MFA login fails with "CSRF token validation failed" due to duplicate form submission #3370

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Issue Description

Logging in with MFA (OTP) enabled fails with:

CSRF token validation failed. Please refresh the page and try again.

The login returns HTTP 403 with EBADCSRFTOKEN. The password step works; the failure happens on the OTP step. It reproduces on a single dashboard instance, in incognito, with no browser extensions, so it is not the multi-replica / sticky-session scenario from #3015.

Steps to reproduce

1. Configure a dashboard user with mfa set.
2. Open the login page, enter username + password, then the one-time password, and submit.
3. Login fails with "CSRF token validation failed".

Actual Outcome

The OTP step issues two POST /login requests. The first authenticates successfully; Passport's req.logIn then calls req.session.regenerate() (session-fixation protection), which destroys the session that holds the CSRF token (csrf-sync stores the token in req.session). The browser cancels that first response and never receives the regenerated session cookie, so the second request lands on the now-destroyed session, finds no CSRF token, and fails validation with 403.

The duplicate submission originates in LoginForm.react.js: the submit control is type="submit" (which natively submits the form) and its onClick also calls this.formRef.current.submit(), producing a second, redundant submission. On the password step both submissions return 302, so it is harmless; only the MFA step exposes it because the first POST regenerates the session.

Expected Outcome

Login with MFA succeeds; the form is submitted exactly once.

Environment

• Parse Dashboard version: 9.1.1 (also present on alpha / HEAD)
• MFA: enabled (OTP)
• Deployment: single instance (not multi-replica)