How Attackers Can Exploit Integer Overflowsīecause of their undefined behavior, integer overflow and underflow errors are notoriously difficult to debug. If a negative value becomes smaller than the minimum value for the current signed type, you get underflow – the negative version of overflow. Because the leftmost bit of a signed integer is 1 only for negative numbers, when a positive value overflows, it can actually become negative. In some specialized hardware, such as signal processors, there is no wraparound or truncation in these cases – the maximum value is simply limited to the biggest representable positive number.įor signed integer types, things get even weirder due to the way negative numbers are represented in binary. For example, if you have a 32-bit integer (unsigned) with the maximum value and increment it by 42, you might get 4,294,967,295 + 42 = 41.
In most modern systems, the value doesn't actually overflow into adjacent memory bits but is wrapped around or truncated using a modulo operation to fit in the variable.įor unsigned integers, this usually means retaining the least significant bits (for 32 bits, this would be the last 10 digits of the decimal value), in effect wrapping the result around zero. The behavior in an integer overflow situation depends on the hardware, compiler, and programming language. If you have a calculation that gives a larger number, not all bits of the result will fit in the 32 bits available for this data type and you get an overflow. In hexadecimal notation, this is 0xFFFFFFFF and here you can clearly see that all bytes already have maximum values (i.e. For example, the biggest number you can store in a 32-bit unsigned integer variable is 4,294,967,295. Let's see why integer overflow errors are possible, how they can be dangerous, and what you can do to prevent them.Īt the most basic level, an integer overflow occurs when the result of an arithmetic operation needs more bits than the target variable has. It is a type of arithmetic overflow error that can not only lead to incorrect results and system instability but also cause buffer overflows and provide an entry point for attackers. An integer overflow happens when a program tries to store an integer value that is too big for the declared integer type.
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