The root cause of the CWE-125 (Out-of-bounds Read) vulnerability in the given code slice is:

The declaration of the 'build_label' array with a fixed size of QCA_FW_BUILD_VER_LEN without proper bounds checking. This can lead to potential buffer overflow or out-of-bounds read issues if the data being read into this array exceeds the allocated size.

To mitigate this vulnerability:
1. Ensure that any operations reading data into 'build_label' are properly bounded.
2. Use dynamic memory allocation if the size of the input is not known in advance.
3. Implement input validation to check the length of data before storing it in 'build_label'.
4. Consider using safer string handling functions that include bounds checking.