit the failover chain, not just the primary feed.

### What's new since the vendored content was written

- **EIP-1153 (transient storage)** — introduced in 2024. New reentrancy classes: transient-storage reads cached across the same transaction can desync from persistent storage. Audit any `tload`/`tstore` usage for read-after-external-call.
- **EIP-7702 (Pectra hard fork 2025)** — added EOA-to-smart-account upgrades. New ATO-like primitives via re-delegation: an EOA signed-once can delegate to a contract that the attacker controls, then signature replay across delegations.
- **Account abstraction (ERC-4337 bundlers)** — paymaster sponsorship abuse and bundler griefing. Paymaster contracts that don't enforce strict sender allowlists drain on first call.
- **ZK-rollup bridge bugs** — proof-replay across rollups, off-chain prover compromise, sequencer censorship leading to forced-inclusion edge cases.
- **LST/LRT depeg dynamics** — liquid-staking and liquid-restaking tokens that assume 1:1 peg under loss conditions; oracle assumes peg, market reflects depeg, liquidation logic breaks.

### Tool stack for 2026

Foundry remains the test framework. `forge test --gas-report --debug` for invariant testing; `forge fuzz` for property-based testing; `forge inspect` for storage-layout audits. Slither + Echidna for static + fuzz. Mythril for symbolic execution on smaller contracts. tenderly.co for forking + simulation (best UX for replicating attacks against mainnet state).

For Solana: anchor framework, sealevel-attacks corpus (curated PoCs by anchor maintainers), soteria-sec / sec3 scanner. For Move (Aptos, Sui): move-prover, aptos-cli `aptos move test`.

For cross-chain: hyperlane and LayerZero each have audit-tooling repos; bridge bugs require simulating both endpoints, not just one.

### Where pre-dive kill signals matter

TVL under $500K isn't worth the audit time unless the bounty floor is high. Audit firm already covered it = low ROI unless you find what they missed — look at the audit-report scope-exclusion section for what they EXPLICITLY didn't audit (oracles, governance, off-chain components, frontend, the admin path).

Multisig signers > 5 + timelock > 48h = low rug-pull risk; if your finding requires team-malicious assumptions, it's low-impact and likely out of scope per Immunefi's "centralization risk" exclusion. Read the program brief — most Immunefi programs explicitly downgrade or reject findings that assume admin malice.

### Reporting discipline

Immunefi requires Foundry PoC. Submission without PoC is auto-rejected. Submission with a PoC that requires manual setup ("first deploy this, then call that") usually gets downgraded — the PoC should be a single `forge test` invocation that proves the impact, with explicit `assertEq` on the drained balance / minted token / corrupted state.

Severity claims must use Immunefi's severity matrix exactly; don't invent severities. The matrix gates on direct economic loss percentage of TVL — a critical against a $500K protocol pays differently than a critical against a $500M one. Read the program's specific severity assignment before claiming Critical.