security(relay): harden internet-facing relay server

Closes the account-takeover chain and related issues found in audit:

- WS auth brute-force protection: per-session lockout (authThrottle) + per-
  socket failure cap that closes the socket (1008); applied to both web-client
  and desktop auth. Failed auth no longer leaves the socket open for unlimited
  guesses.
- WS upgrades now pass an IP-based rate limiter and Origin allowlist before
  handleUpgrade (previously bypassed all HTTP middleware); trust proxy set so
  limiting keys on the real client IP.
- /health no longer leaks live session IDs (counts only).
- .env.example rate-limit fixed (900000ms / 300) from the accidental ~11k rps.
- Credentials moved out of URLs into the X-MacroPad-Password header; images
  fetched via header + blob URLs; login stores creds in sessionStorage.
- Session creation bounded (max sessions, min password length) and TTL-pruned;
  session store writes are now atomic (temp+rename) and debounced.
- Session IDs lengthened to 12 chars with rejection sampling (no modulo bias).
- Enable helmet CSP; restrict CORS to configured origins.
- Request IDs use crypto.randomUUID; drop postinstall build hook and uuid dep.
- Uniform response for unknown session IDs (removes enumeration oracle).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

macropad-relay/src/services/SessionManager.ts

@@ -18,13 +18,38 @@ interface SessionStore {
   sessions: Record<string, Session>;
 }
 
+// Flush pending (debounced) writes at most this often.
+const FLUSH_INTERVAL_MS = 60 * 1000;
+// Prune expired sessions on this cadence.
+const PRUNE_INTERVAL_MS = 60 * 60 * 1000;
+
 export class SessionManager {
   private sessionsFile: string;
   private sessions: Map<string, Session> = new Map();
+  private dirty = false;
+  private flushTimer: NodeJS.Timeout | null = null;
+  private pruneTimer: NodeJS.Timeout | null = null;
 
   constructor() {
     this.sessionsFile = path.join(config.dataDir, 'sessions.json');
     this.load();
+    this.pruneExpired();
+    this.startBackgroundTasks();
+  }
+
+  private startBackgroundTasks(): void {
+    this.flushTimer = setInterval(() => {
+      if (this.dirty) {
+        this.save();
+        this.dirty = false;
+      }
+    }, FLUSH_INTERVAL_MS);
+    if (typeof this.flushTimer.unref === 'function') this.flushTimer.unref();
+
+    this.pruneTimer = setInterval(() => {
+      this.pruneExpired();
+    }, PRUNE_INTERVAL_MS);
+    if (typeof this.pruneTimer.unref === 'function') this.pruneTimer.unref();
   }
 
   private load(): void {
@@ -41,23 +66,69 @@ export class SessionManager {
     }
   }
 
+  // Atomic write: write to a temp file in the same directory, then rename.
   private save(): void {
     try {
       const store: SessionStore = {
         sessions: Object.fromEntries(this.sessions)
       };
-      fs.mkdirSync(path.dirname(this.sessionsFile), { recursive: true });
-      fs.writeFileSync(this.sessionsFile, JSON.stringify(store, null, 2));
+      const dir = path.dirname(this.sessionsFile);
+      fs.mkdirSync(dir, { recursive: true });
+
+      const tmpFile = path.join(
+        dir,
+        `.sessions.${process.pid}.${Date.now()}.tmp`
+      );
+      fs.writeFileSync(tmpFile, JSON.stringify(store, null, 2));
+      fs.renameSync(tmpFile, this.sessionsFile);
     } catch (error) {
       logger.error('Failed to save sessions:', error);
     }
   }
 
+  // Prune sessions that have been unused past the configured TTL.
+  private pruneExpired(): void {
+    const ttlMs = config.sessionTtlDays * 24 * 60 * 60 * 1000;
+    if (ttlMs <= 0) return;
+
+    const now = Date.now();
+    let pruned = 0;
+    for (const [id, session] of this.sessions) {
+      const last = Date.parse(session.lastConnected || session.createdAt);
+      if (!Number.isNaN(last) && now - last > ttlMs) {
+        this.sessions.delete(id);
+        pruned++;
+      }
+    }
+
+    if (pruned > 0) {
+      logger.info(`Pruned ${pruned} expired session(s)`);
+      this.save();
+      this.dirty = false;
+    }
+  }
+
   async exists(sessionId: string): Promise<boolean> {
     return this.sessions.has(sessionId);
   }
 
   async createSession(password: string): Promise<Session> {
+    // Reject empty / short passwords before doing any hashing.
+    if (!password || password.length < config.minPasswordLength) {
+      throw new Error(
+        `Password must be at least ${config.minPasswordLength} characters`
+      );
+    }
+
+    // Cap total sessions to prevent unbounded resource exhaustion.
+    if (this.sessions.size >= config.maxSessions) {
+      // Try to reclaim space by pruning expired sessions first.
+      this.pruneExpired();
+      if (this.sessions.size >= config.maxSessions) {
+        throw new Error('Session limit reached');
+      }
+    }
+
     // Generate unique session ID
     let id: string;
     do {
@@ -75,7 +146,7 @@ export class SessionManager {
     };
 
     this.sessions.set(id, session);
-    this.save();
+    this.save(); // creation is important, persist immediately
 
     logger.info(`Created new session: ${id}`);
     return session;
@@ -90,9 +161,14 @@ export class SessionManager {
     const valid = await bcrypt.compare(password, session.passwordHash);
 
     if (valid) {
-      // Update last connected time
-      session.lastConnected = new Date().toISOString();
-      this.save();
+      // Update last-connected time, but only mark dirty (debounced flush)
+      // if it moved by more than a minute to avoid writing on every auth.
+      const now = Date.now();
+      const prev = Date.parse(session.lastConnected);
+      if (Number.isNaN(prev) || now - prev > 60 * 1000) {
+        session.lastConnected = new Date(now).toISOString();
+        this.dirty = true;
+      }
     }
 
     return valid;
@@ -106,7 +182,7 @@ export class SessionManager {
     const session = this.sessions.get(sessionId);
     if (session) {
       session.lastConnected = new Date().toISOString();
-      this.save();
+      this.dirty = true;
     }
   }
 
@@ -118,4 +194,20 @@ export class SessionManager {
     }
     return deleted;
   }
+
+  // Flush any pending writes and stop background timers.
+  shutdown(): void {
+    if (this.flushTimer) {
+      clearInterval(this.flushTimer);
+      this.flushTimer = null;
+    }
+    if (this.pruneTimer) {
+      clearInterval(this.pruneTimer);
+      this.pruneTimer = null;
+    }
+    if (this.dirty) {
+      this.save();
+      this.dirty = false;
+    }
+  }
 }