myctrl.tools

SC09—Integer Overflow and Underflow

>Control Description

Integer overflow and underflow occur when arithmetic operations produce values outside the representable range of the operand type. In Solidity 0.8+, arithmetic is checked by default and reverts on overflow/underflow. However, explicit `unchecked` blocks, assembly, and custom libraries can disable these checks. On non-EVM platforms (Move, Sui, Solana, Rust-based chains), default overflow semantics differ significantly — some wrap silently, some abort — and incorrect assumptions or flawed custom checks can lead to wrapped values, miscomputed balances, and broken invariants. This affects all contract types performing arithmetic: DeFi protocols (pool invariants, balances, interest calculations, share calculations), NFTs (supply tracking, token IDs), bridges (amounts, sequence numbers), and any logic involving large or user-controlled numeric inputs. **Key Focus Areas:** - EVM/Solidity: use of `unchecked`, assembly, pre-0.8 codebases - Non-EVM chains: Move, Sui, Aptos, Solana, and their default overflow semantics - Multiplication and exponentiation: high risk of overflow with large operands - Subtraction and decrement: underflow when subtrahend exceeds minuend - Casting and type conversion: downcasting (e.g., uint256 to uint128)

>Prevention & Mitigation Strategies

  1. 1.Avoid unchecked arithmetic unless you have strong reasons and comprehensive tests proving safety.
  2. 2.Use explicit checks and custom errors for critical invariants.
  3. 3.Favor well-reviewed math libraries for fixed-point arithmetic, exponentiation, etc.
  4. 4.Upgrade to Solidity 0.8.0+ if using pre-0.8 versions, or implement SafeMath library.
  5. 5.On non-EVM environments (Move, Rust-based chains), understand the language's default overflow semantics and use safe arithmetic constructs.
  6. 6.Test with extreme value ranges (minimum and maximum values for all numeric types).
  7. 7.Implement fuzz tests that target edge cases near boundaries where overflow/underflow is likely.
  8. 8.Conduct formal verification of custom overflow checks.

>Attack Scenarios

#1Cetus Protocol (May 2025, $223M loss)

The Cetus Protocol on Sui was exploited via a flawed checked_shlw function in the shared integer-mate library. The overflow check used the wrong threshold (0xFFFFFFFFFFFFFFFF << 192 instead of 1 << 192), allowing values >= 2^192 to pass. In Move, left shift operations do not abort on overflow — they truncate silently. The flawed check caused get_delta_a to calculate that only 1 token was required to mint enormous liquidity. Attackers exploited this across multiple pools using flash swaps, draining approximately $223M.

>References

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