Securing Shell Scripts From Arbitrary Code Execution

Mitigating the Risks of Unsecure Shell Scripts

Shell scripts provide a convenient way to automate tasks on Linux and Unix-based systems. However, unsecure shell scripts can expose systems to significant risks. Flaws in shell scripts may enable arbitrary code execution, allowing attackers to execute malicious commands and take control of systems.

Arbitrary code execution through shell scripts is unfortunately common. Attackers exploit weaknesses such as untreated user input, unsafe functions like eval(), and excessive permissions. By taking advantage of these flaws, external parties can inject code into scripts and execute commands as the user running the script.

The impacts of arbitrary code execution are severe. Attackers may gain access to sensitive data, carry out destructive activities, or use compromised servers for illegal purposes. Left unchecked, vulnerable shell scripts put an organization’s security posture in jeopardy.

Thankfully, with proper precautions, admins can prevent arbitrary code execution and create secure shell scripts. Best practices such as input sanitization, careful permissions, code review, and restricting vulnerable functions can harden shell scripts.

This article outlines steps and techniques to secure shell scripts from arbitrary remote and local code execution. Following security best practices for shell scripts protects servers, data, and users.

Best Practices for Writing Secure Shell Scripts

Using Function Libraries

Function libraries consolidate reusable shell functions into a single script. Importing dedicated libraries avoids coding custom functions repeatedly. Standard script headers define function libraries to be sourced by other scripts as needed.

For security, script authors should leverage vetted function libraries instead of coding custom solutions. Standard libraries undergo rigorous security review to avoid common pitfalls. Custom functions may not receive the same level of scrutiny, risking overlooked vulnerabilities.

Shell script headers should source system default or trusted third-party function libraries. Debian and Red Hat based systems provide vetted script utility libraries like /usr/share/bash-completion/bash_completion. Sourcing such libraries improves security over ad hoc scripting.

Sanitizing Input Data

Untrusted input data poses one of the biggest risks for arbitrary code execution. Attackers can submit malicious code through insecure user parameters which get injected into sensitive script operations. Script authors must explicitly sanitize all external inputs.

Input validation strategies such as whitelisting, blacklisting, and semantics checks should be applied. The exact sanitization method depends on the permissible parameter format. Numeric data may warrant different checks than free-form text input. Typed parameters also require matching validation.

All supplied parameter data should conform strictly to the expected format. Rejecting improper input prevents injecting unintended payloads. Carefully checking inputs defends against compromised arguments tricking scripts into misbehaving dangerously.

Setting Permissions Carefully

Overly permissive script permissions enable arbitrary code execution if an attacker identifies other weaknesses. Scripts should run with the minimum permissions necessary. Avoid running scripts with root, admin or other elevated rights unless absolutely required.

SetUID and SetGID permissions should also be avoided on scripts. Rather than attempt to safely determine appropriate escalated permissions, create wrapper scripts invoking utilities requiring elevated rights as needed. Such targeted privilege escalation reduces unnecessary exposure.

Scripts must fail safe by default, only granting access if authorized, and denying all else. Restrict use to explicitly allowed purposes through thoughtful permissions for improved security.

Avoiding Eval Statements

Eval statements process string input as live code which is extremely dangerous without sufficient safeguards. Eval can execute attacker-supplied input, allowing arbitrary remote code execution even without script flaws.

Eval should therefore be avoided in most shell scripts. Only use eval if absolutely required, and complement with other mitigate measures like input validation and sanitization. Legitimate use cases for eval are also better served by safer native shell or scripting language alternatives in most situations.

Access control and environment isolation techniques can make eval safer if unavoidable. Minimizing and strictly controlling resources available to eval statements limits damage potential. But avoiding eval altogether is the most prudent approach in shell scripts whenever possible.

Commenting Code Clearly

Code comments explain script logic, flow, dependencies, constraints, and standards to readers. Well commented scripts are easier to maintain, review, and secure.

Comment thoroughly to enable robust security reviews. Call attention to key validation checks, safety measures, and other protections so reviewers understand and can critique risk controls.

Highlight security sensitive sections requiring extra scrutiny like user inputs and external system calls. Provide context so reviewers can identify enforcement gaps compared to expected safeguards.

Clear descriptive comments also help correctly implement script authors’ intent after maintenance or troubleshooting changes. Commenting facilitates security preservation across script evolution.

Real-World Example Scripts

Example 1: Restricting User Access

Sensitive credential files require strict access restrictions, even for privileged users. Scripts providing authorized access must carefully control read permissions.

Input validation checks ensure only pre-approved user accounts receive access credentials per policy. Attempted unauthorized access raises alerts for investigation. Comments draw attention to key protections against tampering or credential leakage.

Example script excerpt:

#!/bin/bash
  
# Function to validate user is permitted access  
verify_authorized_user() {

  # Check user matches allowed list
  if [[ ! " ${ALLOWED_USERS[@]} " =~ " ${1} " ]]; then
    logger "Unauthorized credential access attempt by user $1" 
    exit 1
  fi    
}

# Main script execution 
echo "Fetching credentials..."
  
# Scrub user input to prevent code injection  
user=$({sanitize_input} "$1")  

# Validate accessing user
verify_authorized_user "$user"
  
# Output credentials file
cat "/secured/credentials/${user}_credentials"

Example 2: Validating Input Data

Script parameters sourced from users or external systems may allow arbitrary code execution if left unvalidated. Input scrubbing protects against injection attacks through compromised arguments.

Here a script sanitizes free-form text inputs which get passed to sensitive SQL queries. Only known good characters are permitted using a whitelist, blocking potentially malicious inputs. Comments call out protections against code injection.

Example script excerpt:

  
#!/bin/bash

# Function to scrub input text to prevent code injection  
sanitize_input() {
          
  # Whitelist permitted characters  
  local clean="${1//[^a-zA-Z0-9/]}""
          
  # Return sanitized result
  echo "$clean"  
}

# Main script execution
echo "Running tax report..."  
   
# Scrub free-form user input
name=$(sanitize_input "$1")
category=$(sanitize_input "$2")
  
# Validate data formats
if [[ -z "$name" || -z "$category" ]]; then
   echo "Invalid parameters provided"
   exit 1
fi  

# Run tax report query
run_query "SELECT * FROM tax WHERE name='$name' AND category='$category'"

Additional Steps to Harden Shell Script Security

Enabling Security Features

Shell script interpreters and environments provide built-in security capabilities to leverage. Bash offers options like:

• set -e to exit on errors
• set -u to detect uninitialized variables
• set -o pipefail for safer pipelines
• set -x to debug scripts

Security-enhancing shell options should be set early in scripts. Modern Linux distributions ship with enhanced shell hardening features as well.

Security modules like SELinux also integrate with shell scripts. Where available, leveraging SELinux mandatory access controls strengthens script security.

Monitoring Execution and Access

Runtime monitoring helps detect and respond to suspicious script activity. Logging user access, input parameters, execution targets and other events enables auditing and alerts.

Track access attempts to sensitive scripts to uncover unauthorized usage. Similarly log script inputs and environment variables to catch tampering or poisoning attacks.

Audit logs provide attack early warning and forensic evidence after incidents. Centralize script logging with security information event management tools for easiest analysis and retention.

Other controls like integrity monitoring and anomaly detection also improve script security posture. Multiple visibility techniques make arbitrary code execution much harder to achieve.