mrb_io_s_popen(mrb_state *mrb, mrb_value klass)
{
  mrb_value cmd, io;
  mrb_value mode = mrb_str_new_cstr(mrb, "r");
  mrb_value opt  = mrb_hash_new(mrb);

  struct mrb_io *fptr;
  const char *pname;
  int pid = 0, flags;
  STARTUPINFO si;
  PROCESS_INFORMATION pi;
  SECURITY_ATTRIBUTES saAttr;

  HANDLE ifd[2];
  HANDLE ofd[2];

  int doexec;
  int opt_in, opt_out, opt_err;

  ifd[0] = INVALID_HANDLE_VALUE;
  ifd[1] = INVALID_HANDLE_VALUE;
  ofd[0] = INVALID_HANDLE_VALUE;
  ofd[1] = INVALID_HANDLE_VALUE;

  mrb_get_args(mrb, "S|SH", &cmd, &mode, &opt);
  io = mrb_obj_value(mrb_data_object_alloc(mrb, mrb_class_ptr(klass), NULL, &mrb_io_type));

  pname = mrb_string_value_cstr(mrb, &cmd);
  flags = mrb_io_modestr_to_flags(mrb, mrb_string_value_cstr(mrb, &mode));

  doexec = (strcmp("-", pname) != 0);
  opt_in = option_to_fd(mrb, opt, "in");
  opt_out = option_to_fd(mrb, opt, "out");
  opt_err = option_to_fd(mrb, opt, "err");

  saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
  saAttr.bInheritHandle = TRUE;
  saAttr.lpSecurityDescriptor = NULL;

  if (flags & FMODE_READABLE) {
    if (!CreatePipe(&ofd[0], &ofd[1], &saAttr, 0)
        || !SetHandleInformation(ofd[0], HANDLE_FLAG_INHERIT, 0)) {
      mrb_sys_fail(mrb, "pipe");
    }
  }

  if (flags & FMODE_WRITABLE) {
    if (!CreatePipe(&ifd[0], &ifd[1], &saAttr, 0)
        || !SetHandleInformation(ifd[1], HANDLE_FLAG_INHERIT, 0)) {
      mrb_sys_fail(mrb, "pipe");
    }
  }

  if (doexec) {
    ZeroMemory(&pi, sizeof(pi));
    ZeroMemory(&si, sizeof(si));
    si.cb = sizeof(si);
    si.dwFlags |= STARTF_USESHOWWINDOW;
    si.wShowWindow = SW_HIDE;
    si.dwFlags |= STARTF_USESTDHANDLES;
    if (flags & FMODE_READABLE) {
      si.hStdOutput = ofd[1];
      si.hStdError = ofd[1];
    }
    if (flags & FMODE_WRITABLE) {
      si.hStdInput = ifd[0];
    }
    if (!CreateProcess(
        NULL, (char*)pname, NULL, NULL,
        TRUE, CREATE_NEW_PROCESS_GROUP, NULL, NULL, &si, &pi)) {
      CloseHandle(ifd[0]);
      CloseHandle(ifd[1]);
      CloseHandle(ofd[0]);
      CloseHandle(ofd[1]);
      mrb_raisef(mrb, E_IO_ERROR, "command not found: %S", cmd);
    }
    CloseHandle(pi.hThread);
    CloseHandle(ifd[0]);
    CloseHandle(ofd[1]);
    pid = pi.dwProcessId;
  }

  mrb_iv_set(mrb, io, mrb_intern_cstr(mrb, "@buf"), mrb_str_new_cstr(mrb, ""));

  fptr = mrb_io_alloc(mrb);
  fptr->fd = _open_osfhandle((intptr_t)ofd[0], 0);
  fptr->fd2 = _open_osfhandle((intptr_t)ifd[1], 0);
  fptr->pid = pid;
  fptr->readable = ((flags & FMODE_READABLE) != 0);
  fptr->writable = ((flags & FMODE_WRITABLE) != 0);
  fptr->sync = 0;

  DATA_TYPE(io) = &mrb_io_type;
  DATA_PTR(io)  = fptr;
  return io;
}

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.