keycloak-scim/docbook/auth-server-docs/reference/en/en-US/modules/admin-recovery.xml

<chapter id="admin-recovery">
    <title>Recovering the Master Admin User</title>
    <para>
        It is possible for the "admin" user in the master realm to become inoperable.  This may be because it was
        accidentally deleted, its role mappings were removed, or the password was simply forgotten.
    </para>
    <para>
        To recover the master admin user, just start the server with the following system properties:
        <programlisting><![CDATA[
bin/standalone.sh -Dkeycloak.recover-admin=true -Dkeycloak.temp-admin-password=temppassword
]]></programlisting>
        Then you can log in to the master admin account with your temporary password.  You will then be
        prompted to immediately change this password.
    </para>
</chapter>