2FA bypass is a failure in the authentication flow that lets a user skip or evade a second-factor check. Instead of verifying a password and then demanding a code, push approval, or hardware token response, the application accepts a session, token, or alternate path that should have been blocked. This is usually an application logic flaw, not a break in encryption or the second factor itself.
It matters because second factor protection is often the last barrier before privileged access, especially on web admin portals and other high-value accounts. Attackers look for broken branch handling, legacy login routes, race conditions, or “remember this device” mistakes that let them reuse a session without completing the second step. Defenders should test every authentication path, including recovery, API, and exception flows, and verify that second-factor state is enforced server-side on each sensitive request.