static int fetch_numbers(char *line, void *data)
{
  struct FetchCtx *fc = data;
  anum_t anum;

  if (!line)
    return 0;
  if (sscanf(line, ANUM, &anum) != 1)
    return 0;
  if (anum < fc->first || anum > fc->last)
    return 0;
  fc->messages[anum - fc->first] = 1;
  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.