fix(auth): reject ill-formed local passwords

backend/test/auth-password.test.ts

@@ -1,6 +1,14 @@
 import { readFileSync } from "node:fs";
 import { resolve } from "node:path";
-import { describe, expect, test } from "vitest";
+import { describe, expect, test, vi } from "vitest";
+
+const { argon2SyncSpy } = vi.hoisted(() => ({ argon2SyncSpy: vi.fn() }));
+vi.mock("node:crypto", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:crypto")>();
+  argon2SyncSpy.mockImplementation(actual.argon2Sync);
+  return { ...actual, argon2Sync: argon2SyncSpy };
+});
+
 import { verifyPassword } from "../src/auth/password.js";
 
 interface Argon2Vector {
@@ -27,6 +35,15 @@ describe("local Argon2id password verification", () => {
     }
   });
 
+  test("rejects ill-formed Unicode instead of authenticating as U+FFFD", () => {
+    const replacementPassword = "correct horse battery stap\uFFFD";
+    const loneSurrogatePassword = "correct horse battery stap\uD800";
+    const replacementPasswordHash = "$argon2id$v=19$m=65536,t=3,p=1$AAECAwQFBgcICQoLDA0ODw$+tAXzgaQVnNaonNvgevyG6UKlaKcwyRMi1mESNk0BvQ";
+
+    expect(verifyPassword(replacementPassword, replacementPasswordHash)).toBe(true);
+    expect(verifyPassword(loneSurrogatePassword, replacementPasswordHash)).toBe(false);
+  });
+
   test("rejects malformed and oversized PHC parameters before Argon2 allocation", () => {
     const password = vectors[0].password;
     const digest = vectors[0].phc.split("$")[5];
@@ -40,8 +57,9 @@ describe("local Argon2id password verification", () => {
     ];
 
     for (const phc of cases) {
-      expect(() => verifyPassword(password, phc)).not.toThrow();
+      argon2SyncSpy.mockClear();
       expect(verifyPassword(password, phc)).toBe(false);
+      expect(argon2SyncSpy).not.toHaveBeenCalled();
     }
   });
 });