static CURLcode smtp_statemach_act(struct connectdata *conn)
{
  CURLcode result = CURLE_OK;
  curl_socket_t sock = conn->sock[FIRSTSOCKET];
  struct Curl_easy *data = conn->data;
  int smtpcode;
  struct smtp_conn *smtpc = &conn->proto.smtpc;
  struct pingpong *pp = &smtpc->pp;
  size_t nread = 0;

  /* Busy upgrading the connection; right now all I/O is SSL/TLS, not SMTP */
  if(smtpc->state == SMTP_UPGRADETLS)
    return smtp_perform_upgrade_tls(conn);

  /* Flush any data that needs to be sent */
  if(pp->sendleft)
    return Curl_pp_flushsend(pp);

  do {
    /* Read the response from the server */
    result = Curl_pp_readresp(sock, pp, &smtpcode, &nread);
    if(result)
      return result;

    /* Store the latest response for later retrieval if necessary */
    if(smtpc->state != SMTP_QUIT && smtpcode != 1)
      data->info.httpcode = smtpcode;

    if(!smtpcode)
      break;

    /* We have now received a full SMTP server response */
    switch(smtpc->state) {
    case SMTP_SERVERGREET:
      result = smtp_state_servergreet_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_EHLO:
      result = smtp_state_ehlo_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_HELO:
      result = smtp_state_helo_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_STARTTLS:
      result = smtp_state_starttls_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_AUTH:
      result = smtp_state_auth_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_COMMAND:
      result = smtp_state_command_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_MAIL:
      result = smtp_state_mail_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_RCPT:
      result = smtp_state_rcpt_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_DATA:
      result = smtp_state_data_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_POSTDATA:
      result = smtp_state_postdata_resp(conn, smtpcode, smtpc->state);
      break;

    case SMTP_QUIT:
      /* fallthrough, just stop! */
    default:
      /* internal error */
      state(conn, SMTP_STOP);
      break;
    }
  } while(!result && smtpc->state != SMTP_STOP && Curl_pp_moredata(pp));

  return result;
}

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.