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 when the vulnerability is part of a DeFi protocol exploit (flash loans, oracle manipulation, governance attacks)
• deserialization-insecure when the target is off-chain infrastructure deserializing blockchain data

Advanced Reference

Also load 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

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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:

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

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2. INTEGER OVERFLOW / UNDERFLOW

Pre-Solidity 0.8

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

Post-Solidity 0.8

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

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

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3. ACCESS CONTROL

tx.origin vs msg.sender

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

Common Patterns

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4. RANDOMNESS MANIPULATION

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

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

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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.

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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

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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

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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.

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9. CREATE2 & DETERMINISTIC ADDRESS EXPLOITATION

CREATE2 address = keccak256(0xff ++ deployer ++ salt ++ keccak256(initCode)).

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10. FLASH LOAN ATTACK PATTERNS

Single transaction:
├── Borrow large amount (no collateral)
├── Manipulate state (price oracle, governance, etc.)
├── Extract profit from manipulated state
├── Repay loan + fee
└── Keep profit

Key: entire sequence must succeed atomically or the whole tx reverts.

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11. SHORT ADDRESS ATTACK

EVM pads missing bytes in ABI-encoded calldata with zeros. If transfer(address, uint256) is called with a 19-byte address, the uint256 amount shifts left by 8 bits → multiplied by 256.

Mitigation: validate calldata length; modern Solidity compilers add checks.

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12. TOOLS

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13. DECISION TREE

Auditing a smart contract?
├── Is it a proxy pattern?
│   ├── Yes → Check storage layout collision (Section 5)
│   │   ├── Compare slot assignments between proxy and implementation
│   │   ├── Check for function selector collision
│   │   └── Verify initializer cannot be called twice
│   └── No → Continue
├── Does it make external calls?
│   ├── Yes → Check reentrancy (Section 1)
│   │   ├── State updated before call? → CEI pattern OK
│   │   ├── ReentrancyGuard present? → Check all entry points
│   │   ├── Cross-function state sharing? → Cross-function reentrancy risk
│   │   └── View functions read during callback? → Read-only reentrancy
│   └── No → Continue
├── Does it handle tokens/ETH?
│   ├── Yes → Check integer overflow (Section 2)
│   │   ├── Solidity < 0.8? → All arithmetic suspect
│   │   ├── unchecked{} blocks? → Verify no user-influenced values
│   │   └── Casting between uint sizes? → Truncation risk
│   └── Also check self-destruct force-send (Section 8)
├── Does it use signatures?
│   ├── Yes → Check replay (Section 7)
│   │   ├── Nonce included? → Verify incremented
│   │   ├── ChainId included? → Cross-chain safe
│   │   └── ecrecover result checked for address(0)? → OK
│   └── No → Continue
├── Does it use on-chain randomness?
│   ├── Yes → Predictable (Section 4)
│   │   └── Recommend Chainlink VRF or commit-reveal with bond
│   └── No → Continue
├── Does it interact with DeFi protocols?
│   ├── Yes → Load [defi-attack-patterns](../defi-attack-patterns/SKILL.md)
│   │   ├── Flash loan vectors
│   │   ├── Oracle manipulation
│   │   └── MEV exposure
│   └── No → Continue
├── Does it use CREATE2?
│   ├── Yes → Check deterministic address exploitation (Section 9)
│   └── No → Continue
└── Run automated tools (Section 12)
    ├── Slither for static analysis
    ├── Mythril for symbolic execution
    └── Echidna for fuzzing invariants