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 variable `pos` is used as an offset to access the string `str` without checking if `pos` is within the bounds of the string. If `pos` is greater than or equal to the length of the string, then accessing the string at `RSTRING_PTR(str) + pos` will result in an out-of-bounds write vulnerability, which is CWE-787.