The root cause of the CWE-125 (Out-of-bounds Read) vulnerability at line 187 is:

1. Lack of boundary checking: The code attempts to access buf[len - 1] without first verifying that len is greater than 0. This can lead to an out-of-bounds read if len is 0.

2. Incorrect assumption: The code assumes that there's always at least one character in the buffer (buf) and that len is always greater than 0. This assumption may not hold true in all cases, especially if the attribute value is empty or consists only of whitespace characters that were normalized.

3. Improper handling of edge cases: The code doesn't properly handle the case where all characters in the buffer are spaces (0x20) and need to be removed during normalization. In such a case, len could be decremented to 0, leading to an invalid memory access in the next iteration.

4. Missing safeguard: There's no check to ensure that len remains non-negative during the while loop execution.

To fix this vulnerability, the code should include a check to ensure len is greater than 0 before entering the while loop, and should break the loop if len becomes 0. For example:

```c
if (len > 0) {
    while (len > 0 && buf[len - 1] == 0x20) len--;
}
```

This modification would prevent the out-of-bounds read by ensuring that the buffer access is always within valid bounds.