Reentrancy Pattern Analysis

1. CHECKS   — Validate all conditions (require statements, access control)
2. EFFECTS  — Update all state variables
3. INTERACTIONS — Make external calls (ETH transfers, token calls, cross-contract)

// VULNERABLE
function withdraw(uint256 amount) public {
    require(balances[msg.sender] >= amount);
    (bool success, ) = msg.sender.call{value: amount}(""); // INTERACTION before EFFECT
    require(success);
    balances[msg.sender] -= amount; // State update AFTER external call
}

function withdraw(uint256 amount) public {
    require(balances[msg.sender] >= amount);
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success);
    balances[msg.sender] -= amount;
}

// Attacker re-enters HERE during withdraw's external call
function transfer(address to, uint256 amount) public {
    require(balances[msg.sender] >= amount);
    balances[msg.sender] -= amount;
    balances[to] += amount;
}

// Contract A
function withdrawFromVault() public {
    uint256 shares = vault.balanceOf(msg.sender);
    vault.burn(msg.sender, shares);
    // External call — attacker can re-enter Contract B
    (bool success, ) = msg.sender.call{value: shares * pricePerShare}("");
    require(success);
}

// Contract B (attacker re-enters here)
function borrow() public {
    uint256 collateral = vault.balanceOf(msg.sender); // Reads stale state!
    // Shares not yet burned, so collateral appears inflated
    uint256 loanAmount = collateral * maxLTV;
    token.transfer(msg.sender, loanAmount);
}

// Pool contract
function removeLiquidity() external {
    uint256 shares = balances[msg.sender];
    // Burns LP tokens (updates internal accounting)
    _burn(msg.sender, shares);
    // External call BEFORE updating reserves
    (bool success, ) = msg.sender.call{value: ethAmount}("");
    // Reserves updated AFTER the call
    totalReserves -= ethAmount;
}

// This view function returns stale data during the callback
function getRate() public view returns (uint256) {
    return totalReserves / totalSupply(); // totalReserves not yet updated!
}

// Third-party contract reads the inflated rate
function priceOracle() external view returns (uint256) {
    return pool.getRate(); // Returns wrong value during reentrancy
}

// ERC-777: tokensReceived() hook called on recipient
// ERC-1155: onERC1155Received() hook called on recipient
// ERC-721: onERC721Received() hook called on recipient

function deposit(uint256 amount) public {
    token.transferFrom(msg.sender, address(this), amount); // Triggers callback!
    // If token is ERC-777, msg.sender's tokensReceived() runs HERE
    balances[msg.sender] += amount; // State update after callback
}

Function: withdraw()
├── External Calls:
│   ├── msg.sender.call{value: amount}("") at line 45
│   ├── token.transfer(user, amount) at line 48
│   └── oracle.getPrice() at line 42
├── State Writes:
│   ├── balances[msg.sender] -= amount at line 50
│   └── totalWithdrawn += amount at line 51
├── State Reads (in requires):
│   └── balances[msg.sender] at line 41
└── Modifiers:
    └── nonReentrant: NO

For each function F with external calls:
  1. E = set of all state variables written by F
  2. C = set of all state variables read in require/if checks
  3. I = position of each external call in F
  4. For each external call at position P:
     a. W_after = state writes that occur AFTER position P
     b. If W_after ∩ (E ∪ C) ≠ ∅:
        → CEI VIOLATION: state modified after external call
     c. Classify violation:
        - W_after ∩ C ≠ ∅ → Classic reentrancy (check variable modified after call)
        - W_after ∩ E ≠ ∅ → State inconsistency window

For each external call in function F at position P:
  W_before = state variables NOT yet updated at position P
  For each OTHER public function G:
    R_G = state variables read by G
    W_G = state variables written by G
    If R_G ∩ W_before ≠ ∅ OR W_G ∩ W_before ≠ ∅:
      → CROSS-FUNCTION REENTRANCY: G can be called during F's external call
         with inconsistent state