Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-787 (out-of-bounds write) vulnerability exists in the code because the `frame` array may not have enough space to store the data written to it. This can happen if the `frame_end` is less than the `frame` plus one, which is the size of the data being written to the `frame` array. This can result in writing data beyond the bounds of the `frame` array, causing a buffer overflow vulnerability, which is CWE-787.