In mintplex-labs/anything-llm, an attacker can exploit improper input validation by sending a malformed JSON payload to the '/system/enable-multi-user' endpoint. This triggers an error that is caught by a catch block, which in turn deletes all users and disables the 'multi_user_mode'. The vulnerability allows an attacker to remove all existing users and potentially create a new admin user without requiring a password, leading to unauthorized access and control over the application.