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We strongly recommend to not make use of
Guest URLs that anyone can use to issue certificates. Any form of check is completely bypassed when using Guest URLs.
API keys unless you want to write your own interface.
Under Authentication Settings you can enable the two factors of authentication for login (2FA). Both client certificates and One Time Passwords (OTP) are available. Refer to the DigiCert Two-Factor Authentication section in the user guide (Documents section in this wiki) for more info.