Script Audit

Introduction

Because Bash scripts often run as the root user to perform administrative tasks, a vulnerability in a script can compromise the entire server. Auditing your scripts for common security flaws is a critical step before deploying them to production.

1. Quote Your Variables!

The single most common vulnerability in Bash scripting is unquoted variables. If a variable contains spaces or special characters, Bash will perform "Word Splitting", which can lead to disastrous execution of unintended commands.

Vulnerable:

USER_INPUT="some_file; rm -rf /"
# This will try to cat some_file, and then execute rm -rf / !!!
cat $USER_INPUT

Secure:

# Always surround variables in double quotes!
# This tries to read a single file literally named "some_file; rm -rf /"
cat "$USER_INPUT"

2. Never Use eval

The eval command takes a string and executes it as Bash code. It is incredibly dangerous. If any part of that string is derived from user input, you have introduced an Arbitrary Code Execution vulnerability. There is almost always a safer way to achieve your goal without using eval.

3. Use ShellCheck

You do not have to audit your scripts manually. ShellCheck is an open-source static analysis tool that automatically finds bugs, edge cases, and security vulnerabilities in your Bash scripts.

# Install ShellCheck
sudo apt install shellcheck

# Run it against your script
shellcheck my_script.sh

ShellCheck acts as a strict linter. It will warn you if you forgot to quote variables, if you are using deprecated syntax, or if your pipes are unsafe. It is highly recommended to integrate ShellCheck into your CI/CD pipeline!

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