By default, the ASA generates a self-signed X.509 certificate upon startup. This certificate is used in order to serve client connections by default. It is not recommended to use this certificate because its authenticity cannot be verified by the browser. Furthermore, this certificate is regenerated upon each reboot so it changes after each reboot.
https://www.cisco.com/c/en/us/support/docs/security-vpn/webvpn-ssl-vpn/119417-config-asa-
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