ASP.NET Core のギャラリーフォルダー：安全に閲覧・作成する

// POST /rte/gallery   action=create-folder, folder, name
[HttpPost("gallery")]
public IActionResult CreateFolder([FromForm(Name = "action")] string? op, [FromForm] string? folder, [FromForm] string? name) =>
    Send(RteFiles.CreateFolder(HttpContext, op, folder, name));

// POST /rte/gallery   action=create-folder, folder=<parent>, name=<new folder>
public static RteResult CreateFolder(HttpContext http, string? action, string? folder, string? name) => Run(() =>
{
    RequireSameOrigin(http.Request);
    if (action != "create-folder") throw new RteError(400, "Unknown action.");
    var root = RootFor(http);
    folder ??= "";
    var parent = Resolve(root, folder);
    if (!IsValidName(name))
        throw new RteError(400, "Use 1-64 letters, digits, spaces, '-', '_' or '.', not starting with '.'.");
    var path = Path.Combine(parent, name!);
    if (Directory.Exists(path) || File.Exists(path))
        throw new RteError(409, "A folder with that name already exists.");
    Directory.CreateDirectory(path);
    return Listing(root, folder, parent); // the parent's listing, so the new folder shows
});

// Folder names: 1-64 ASCII letters, digits, space, - _ . ; not starting with "." (no "..", no hidden
// folders). Also refused because Windows mangles them: a trailing "." or space, a leading space,
// and device names such as "con" or "nul.txt".
public static bool IsValidName(string? name) =>
    name is { Length: >= 1 and <= 64 }
    && name.All(c => char.IsAsciiLetterOrDigit(c) || c is ' ' or '-' or '_' or '.')
    && !name.StartsWith('.') && !name.StartsWith(' ')
    && !name.EndsWith('.') && !name.EndsWith(' ')
    && !WindowsDeviceNames.Contains(name.Split('.')[0]);

// A virtual folder ("", "products", "products/2026") becomes a disk path inside the root.
// Every segment must be a valid folder name, so "..", "C:", "\", "%2e%2e" and NUL never reach the disk.
static string Resolve(RteRoot root, string folder)
{
    var rootPath = Path.GetFullPath(root.Disk);
    Directory.CreateDirectory(rootPath);
    if (folder.Length == 0) return rootPath;

    var segments = folder.Split('/');
    if (!segments.All(IsValidName)) throw new RteError(400, "Invalid folder.");

    // Second line of defence: the resolved path must still be inside the root. Compare against the
    // root WITH a trailing separator, or "C:\app\uploads-old" would pass as inside "C:\app\uploads".
    var path = Path.GetFullPath(Path.Combine([rootPath, .. segments]));
    if (!path.StartsWith(Path.TrimEndingDirectorySeparator(rootPath) + Path.DirectorySeparatorChar, StringComparison.Ordinal))
        throw new RteError(400, "Invalid folder.");
    if (!Directory.Exists(path)) throw new RteError(404, "Folder not found.");
    return path;
}

curl -F "action=create-folder" -F "folder=blog" -F "name=2026" http://localhost:5302/rte/gallery
{"currentFolder":"blog","currentFolderDisplay":"/blog","parentFolder":"","folders":[{"folder":"blog/2026","name":"2026"}],"images":[...]}

curl -F "action=create-folder" -F "folder=blog" -F "name=2026" http://localhost:5302/rte/gallery
{"error":"A folder with that name already exists."}

curl -F "action=create-folder" -F "folder=" -F "name=.." http://localhost:5302/rte/gallery
{"error":"Use 1-64 letters, digits, spaces, '-', '_' or '.', not starting with '.'."}

curl "http://localhost:5302/rte/gallery?folder=..%2F.."
{"error":"Invalid folder."}

curl "http://localhost:5302/rte/gallery?folder=blog%2Fnope"
{"error":"Folder not found."}