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Practical UNIX and Internet Security, 3rd Edition by Alan Schwartz, Gene Spafford, Simson Garfinkel

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Restrictions on the Superuser

Because the superuser account is occasionally compromised—for example, by somebody sharing the superuser password with a friend—there have been numerous attempts to limit the availability and the power of the Unix superuser account.

Secure Terminals: Limiting Where the Superuser Can Log In

Most versions of Unix allow you to configure certain terminals so that users can’t log in as the superuser from the login: prompt. Anyone who wishes to have superuser privileges must first log in as himself and then su to root. This feature makes tracking who is using the root account easier because the su command logs the username of the person who runs it and the time that it was run.[61] Unix also requires that the root user’s password be provided when booting in single-user mode if the console is not listed as being secure.

Secure consoles add to overall system security because they force people to know two passwords to gain superuser access to the system. Network virtual terminals should not be listed as secure to prevent users from logging into the root account remotely using telnet. (Of course, telnet should also be disabled, which it isn’t in some environments.) The Secure Shell server ignores the terminal security attribute, but it has its own directive (PermitRootLogin in sshd_config) that controls whether users may log in as root remotely.

On BSD-derived systems, terminal security is specified in the /etc/ttys file. In this excerpt from the file, the ...

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