When a browser session predates a server restart with a fresh
secret.key, the old __csrf HMAC cookie is no longer verifiable
under the new secret. Every action POST returns 403 until the user
manually clears cookies, because the existing-pair check on the
front controller was 'is the cookie present?' rather than 'does
the cookie verify under the current secret?'.
The new check at the top of the front controller computes the
expected HMAC of the raw-token cookie under the current secret and
constant-time-compares it to the __csrf cookie. If they don't match,
the controller re-mints a fresh pair on the response, replacing the
browser's stale cookies with a working set. The next click of 'Play
bundled' then succeeds without user intervention.
Test coverage in tests/Integration/StaleCookieRotationTest.php:
- testStaleCookiesAreReplacedOnNextRequest
- testValidCookiesAreNotReIssuedOnNextRequest
- testMissingTokenCookieIsFilledInOnNextRequest
