Classic Shell Scripting

Chapter 15. Secure Shell Scripts: Getting Started

Unix security is a problem of legendary notoriety. Just about every aspect of a Unix system has some security issue associated with it, and it's usually the system administrator's job to worry about this issue.

In this chapter, we first present a list of "tips" for writing shell scripts that have a better chance of avoiding security problems. Next we cover the restricted shell, which attempts to put a straitjacket around the user's environment. Then we present the idea of a "Trojan horse," and why such things should be avoided. Finally we discuss setuid shell scripts, including the Korn shell's privileged mode.

Tip

This is not a textbook on Unix system security. Be aware that this chapter merely touches the tip of the iceberg and that there are myriad other aspects to Unix system security besides how the shell is set up.

If you would like to learn more about Unix security, we recommend Practical UNIX & Internet Security (O'Reilly).

Tips for Secure Shell Scripts

Here are some tips for writing more-secure shell scripts, courtesy of Professor Eugene (Gene) Spafford, the director of Purdue University's Center for Education and Research in Information Assurance and Security:^[1]

Don't put the current directory (dot) in PATH

Executable programs should come only from standard system directories. Having the current directory (dot) in PATH opens the door wide for "Trojan horses," described in Section 15.3.

Protect bin directories

Make sure that ...