smart-contract-vulnerabilities

What this skill does

# 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 optimization" — but if i can be influenced by user input, overflow returns
    for (uint i = start; i < end; i++) { ... }
}
```

### SafeMath Bypass Scenarios

- Casting: `uint256` → `uint128` truncation before SafeMath check
- Assembly blocks: `mstore` / `add` bypass Solidity-level checks
- Intermediate multiplication overflow before division: `(a * b) / c` where `a * b` overflows

---

## 3. ACCESS CONTROL

### tx.origin vs msg.sender

| Property | `msg.sender` | `tx.origin` |
|---|---|---|
| Value | Immediate caller | EOA that initiated the tx |
| Safe for auth | Yes | **No** — phishing contract can inherit tx.origin |

Attack: trick owner into calling attacker contract → attacker contract calls victim with owner's `tx.origin`.

### Common Patterns

| Issue | Impact |
|---|---|
| Missing `onlyOwner` on critical functions | Anyone can call admin functions |
| Unprotected `selfdestruct` | Anyone can destroy the contract, force-send ETH |
| Unprotected `delegatecall` | Attacker executes arbitrary code in victim's context |
| Default visibility (pre-0.6.0) | Functions default to `public` |
| Missing zero-address checks | Ownership transferred to `address(0)` |

---

## 4. RANDOMNESS MANIPULATION

On-chain randomness sources are predictable to miners/validators:

| Source | Predictability |
|---|---|
| `block.timestamp` | Miner has ~15s window to manipulate |
| `blockhash(block.number - 1)` | Known to all at execution time |
| `blockhash(block.number)` | Always returns 0 (current block hash unknown) |
| `block.difficulty` / `block.prevrandao` | Post-merge: known beacon chain value |

**Commit-reveal bypass**: If reveal phase doesn't enforce timeout or bond, attacker can choose not to reveal unfavorable outcomes (selective abort attack).

---

## 5. DELEGATECALL VULNERABILITIES

`delegatecall` executes callee's code in caller's storage context. Storage slot layout must match exactly.

### Storage Layout Collision

```
Proxy (storage):         Implementation (code):
slot 0: owner            slot 0: someVariable
slot 1: implementation   slot 1: anotherVariable
```

Implementation writes to `someVariable` (slot 0) → overwrites proxy's `owner`. Attacker calls implementation function that writes slot 0 → becomes proxy owner.

### Function Selector Collision

4-byte function selectors can collide. If proxy's `admin()` selector collides with implementation's `transfer()`, calling `admin()` on the proxy executes `transfer()` logic.

Tool: `cast selectors <bytecode>` (Foundry) to enumerate selectors.

---

## 6. FRONT-RUNNING / MEV

### Transaction Ordering Manipulation

```
Victim submits DEX swap tx (visible in mempool)
├── Front-runner: buy token before victim (raise price)
├── Victim tx executes at worse price
└── Back-runner: sell token after victim (profit from spread)
= Sandwich attack
```

### Protection Patterns

| Defense | Mechanism |
|---|---|
| Commit-reveal | Hide transaction intent until reveal |
| Flashbots / private mempool | Submit tx directly to block builder |
| Slippage protection | Set `minAmountOut` to limit MEV extraction |
| Time-lock | Delay execution to reduce predictability |

---

## 7. SIGNATURE REPLAY

### Missing Nonce

Reuse a valid signature to repeat the action (e.g., transfer) multiple times.

### Cross-Chain Replay

Same contract deployed on multiple chains with same address → signature valid on all chains. Must include `block.chainid` in signed message.

### EIP-712 Implementation Errors

| Error | Consequence |
|---|---|
| Missing `DOMAIN_SEPARATOR` with chainId | Cross-chain replay |
| Domain separator cached at deploy | Breaks after hard fork changing chainId |
| Missing nonce in struct hash | Signature replay |
| `ecrecover` returns `address(0)` on invalid sig | Passes `== address(0)` owner check |

---

## 8. SELF-DESTRUCT & FORCE-SEND ETH

`selfdestruct(recipient)` force-sends all contract ETH to recipient — bypasses `receive()` and `fallback()`, cannot be rejected.

Breaks contracts that rely on `address(this).balance` for logic (e.g., `require(balance == expected)`).

Post-EIP-6780 (Dencun): `selfdestruct` only sends ETH; code/storage deletion only if called in same tx as creation.

---

## 9. CREATE2 & DETERMINISTIC ADDRESS EXPLOITATION

`CREATE2` address = `kec