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.