Given the summary of the `card->sm_ctx.ops.read_binary` function, the root cause of the CWE-787 (Out-of-bounds Write) vulnerability at line 18 becomes more apparent.

The vulnerability lies in the fact that the `sc_read_binary` function calls the `card->sm_ctx.ops.read_binary` function without any additional checks or safeguards. If the `card->sm_ctx.ops.read_binary` function does not perform proper bounds checking or relies on untrusted input for the `count` parameter, it can lead to a buffer overflow vulnerability.

Specifically, if the `card->sm_ctx.ops.read_binary` function writes more data than the size of the provided `buf` buffer, it will result in an out-of-bounds write, which is a CWE-787 vulnerability.

The root cause can be summarized as follows:

1. The `sc_read_binary` function calls the `card->sm_ctx.ops.read_binary` function without verifying the implementation details or performing additional bounds checking.
2. If the `card->sm_ctx.ops.read_binary` function does not properly validate the `count` parameter or does not ensure that the write operation stays within the bounds of the `buf` buffer, it can lead to a buffer overflow.
3. The lack of proper input validation and bounds checking in the `card->sm_ctx.ops.read_binary` function is the root cause of the CWE-787 vulnerability.

To mitigate this vulnerability, the `sc_read_binary` function should either:

1. Implement additional checks and safeguards before calling the `card->sm_ctx.ops.read_binary` function to ensure that the `count` parameter is within safe bounds.
2. Verify that the `card->sm_ctx.ops.read_binary` function has proper input validation and bounds checking implemented, or refactor the code to use a safer alternative function.

Without access to the implementation details of the `card->sm_ctx.ops.read_binary` function, it is difficult to provide a more specific root cause analysis or suggest a concrete fix. However, the root cause lies in the lack of proper input validation and bounds checking in the called function, which can lead to an out-of-bounds write vulnerability.