The CWE-787 vulnerability is a "Out-of-bounds Write" vulnerability. 

In this specific case, the vulnerability is likely due to the fact that the `strbuf` struct is not properly initialized or checked for buffer overflow before being used.

The `STRBUF_INIT` macro is used to initialize the `strbuf` struct, but it does not guarantee that the buffer is large enough to hold the data that will be written to it.

The root cause of the vulnerability is likely that the `realpath` buffer is not properly checked for buffer overflow before being used to store the result of a `realpath` function call (not shown in the provided code slice). 

If the `realpath` function returns a path that is longer than the buffer size, it will overflow the buffer, causing undefined behavior and potentially allowing an attacker to execute arbitrary code.

To fix this vulnerability, the code should check the buffer size before writing to it, and ensure that the buffer is large enough to hold the data. Additionally, the code should use safe functions that prevent buffer overflows, such as `strbuf_addstr` or `strbuf_addpath`, instead of directly writing to the buffer.

Here is an example of how the code could be fixed:
```c
struct strbuf realpath = STRBUF_INIT;
if (strbuf_realpath(&realpath, src->buf) < 0) {
    die("Failed to resolve path: %s", src->buf);
}
```
In this example, the `strbuf_realpath` function is used to resolve the path, and it checks the buffer size before writing to it. If the buffer is too small, it returns an error.