Test: malformed URL must not reach a sibling prefix's not-found handler (GHSA-p68q-wchp-6fh7)

Red on fastify 5.8.5, the live lockfile.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/fastify-malformed-url.test.ts

@@ -0,0 +1,47 @@
+import Fastify, { type FastifyReply, type FastifyRequest } from "fastify";
+import { afterEach, describe, expect, it } from "vitest";
+
+// GHSA-p68q-wchp-6fh7 (fastify < 5.12.2): a malformed request target under one
+// plugin prefix, with a method that has no route in the main router, was
+// dispatched to the not-found handler of a sibling plugin, skipping that
+// handler's preHandler. This pins the framework fix; it is not about a Heimdall
+// route.
+describe("fastify malformed URL under sibling not-found handlers", () => {
+  const apps: Array<ReturnType<typeof Fastify>> = [];
+  afterEach(async () => {
+    await Promise.all(apps.splice(0).map((app) => app.close()));
+  });
+
+  async function build() {
+    const app = Fastify();
+    apps.push(app);
+    await app.register(async (pub) => {
+      pub.get("/ping", async () => "pong");
+    }, { prefix: "/public" });
+    await app.register(async (priv) => {
+      priv.setNotFoundHandler(
+        {
+          preHandler: async (request: FastifyRequest, reply: FastifyReply) => {
+            if (!request.headers.authorization) return reply.code(401).send({ error: "auth" });
+          },
+        },
+        (_request: FastifyRequest, reply: FastifyReply) => reply.code(404).send({ secret: "PRIVATE-TENANT-DATA" }),
+      );
+      priv.get("/x", async () => "x");
+    }, { prefix: "/private" });
+    return app;
+  }
+
+  it("fails closed with 400 and never reaches another prefix's not-found handler", async () => {
+    const app = await build();
+    const response = await app.inject({ method: "DELETE", url: "/public/%zz" });
+    expect(response.body).not.toContain("PRIVATE-TENANT-DATA");
+    expect(response.statusCode).toBe(400);
+  });
+
+  it("still runs the authenticated not-found preHandler for a well-formed URL", async () => {
+    const app = await build();
+    const response = await app.inject({ method: "DELETE", url: "/private/nothing" });
+    expect(response.statusCode).toBe(401);
+  });
+});