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 `buf` array is accessed at index `len` without checking if `len` is within the bounds of the `buf` array.

The `usb_string` function (whose definition is not provided) returns the length of the string it copies into `buf`. However, there is no check to ensure that `len` is less than `maxlen - 1`. If `len` is equal to `maxlen - 1`, then `buf[len]` will be an out-of-bounds write, as the valid indices for `buf` are from 0 to `maxlen - 2`.

To mitigate this vulnerability, the code should check if `len` is less than `maxlen - 1` before writing `buf[len] = 0;`.