Unsafe Downcasting Remediation

Overview

Related Detector: Unsafe Downcasting

Casting a wider integer to a narrower one — uint256 to uint128, uint64, uint32 — truncates the high bits with no revert, even under Solidity 0.8’s checked arithmetic (the overflow checks do not cover explicit casts). A value that exceeds the target width wraps to a small number, corrupting balances, timestamps, or packed storage. The remediation is a checked cast that reverts when the value does not fit.

Recommended Fix

Before (Vulnerable)

function record(uint256 amount) external {
    // If amount > type(uint128).max, the high bits are silently dropped.
    packedAmount = uint128(amount);
}

After (Fixed — checked cast)

// OpenZeppelin SafeCast reverts when the value does not fit the target type.
using SafeCast for uint256;

function record(uint256 amount) external {
    packedAmount = amount.toUint128();
}

If you cannot add a dependency, write the bound explicitly: require(amount <= type(uint128).max, "overflow"); packedAmount = uint128(amount);.

Common Mistakes

Believing Solidity 0.8’s built-in overflow checks cover downcasts — they do not; only arithmetic operators are checked.
Downcasting a signed/unsigned boundary (int256 to uint128) and losing the sign as well as the magnitude.
Packing several values into one slot with raw casts, so one oversized field silently corrupts its neighbours.