A vulnerable authentication system could allow attackers to impersonate as a legitimate users by compromising passwords, session tokens etc. The technical impact is very worse if you could log in as anybody else and then  you would have access to all resources on their website or application as per the compromised user account privileges.

Vulnerabilities in authentication systems give attackers access to compromised user accounts and even the ability to compromise an entire system using an admin account and can have access to all other users credentials.