Next.js での安全な画像アップロード

// Next.js checks the Origin of Server Actions, not of route handlers. A browser sends Origin
// with these POSTs, so refuse one from another site. Requests without Origin (curl) pass.
export function checkOrigin(request: Request): void {
  const origin = request.headers.get("origin");
  if (!origin) return;
  const host = request.headers.get("x-forwarded-host") ?? request.headers.get("host");
  let originHost: string | null = null;
  try {
    originHost = new URL(origin).host;
  } catch {}
  if (originHost !== host) throw new GalleryError(403, "Cross-site request refused.");
}

// Allowed extensions, each with a check of the file's first bytes (its signature).
const SIGNATURES: Record<string, (bytes: Buffer) => boolean> = {
  ".jpg": (b) => hex(b, 0, 3) === "ffd8ff",
  ".jpeg": (b) => hex(b, 0, 3) === "ffd8ff",
  ".png": (b) => hex(b, 0, 8) === "89504e470d0a1a0a",
  ".gif": (b) => ascii(b, 0, 6) === "GIF87a" || ascii(b, 0, 6) === "GIF89a",
  ".webp": (b) => ascii(b, 0, 4) === "RIFF" && ascii(b, 8, 12) === "WEBP",
};
const hex = (bytes: Buffer, start: number, end: number) => bytes.subarray(start, end).toString("hex");
const ascii = (bytes: Buffer, start: number, end: number) => bytes.subarray(start, end).toString("latin1");

// True for "chair.jpg", false for ".chair.jpg", "chair.svg" or "chair".
export function isImageName(name: string): boolean {
  return !name.startsWith(".") && Object.hasOwn(SIGNATURES, extensionOf(name));
}

// A folder or file name: 1-64 letters, digits, space, - _ . and not starting with a dot.
// Also refused: a trailing dot or space (Windows drops them, so "a." would become "a")
// and Windows device names such as CON or NUL.
export function isValidName(name: unknown): name is string {
  return (
    typeof name === "string" &&
    /^[A-Za-z0-9 _.-]{1,64}$/.test(name) &&
    !name.startsWith(".") &&
    !/[. ]$/.test(name) &&
    !/^(con|prn|aux|nul|com[1-9]|lpt[1-9])(\.|$)/i.test(name)
  );
}

// /uploads/<parts> -> the image file on disk, or a 404 GalleryError.
export async function resolveUpload(parts: string[]): Promise<string> {
  const name = parts.at(-1) ?? "";
  if (!isImageName(name) || /[\\/\0]/.test(name)) {
    throw new GalleryError(404, "Not found.");
  }
  const dir = await resolveFolder(UPLOADS_DIR, parts.slice(0, -1).join("/")).catch(() => {
    throw new GalleryError(404, "Not found.");
  });
  const file = path.join(dir, name);
  const info = await lstat(file).catch(() => null);
  if (!info || !info.isFile()) {
    throw new GalleryError(404, "Not found.");
  }
  return file;
}

curl -H "Origin: https://evil.example" -F action=create-folder -F folder= -F name=x http://localhost:5102/rte/gallery
{"error":"Cross-site request refused."}

printf '<?php echo 1; ?>' > fake.jpg
curl -F "file=@fake.jpg" http://localhost:5102/rte/upload
{"error":"The file content is not a valid JPG image."}

curl -F "file=@photo.jpg;filename=x.svg" http://localhost:5102/rte/upload
{"error":"Only .jpg, .jpeg, .png, .gif and .webp images can be uploaded."}