Smart Contract Vulnerabilities

# SKILL: Smart Contract Vulnerabilities — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Expert smart contract audit techniques. Covers reentrancy (single, cross-function, cross-contract, read-only), integer overflow, access control, delegatecall, randomness manipulation, flash loans, signature replay, front-running/MEV, and CREATE2 exploitation. Base models miss subtle cross-contract reentrancy and storage layout collisions in proxy patterns.

## 0. RELATED ROUTING

- [defi-attack-patterns](../defi-attack-patterns/SKILL.md) when the vulnerability is part of a DeFi protocol exploit (flash loans, oracle manipulation, governance attacks)
- [deserialization-insecure](../deserialization-insecure/SKILL.md) when the target is off-chain infrastructure deserializing blockchain data

### Advanced Reference

Also load [SOLIDITY_VULN_PATTERNS.md](./SOLIDITY_VULN_PATTERNS.md) when you need:
- Side-by-side vulnerable vs fixed code patterns for each vulnerability class
- Gas optimization traps that introduce vulnerabilities
- Proxy pattern storage collision examples with slot calculations

---

## 1. REENTRANCY

The most iconic smart contract vulnerability. External calls transfer execution control; if state is not updated before the call, the callee can re-enter.

### 1.1 Classic Reentrancy (Single-Function)

```
Victim.withdraw()
  ├── checks balance[msg.sender] > 0          ✓
  ├── msg.sender.call{value: balance}("")     ← external call
  │   └── Attacker.receive()
  │       └── Victim.withdraw()               ← re-enters before state update
  │           ├── checks balance[msg.sender]   ← still > 0!
  │           └── sends ETH again
  └── balance[msg.sender] = 0                 ← too late
```

### 1.2 Cross-Function Reentrancy

Two functions share state; attacker re-enters a different function during callback:

| Step | Execution | State |
|---|---|---|
| 1 | Call `withdraw()` → external call | balance still positive |
| 2 | Attacker fallback calls `transfer(attacker2)` | balance used before reset |
| 3 | `transfer` reads stale balance → moves funds | attacker2 receives tokens |
| 4 | Original `withdraw` completes, zeroes balance | damage done |

### 1.3 Cross-Contract Reentrancy

Contract A calls Contract B, which calls back into Contract A (or Contract C that reads A's stale state). Especially dangerous in DeFi protocols where multiple contracts share state.

### 1.4 Read-Only Reentrancy

The re-entered function is a `view` function used by a third-party contract for price calculation. No state modification in the victim, but the stale intermediate state misleads the reader.

**Real-world**: Curve pool `get_virtual_price()` read during `remove_liquidity()` callback → inflated price → profit on dependent lending protocol.

### Mitigations

| Pattern | Protection Level |
|---|---|
| Checks-Effects-Interactions (CEI) | Core defense; update state before external call |
| `ReentrancyGuard` (OpenZeppelin) | Mutex lock; prevents same-tx re-entry |
| Pull payment pattern | Eliminate external calls in state-changing functions |
| CEI + guard on all public functions | Defense-in-depth against cross-function |

---

## 2. INTEGER OVERFLOW / UNDERFLOW

### Pre-Solidity 0.8

Arithmetic silently wraps: `uint8(255) + 1 == 0`, `uint8(0) - 1 == 255`.

| Attack | Example |
|---|---|
| Balance underflow | `balances[attacker] -= amount` when amount > balance → huge balance |
| Supply overflow | `totalSupply + mintAmount` wraps → bypass cap checks |
| Timelock bypass | `lockTime[msg.sender] + extend` wraps to past → early unlock |

### Post-Solidity 0.8

Default checked arithmetic reverts on overflow. But `unchecked{}` blocks reintroduce risk:

```solidity
unchecked {
    // "gas optim