Harden the static file server against path traversal (security review)

The internet-facing static handler now decodes the URL (rejecting bad
encodings and null bytes), anchors the containment check with a
trailing separator so sibling-prefix directories cannot slip past,
and realpath-resolves the final file to defeat symlink escapes —
serving only what provably lives inside the built client directory.
Deployed and probed live: normal requests 200, literal and
percent-encoded traversal attempts both 403.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/server/src/index.ts

@@ -12,8 +12,8 @@
 //     {type:"error", message}
 
 import { createServer } from "node:http";
-import { readFileSync, existsSync } from "node:fs";
-import { extname, join, normalize } from "node:path";
+import { readFileSync, existsSync, realpathSync } from "node:fs";
+import { extname, join, normalize, sep } from "node:path";
 import { WebSocketServer, WebSocket } from "ws";
 import type { Command, PlayerId } from "@wizwar/engine";
 import {
@@ -42,17 +42,34 @@ const MIME: Record<string, string> = {
   ".png": "image/png", ".svg": "image/svg+xml", ".ico": "image/x-icon",
   ".woff2": "font/woff2", ".json": "application/json",
 };
+const staticRoot = realpathSync(normalize(STATIC_DIR));
 const httpServer = createServer((req, res) => {
   try {
-    const url = (req.url ?? "/").split("?")[0]!;
-    let file = normalize(join(STATIC_DIR, url === "/" ? "index.html" : url));
-    if (!file.startsWith(normalize(STATIC_DIR))) {
+    let url: string;
+    try {
+      url = decodeURIComponent((req.url ?? "/").split("?")[0]!);
+    } catch {
+      res.writeHead(400).end();
+      return;
+    }
+    if (url.includes("\0")) {
+      res.writeHead(400).end();
+      return;
+    }
+    let file = normalize(join(staticRoot, url === "/" ? "index.html" : url));
+    if (file !== staticRoot && !file.startsWith(staticRoot + sep)) {
       res.writeHead(403).end();
       return;
     }
-    if (!existsSync(file)) file = join(STATIC_DIR, "index.html"); // SPA fallback
-    const body = readFileSync(file);
-    res.writeHead(200, { "content-type": MIME[extname(file)] ?? "application/octet-stream" });
+    if (!existsSync(file)) file = join(staticRoot, "index.html"); // SPA fallback
+    // Resolve symlinks and re-verify the real location stays inside the root.
+    const real = realpathSync(file);
+    if (real !== staticRoot && !real.startsWith(staticRoot + sep)) {
+      res.writeHead(403).end();
+      return;
+    }
+    const body = readFileSync(real);
+    res.writeHead(200, { "content-type": MIME[extname(real)] ?? "application/octet-stream" });
     res.end(body);
   } catch {
     res.writeHead(500).end();