Root Cause Analysis: The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in the given code is not clear due to the lack of information about the `_stream->send` function. The vulnerability could potentially exist if the `_stream->send` function does not properly validate or handle the `size` parameter, leading to a buffer overflow or out-of-bounds write.

To better analyze the root cause, we need more information about the implementation of the `_stream->send` function. Please provide the summary of the function `_stream->send`.