grav-plugin-login (the Grav CMS Login plugin) versions >= 3.8.7 and < 3.9.7 allow the two-factor authentication challenge to be bypassed for content gated by the authenticated() Twig function or the [authenticated] shortcode. On sites with 2FA enabled, Login::isAuthenticated() checked only the session flag indicating that the password step had succeeded, not the flag indicating that login had completed, so a session sitting at the 2FA code prompt was treated as fully authenticated. An attacker who knows a members password but cannot answer that members second factor can therefore read member-only content rendered by the no-argument authenticated() or group authenticated(null, group) forms and by [authenticated]; the inverse [guest] shortcode is likewise evaluated too early. Impact is limited to disclosure of that content: the attacker does not obtain a completed session, cannot access pages protected by an access: rule, and cannot act as the user. The authenticated(some.permission) form, which goes through UserObject::authorize(), is not affected. Fixed in grav-plugin-login 3.9.7.
Weakness
The product implements an authentication technique, but it skips a step that weakens the technique.