Based on the provided summary of the `sc_read_binary` function, the root cause of the CWE-787 (out-of-bounds write) vulnerability at line 18 is likely due to a lack of proper bounds checking or a buffer overflow vulnerability in the `card->sm_ctx.ops.read_binary` function.

The `sc_read_binary` function itself appears to have proper bounds checking and error handling mechanisms in place. However, when it calls the `card->sm_ctx.ops.read_binary` function at line 18, it relies on that function to perform the actual read operation correctly and safely.

If the `card->sm_ctx.ops.read_binary` function does not validate the `count` parameter or does not ensure that the `buf` buffer has sufficient space to hold the requested number of bytes, it can lead to an out-of-bounds write vulnerability (CWE-787).

The root cause can be summarized as follows:

1. The `sc_read_binary` function calls the `card->sm_ctx.ops.read_binary` function at line 18 to perform the actual read operation.
2. If the `card->sm_ctx.ops.read_binary` function does not properly validate the `count` parameter or does not ensure that the `buf` buffer has sufficient space to hold the requested number of bytes, it can lead to a buffer overflow.
3. This buffer overflow can result in writing data beyond the bounds of the `buf` buffer, causing an out-of-bounds write vulnerability (CWE-787).

To mitigate this vulnerability, the `card->sm_ctx.ops.read_binary` function should be reviewed and modified to include proper bounds checking and validation of the input parameters, especially the `count` parameter and the size of the `buf` buffer. Additionally, it should ensure that the write operation does not exceed the bounds of the `buf` buffer.