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

The line `wStream sbuffer = { 0 };` initializes a wStream structure on the stack with all members set to zero. This can lead to an out-of-bounds write vulnerability if the wStream structure is later used to write data without proper bounds checking or allocation.

Key points:

1. Stack allocation: The wStream object is created on the stack with a fixed size.

2. Zero initialization: All members are set to zero, including any pointers or size fields within the structure.

3. Potential misuse: If the code later attempts to write data to this stream without properly allocating memory or checking bounds, it could lead to buffer overflow or memory corruption.

4. Lack of dynamic allocation: The code doesn't allocate memory dynamically for the stream buffer, which may be necessary for handling variable-sized data.

To mitigate this vulnerability:

1. Properly initialize the wStream object with adequate buffer size and memory allocation.
2. Implement bounds checking when writing to the stream.
3. Use dynamic memory allocation if the size of the data is not known at compile-time.
4. Ensure proper error handling and resource cleanup.

By addressing these issues, the risk of out-of-bounds writes can be significantly reduced.