The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability at line 15 is:

1. Lack of null pointer check: The code does not verify if 'fw' is a valid non-null pointer before dereferencing it to access 'fw->len'.

2. Potential null pointer assignment: The 'fw' parameter is passed into the function, but there's no check to ensure it's not NULL before using it throughout the function.

3. Inconsistent error handling: The function doesn't appear to have proper error handling or return value checking for the kmemdup() call on line 12, which could potentially lead to 'fw->img' being NULL.

4. Implicit assumption of valid data: The code assumes that 'fw' and its members are always valid, without explicit validation.

To fix this vulnerability, the function should:

1. Add a null pointer check for 'fw' at the beginning of the function.
2. Implement proper error handling for memory allocation operations.
3. Validate all input parameters before use.

These measures would prevent potential null pointer dereferences and improve the overall robustness of the code.