The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in this code slice is:

1. Unbounded loop iteration:
   The for loop on line 25 iterates over `cinfo->num_components`, which could potentially be larger than the fixed-size array `cinfo->cur_comp_info`.

2. Fixed-size array access:
   The comment indicates that `cinfo->cur_comp_info` has a length of `MAX_COMPS_IN_SCAN`, which is a predefined constant.

3. Lack of bounds checking:
   There is no check to ensure that `i` does not exceed the size of `cinfo->cur_comp_info` before accessing it.

4. Potential for out-of-bounds write:
   If `cinfo->num_components` is greater than `MAX_COMPS_IN_SCAN`, the loop will attempt to write beyond the bounds of the `cur_comp_info` array, causing a buffer overflow.

To fix this vulnerability, the code should include a bounds check to ensure that `i` does not exceed `MAX_COMPS_IN_SCAN` before accessing `cinfo->cur_comp_info[i]`. Alternatively, the loop could be limited to iterate only up to `MAX_COMPS_IN_SCAN` or the minimum of `cinfo->num_components` and `MAX_COMPS_IN_SCAN`.