Step-up re-authentication does not rotate the session id (CSRF token does rotate) #369

Summary

When step-up (SUF-02) re-runs the passkey login ceremony while the user is already authenticated, the servlet session id (JSESSIONID) is not rotated. The CSRF token is rotated. This was verified in a browser as part of SpringUserFrameworkDemoApp#75, whose acceptance criteria expected the session id to change on re-assertion.

Evidence (browser, library 5.3.3, step-up enabled)

Captured immediately before submitting a gated operation and immediately after the step-up ceremony (/login/webauthn re-run mid-session):

JSESSIONID before = 562CB176BDC07A3C3910BEAE8BE5BE9C
JSESSIONID after  = 562CB176BDC07A3C3910BEAE8BE5BE9C   (unchanged)

_csrf before = 5918qjK-pVpDMMp00naRBGQJb-5In7sVocHP3LlHgGRPzEV...
_csrf after  = hYOlzFKos_wbhhMEBPW3qm8UwP5PLxcDAPiIgaixc3cY8_9...  (rotated)

Why this happens

Step-up depends on factor merging (setMfaEnabled(true) on the authentication processing filters, MfaFilterMergingConfiguration). In MFA/merge mode the filter merges the new factor onto the already-authenticated principal rather than running a fresh authentication, so the session-fixation strategy (ChangeSessionIdAuthenticationStrategy) does not rotate the id. CsrfAuthenticationStrategy still runs, so the CSRF token rotates.

Is it a problem?

Arguably not a vulnerability: the principal is unchanged and the session was already authenticated, so preserving the id introduces no fixation vector (there is no pre-auth id being "upgraded"). But it contradicts the reasonable expectation — stated in the demo issue's acceptance criteria — that a full re-login rotates the session id, and it is worth documenting the intended behavior either way.

Ask

• Confirm whether preserving the session id across a step-up re-assertion is intended, and document it (CONFIG.md / MIGRATION.md) alongside the note that CSRF rotates.
• If id rotation is desired for step-up, decide how that interacts with factor merging (which deliberately does not rotate on each factor).

Client consequence already handled in the demo: because CSRF rotates, a client that stays on the page after step-up must refresh its CSRF token before retrying (SpringUserFrameworkDemoApp#75 adds a GET /csrf for exactly this).