diff --git a/apps/pwa/src/lib/crypto.mjs b/apps/pwa/src/lib/crypto.mjs
index d09a6f1..e5755b4 100644
--- a/apps/pwa/src/lib/crypto.mjs
+++ b/apps/pwa/src/lib/crypto.mjs
@@ -32,7 +32,9 @@ export function sign(value) {
 
 export function unsign(signed) {
   if (!signed || typeof signed !== "string" || !signed.includes(".")) return null;
-  const [payload, mac] = signed.split(".");
+  const parts = signed.split(".");
+  if (parts.length !== 2) return null;
+  const [payload, mac] = parts;
   const expected = crypto.createHmac("sha256", config.sessionSecret).update(payload).digest("base64url");
   const a = Buffer.from(mac);
   const b = Buffer.from(expected);
diff --git a/apps/pwa/test/crypto-cookie.test.mjs b/apps/pwa/test/crypto-cookie.test.mjs
new file mode 100644
index 0000000..8143b21
--- /dev/null
+++ b/apps/pwa/test/crypto-cookie.test.mjs
@@ -0,0 +1,15 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { sign, unsign } from "../src/lib/crypto.mjs";
+
+test("unsign accepts an untouched signed cookie value", () => {
+  const signed = sign({ step: "passkey", nonce: "abc" });
+
+  assert.deepEqual(unsign(signed), { step: "passkey", nonce: "abc" });
+});
+
+test("unsign rejects signed cookie values with extra segments", () => {
+  const signed = sign({ step: "passkey", nonce: "abc" });
+
+  assert.equal(unsign(`${signed}.junk`), null);
+});