Bash Shell Script Security Best Practices

In the world of Linux, Bash (Bourne Again SHell) is a predominant figure, serving as the default shell for many distributions. It plays a critical role in system administration, scripting, and automation. However, the power of Bash also comes with the responsibility to ensure scripts are secure. This blog post will guide you through essential security best practices for Bash shell scripting and detail how to update and install necessary tools using different package managers like apt (for Debian-based systems), dnf (for Fedora and RHEL-based systems), and zypper (for openSUSE).

1. Use Safe Paths

Always use absolute paths to commands in scripts to avoid the risk of executing rogue scripts due to alterations in the PATH environment. For example, instead of using cat filename, use /bin/cat filename, thereby ensuring the intended system command is executed.

Updating System and Installing Core Utilities:

• APT (Debian-based):

  sudo apt update
  sudo apt install coreutils
  

• DNF (Fedora, RHEL-based):

  sudo dnf check-update
  sudo dnf install coreutils
  

• ZYPPER (openSUSE):

  sudo zypper refresh
  sudo zypper install coreutils
  

2. Validate Input Data

Always validate external inputs to your scripts to prevent injection and other malicious attacks. This can include data passed via arguments, input from users, or input from files.

3. Use Quotation Marks for Variables

When referencing variables, especially those that could potentially contain spaces or special characters, always enclose them in quotes. This practice prevents globbing and word splitting. For instance, use "${VAR}" instead of ${VAR}.

4. Secure Bash Script by Restricting Privileges

Run your script with the lowest privileges necessary for the task. For operations requiring elevation, use specific tools like sudo to elevate privileges on a per-command basis, rather than running the entire script as a superuser.

Installing sudo:

• APT (Debian-based):

  sudo apt install sudo
  

• DNF (Fedora, RHEL-based):

  sudo dnf install sudo
  

• ZYPPER (openSUSE):

  sudo zypper install sudo
  

5. Handle Errors and Signals Gracefully

Implement error checking in your scripts. Use Bash's set commands like set -o nounset, set -o errexit, and set -o pipefail to handle unset variables, any error, and errors in pipelines, respectively. Also, consider adding signal handling for clean termination of your scripts (trap command).

6. Avoid Using eval

Avoid using eval due to the high risk of executing arbitrary code unintentionally. If you must use it, ensure the data is well-sanitized and absolutely safe.

7. Regularly Update Script Dependencies

Keep all your system's software, especially those that your scripts depend on, up to date to protect against vulnerabilities.

System-wide Updates:

• APT (Debian-based):

  sudo apt update && sudo apt upgrade
  

• DNF (Fedora, RHEL-based):

  sudo dnf upgrade --refresh
  

• ZYPPER (openSUSE):

  sudo zypper update
  

8. Use Official Sources for External Scripts

Always download scripts from trusted, official sources. Verify their integrity using cryptographic signatures or hashes if available.

9. Readability and Maintainability

Write clear, readable code and comment generously. Secure scripts are not just about thwarting attacks but also about making sure that future maintainers (or "future you") understand how and why your scripts work.

10. Use Version Control

Using version control for your scripts helps track changes and rollback when necessary. It's also beneficial for auditing and collaborative work.

By following these security best practices, you can help safeguard your Bash scripts against common vulnerabilities and maintain a robust, secure scripting environment. Remember, security is an ongoing endeavor, so consistently review and improve your scripts and systems!