header_cache_t *imap_hcache_open(struct ImapData *idata, const char *path)
{
  struct ImapMbox mx;
  struct Url url;
  char cachepath[PATH_MAX];
  char mbox[PATH_MAX];

  if (path)
    imap_cachepath(idata, path, mbox, sizeof(mbox));
  else
  {
    if (!idata->ctx || imap_parse_path(idata->ctx->path, &mx) < 0)
      return NULL;

    imap_cachepath(idata, mx.mbox, mbox, sizeof(mbox));
    FREE(&mx.mbox);
  }

  if (strstr(mbox, "/../") || (strcmp(mbox, "..") == 0) || (strncmp(mbox, "../", 3) == 0))
    return NULL;
  size_t len = strlen(mbox);
  if ((len > 3) && (strcmp(mbox + len - 3, "/..") == 0))
    return NULL;

  mutt_account_tourl(&idata->conn->account, &url);
  url.path = mbox;
  url_tostring(&url, cachepath, sizeof(cachepath), U_PATH);

  return mutt_hcache_open(HeaderCache, cachepath, imap_hcache_namer);
}
The software uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the software does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.