feat(auth): add csrfOriginCheck middleware

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/web/middleware/csrf.ts

@@ -0,0 +1,35 @@
+import type { Request, Response, NextFunction } from "express";
+
+const SAFE_METHODS = new Set(["GET", "HEAD", "OPTIONS"]);
+
+/**
+ * Same-origin CSRF protection. For mutating requests, the Origin or Referer
+ * header must indicate a host equal to the request's own host.
+ *
+ * SameSite=Lax on the session cookie blocks classic cross-site form posts;
+ * this header check covers the remaining attack surface.
+ */
+export function csrfOriginCheck(req: Request, res: Response, next: NextFunction): void {
+  if (SAFE_METHODS.has(req.method)) {
+    next();
+    return;
+  }
+  const expectedHost = req.get("host");
+  const originHeader = req.get("origin");
+  const refererHeader = req.get("referer");
+  const headerHost = hostOf(originHeader) ?? hostOf(refererHeader);
+  if (!headerHost || !expectedHost || headerHost !== expectedHost) {
+    res.status(403).json({ error: "bad origin" });
+    return;
+  }
+  next();
+}
+
+function hostOf(url: string | undefined): string | null {
+  if (!url) return null;
+  try {
+    return new URL(url).host;
+  } catch {
+    return null;
+  }
+}