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

The function DecodeBase64Block is performing a base64 decoding operation without proper bounds checking on the input or output arrays. Specifically:

1. There's no validation of the size or contents of the input array b64.
2. There's no check to ensure that ascii[0] is a valid index within the ascii array.

The vulnerability occurs because:

1. If b64[0] or b64[1] contain unexpected values, it could lead to writing an unintended value to ascii[0].
2. If the ascii array is smaller than expected or not properly allocated, writing to ascii[0] could result in an out-of-bounds write.

To mitigate this vulnerability:

1. Implement input validation to ensure b64 contains valid base64 characters.
2. Add bounds checking to verify that ascii has sufficient space for the decoded output.
3. Consider using a safer decoding function that includes built-in bounds checking and error handling.

By addressing these issues, the risk of out-of-bounds writes can be significantly reduced, improving the overall security and reliability of the code.