A client/server product performs authentication within client code but not in server code, allowing server-side authentication to be bypassed via a modified client that omits the authentication check.
Extended description
Client-side authentication is extremely weak and may be breached easily. Any attacker may read the source code and reverse-engineer the authentication mechanism to access parts of the application which would otherwise be protected.
Best practices to prevent this CWE
Phase: Architecture and Design
Do not rely on client side data. Always perform server side authentication.