Verify session tokens before trusting server-side identity

apps/cursor/src/app/auth/callback/route.ts

@@ -1,10 +1,23 @@
 import { NextResponse } from "next/server";
 import { createClient } from "@/utils/supabase/server";
 
+// `next` comes from the query string and is appended to a base URL, so values
+// like `@evil.example` or `//evil.example` would otherwise leave the site.
+function safeNextPath(next: string | null, origin: string): string {
+  if (!next) return "/";
+  try {
+    const url = new URL(next, origin);
+    if (url.origin !== origin) return "/";
+    return `${url.pathname}${url.search}${url.hash}`;
+  } catch {
+    return "/";
+  }
+}
+
 export async function GET(request: Request) {
   const { searchParams, origin } = new URL(request.url);
   const code = searchParams.get("code");
-  const next = searchParams.get("next") ?? "/";
+  const next = safeNextPath(searchParams.get("next"), origin);
 
   if (code) {
     const supabase = await createClient();

apps/cursor/src/utils/supabase/auth.ts

@@ -1,9 +1,38 @@
 import { createClient } from "./server";
 
-export async function getSession() {
+export type VerifiedSession = {
+  user: {
+    id: string;
+    email: string | undefined;
+    user_metadata: Record<string, unknown>;
+  };
+};
+
+/**
+ * The signed-in user, read from the access token's verified claims.
+ *
+ * `supabase.auth.getSession()` returns whatever the session cookie contains
+ * without checking the token signature, so its `user` must never back an
+ * authorization decision on the server. `getClaims()` verifies the JWT
+ * (against the project's JWKS, or via the Auth server for symmetric keys)
+ * before we trust `sub`.
+ */
+export async function getSession(): Promise<VerifiedSession | null> {
   const supabase = await createClient();
-  const {
-    data: { session },
-  } = await supabase.auth.getSession();
-  return session;
+  // getClaims throws, rather than returning an error, for an expired token.
+  const result = await supabase.auth.getClaims().catch(() => null);
+
+  if (!result || result.error || !result.data?.claims.sub) {
+    return null;
+  }
+
+  const { claims } = result.data;
+
+  return {
+    user: {
+      id: claims.sub,
+      email: claims.email,
+      user_metadata: claims.user_metadata ?? {},
+    },
+  };
 }