int Curl_protocol_getsock(struct connectdata *conn,
                          curl_socket_t *socks,
                          int numsocks)
{
  if(conn->handler->proto_getsock)
    return conn->handler->proto_getsock(conn, socks, numsocks);
  /* Backup getsock logic. Since there is a live socket in use, we must wait
     for it or it will be removed from watching when the multi_socket API is
     used. */
  socks[0] = conn->sock[FIRSTSOCKET];
  return GETSOCK_READSOCK(0) | GETSOCK_WRITESOCK(0);
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.