A: Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-476 (NULL Pointer Dereference) vulnerability exists in the code because the `strchr` function could return NULL if the character '/' is not found in the string `ip_strv4`. In this case, the assignment to `mask_str` will result in a NULL pointer and will lead to a NULL pointer dereference in subsequent operations on `mask_str`.