CWE 196

Requirements specification: Choose a language which is not subject to these casting flaws.

Phase: Architecture and Design

Design object accessor functions to implicitly check values for valid sizes. Ensure that all functions which will be used as a size are checked previous to use as a size. If the language permits, throw exceptions rather than using in-band errors.

Phase: Implementation

Error check the return values of all functions. Be aware of implicit casts made, and use unsigned variables for sizes if at all possible.

Often, functions will return negative values to indicate a failure state. In the case of functions which return values which are meant to be used as sizes, negative return values can have unexpected results. If these values are passed to the standard memory copy or allocation functions, they will implicitly cast the negative error-indicating value to a large unsigned value. In the case of allocation, this may not be an issue; however, in the case of memory and string copy functions, this can lead to a buffer overflow condition which may be exploitable. Also, if the variables in question are used as indexes into a buffer, it may result in a buffer underflow condition.

Although less frequent an issue than signed-to-unsigned casting, unsigned-to-signed casting can be the perfect precursor to dangerous buffer underwrite conditions that allow attackers to move down the stack where they otherwise might not have access in a normal buffer overflow condition. Buffer underwrites occur frequently when large unsigned values are cast to signed values, and then used as indexes into a buffer or for pointer arithmetic.