Integer Overflow Vulnerabilities and Their Protection

Integer overflow is a common issue in programming. In most programming languages, integer values are stored in fixed-length memory. Integers are divided into two types: unsigned and signed, with the distinction being whether the highest bit is used as the sign bit. For example, a 32-bit memory can store unsigned integers in the range of 0 to 4,294,967,295 (uint32), or signed integers in the range of -2,147,483,648 to 2,147,483,647 (int32).

When the calculation result exceeds the range that the integer type can represent, an overflow occurs. Most programming languages and compilers do not check for such errors, but simply perform modulo operations or produce undefined behavior. This can lead to unexpected results in program execution. In blockchain smart contract development, especially in the decentralized finance sector, integer calculations are common, so special attention must be paid to integer overflow vulnerabilities.

Integer overflow can be divided into two cases: overflow and underflow.

1. Overflow: The calculation result exceeds the maximum value that the type can represent. For example, adding 1 to uint32's 0xFFFFFFFF will result in 0x00000000.

2. Underflow: The result of the calculation is less than the minimum value that the type can represent. For example, subtracting 1 from 0 in uint32 will result in 0xFFFFFFFF.

In April 2018, the BeautyChain(BEC) token contract was attacked due to an integer overflow vulnerability, resulting in the attacker gaining a large amount of tokens. This vulnerability occurred in the batchTransfer function, where the lack of multiplication overflow checks allowed a small token balance to result in the transfer of a large number of tokens.

To prevent integer overflow, the following measures can be taken:

1. Configure Rust compilation options to check for integer overflow and trigger panic even in release mode.

2. Use the uint crate to support larger integer types, such as U256, U512, etc.

3. Use the conversion function of uint type to detect overflow, such as as_u128().

4. Use Safe Math functions like checked_add() to check for overflow in calculations.

By using these methods, integer overflow vulnerabilities can be effectively avoided, enhancing the security of smart contracts. When writing contracts that involve large number calculations, it is essential to handle integer overflow issues with caution.