Integer Overflow and Underflow

Why this rank in 2026

Held mid-list despite producing the largest single-category dollar total in 2025 ($260.4M, ~29% of all 2025 losses) - driven almost entirely by the Cetus Move bug on Sui. OWASP keeps it ranked #9 because Solidity 0.8+ has effectively closed the EVM half of the category; the remaining risk is non-EVM (Move, Rust, Cairo).

Vulnerable vs secure code

// Vulnerable: pre-0.8 wrap-around - withdrawing more than balance underflows to max-uint
contract OldVault {
    mapping(address => uint256) public balance;

    function withdraw(uint256 amount) external {
        balance[msg.sender] = balance[msg.sender] - amount;   // pre-0.8: underflows silently
        payable(msg.sender).transfer(amount);
    }
}

// Secure: Solidity 0.8+ with explicit downcast check
contract Vault {
    mapping(address => uint128) public balance;

    function deposit(uint256 amount) external {
        require(amount <= type(uint128).max - balance[msg.sender], "overflow");
        balance[msg.sender] = uint128(uint256(balance[msg.sender]) + amount);
    }
}

Prevention checklist

• Use Solidity 0.8.0+ for automatic overflow checks; document any unchecked block with a formal proof.
• On non-EVM platforms (Move, Solana, Sui, Cairo): document overflow semantics, bounds-check before arithmetic, assert invariants, fuzz with extreme values.
• Test extreme values on multiplication and exponentiation.
• Validate downcasts (uint256 → uint128) explicitly.
• Apply extra scrutiny to shared library functions; formal-verify critical math.

Deep dive: related SCH attack pages

Sources & methodology

Every claim on this page traces back to a primary, citable source. Open the originals if you want to verify the numbers or read the underlying methodology.

Quoted OWASP descriptions are used under CC BY-NC-SA 4.0. SCH editorial content (analysis, code, prevention prose, FAQ, AI-vs-human rates) is © Smart Contract Hacking.