mm_key_sign(Key *key, u_char **sigp, u_int *lenp,
    const u_char *data, u_int datalen)
{
	struct kex *kex = *pmonitor->m_pkex;
	Buffer m;

	debug3("%s entering", __func__);

	buffer_init(&m);
	buffer_put_int(&m, kex->host_key_index(key, 0, active_state));
	buffer_put_string(&m, data, datalen);

	mm_request_send(pmonitor->m_recvfd, MONITOR_REQ_SIGN, &m);

	debug3("%s: waiting for MONITOR_ANS_SIGN", __func__);
	mm_request_receive_expect(pmonitor->m_recvfd, MONITOR_ANS_SIGN, &m);
	*sigp  = buffer_get_string(&m, lenp);
	buffer_free(&m);

	return (0);
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.