TL;DR
Yes, there are significant security concerns with using pure bash scripts as CGI programs. Bash isn’t designed for secure web application development and is prone to vulnerabilities like command injection, path traversal, and information disclosure. You should strongly consider using a more appropriate language (Python, PHP, Ruby, etc.) or carefully sanitise all input and restrict script execution environments.
Understanding the Risks
CGI scripts are programs that run on a web server to generate dynamic content. Bash, while powerful for system administration, has weaknesses when exposed directly to user-supplied data via HTTP requests. Here’s why:
- Command Injection: The biggest risk. If you don’t carefully handle input, malicious users can inject commands into your script that will be executed by the server.
- Path Traversal: Users might manipulate file paths to access sensitive files outside of intended directories.
- Information Disclosure: Errors or verbose output from bash scripts can reveal internal system details.
- Lack of Built-in Security Features: Bash lacks the security features found in languages designed for web development (e.g., built-in input validation, output encoding).
Securing Bash CGI Scripts – A Step-by-Step Guide
If you absolutely must use bash for CGI scripts, follow these steps to mitigate risks:
1. Input Validation and Sanitisation
- Whitelist Valid Characters: Don’t try to block bad characters; define what’s allowed instead.
- Escape Special Characters: Properly escape any user input before using it in commands or file paths. This is crucial!
- Use Parameterised Queries (where possible): While bash doesn’t have true parameterised queries, you can simulate them by carefully constructing commands with fixed parts and only substituting validated variables.
Example: Sanitising a username for use in a command.
username="$(sed 's/[^a-zA-Z0-9_-]//g' <<< "$QUERY_STRING")"This example removes any characters that aren’t letters, numbers, underscores or hyphens from the QUERY_STRING variable. It is a basic example; adjust the regular expression to suit your needs.
2. Restrict Script Execution Environment
- Use a Dedicated User: Run CGI scripts under a user account with minimal privileges. This limits damage if a script is compromised.
- Chroot Jail (Advanced): Consider using a chroot jail to further isolate the script's environment, restricting its access to only specific directories.
- Disable Unnecessary Features: Turn off features in bash that aren’t needed for your scripts (e.g., history).
3. Secure File Handling
- Absolute Paths: Always use absolute paths when accessing files to prevent path traversal attacks.
- Check Permissions: Ensure that the script only has access to the files it needs and that those files have appropriate permissions.
- Avoid Using User-Supplied File Names Directly: Never directly incorporate user input into file names without thorough validation.
Example: Constructing a safe file path.
filepath="/var/www/cgi-bin/data/${sanitized_filename}.txt"This uses an absolute path and incorporates a validated filename. Ensure ${sanitized_filename} has been properly sanitised as described in step 1.
4. Error Handling & Logging
- Suppress Verbose Errors: Avoid displaying detailed error messages to the user, as they can reveal sensitive information.
- Log All Input and Actions: Log all user input and script actions for auditing purposes. This helps identify and investigate potential attacks.
- Centralised Logging: Send logs to a central logging server for easier analysis.
5. Keep Scripts Simple
The more complex your bash script, the harder it is to secure. Aim for simplicity and clarity.
Alternatives
Seriously consider using a language designed for web development:
- Python: With frameworks like Flask or Django, Python offers excellent security features and ease of use.
- PHP: A widely used language with many security resources available.
- Ruby: Ruby on Rails provides a robust framework for building secure web applications.