Security vulnerabilities exist in any authentication server. See the Internet Engineering Task Force's (IETF) https://datatracker.ietf.org/doc/html/rfc6819[OAuth 2.0 Threat Model] and the https://datatracker.ietf.org/doc/html/draft-ietf-oauth-security-topics-15[OAuth 2.0 Security Best Current Practice] for more information.