The root cause of the CWE-190 (Integer Overflow or Wraparound) vulnerability in the given code is that the `len` parameter is of type `int`, which is a signed integer type. If a negative value is passed as the `len` parameter, it can lead to an integer overflow when used in the `memcpy` function, causing unexpected behavior or security vulnerabilities.

The `memcpy` function is a standard C library function that copies a specified number of bytes (`len`) from the source memory location (`data`) to the destination memory location (`start`). However, if `len` is a negative value, the behavior of `memcpy` is undefined, and it may result in an integer overflow or wraparound, potentially leading to buffer overflows, data corruption, or other security vulnerabilities.

To mitigate this vulnerability, the function should validate the `len` parameter to ensure that it is a non-negative value before calling `memcpy`. Additionally, using an unsigned integer type for the `len` parameter can also help prevent integer overflows or wraparounds.