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3.3. Do not plug to the Internet until ready

The system should not be immediately connected to the Internet during installation. This could sound stupid but network installation is a common method. Since the system will install and activate services immediately, if the system is connected to the Internet and the services are not properly configured you are opening it to attack.
Also note that some services might have security vulnerabilities not fixed in the packages you are using for installation. This is usually true if you are installing from old media (like CD-ROMs). In this case, the system could even be compromised before you finish installation!
Since Debian installation and upgrades can be done over the Internet you might think it is a good idea to use this feature on installation. If the system is going to be directly connected to the Internet (and not protected by a firewall or NAT), it is best to install without connection to the Internet, using a local packages mirror for both the Debian package sources and the security updates. You can set up package mirrors by using another system connected to the Internet with Debian-specific tools (if it's a Debian system) like apt-move or apt-proxy, or other common mirroring tools, to provide the archive to the installed system. If you cannot do this, you can set up firewall rules to limit access to the system while doing the update (see Section B.6, “Security update protected by a firewall”).