Skip to main content

OS Command Injection

Need​

Prevention of OS command injection attacks

Context​

  • Usage of Ruby for building web applications and scripting
  • Usage of params for handling request parameters in a Node.js application
  • Usage of render for server-side rendering in web development

Description​

Non compliant code​

def execute_system_command
  command = params[:command]
  result = `#{command}`
  render plain: result
end

In the above code, we have a function execute_system_command which is designed to execute a system command that is passed in as a parameter from the user. The function takes the user-provided command, executes it directly in the system shell, and then returns the result.

The vulnerability here is that the command to be executed is taken directly from the user input (params[:command]) without any form of sanitization or validation. This is a classic example of an OS Command Injection vulnerability.

An attacker can exploit this vulnerability by passing in malicious commands. For example, they could pass in a command to list all files in the directory, delete files, or even execute arbitrary code that could compromise the server. This vulnerability could lead to a full system compromise depending on the permissions of the process running the Ruby on Rails server.

This code is vulnerable because it trusts user input without validation. In secure coding, one of the primary rules is "Never trust user input". Any input coming from the user should be treated as potentially malicious and should be properly sanitized and validated before using it in a sensitive context such as command execution.

Steps​

  • Validate and sanitize user input: Ensure that the 'command' parameter is properly validated and sanitized before executing it as a system command. Use input validation techniques such as whitelisting or regular expressions to only allow safe characters and commands.
  • Use parameterized queries or prepared statements: If the 'command' parameter is used in a database query, make sure to use parameterized queries or prepared statements to prevent SQL injection attacks.
  • Implement a command execution whitelist: Instead of allowing arbitrary commands to be executed, create a whitelist of allowed commands and validate the 'command' parameter against this whitelist. Only execute the command if it matches an entry in the whitelist.
  • Limit privileges: Ensure that the code executing the system command has the minimum necessary privileges. Avoid running commands with root/administrator privileges if possible.
  • Use a secure shell escape mechanism: If the application needs to execute system commands, use a secure shell escape mechanism provided by the programming language or framework. This can help prevent command injection vulnerabilities.
  • Implement strong access controls: Ensure that only authorized users have access to the functionality that allows executing system commands. Implement strong access controls and authentication mechanisms to prevent unauthorized access.
  • Regularly update dependencies: Keep the Ruby on Rails framework and other dependencies up to date to benefit from security patches and fixes.
  • Perform security testing: Regularly perform security testing, including penetration testing and code reviews, to identify and fix vulnerabilities in the application.

Compliant code​

def execute_system_command
  whitelist = ['ls', 'date', 'uptime'] # Add your safe commands here
  command = params[:command]

  # Validate and sanitize user input
  if whitelist.include?(command)
    result = `#{Shellwords.escape(command)}`
    render plain: result
  else
    render plain: "Invalid command"
  end
end

The updated code includes a whitelist of safe commands that can be executed. The 'command' parameter is checked against this whitelist, and the command is only executed if it matches an entry in the whitelist. This prevents the execution of arbitrary commands, reducing the risk of command injection attacks.

The Shellwords.escape(command) function is used to escape any potentially dangerous characters in the 'command' parameter. This is a secure shell escape mechanism provided by Ruby, which can help prevent command injection vulnerabilities.

If the 'command' parameter does not match an entry in the whitelist, the function returns a message "Invalid command", instead of executing the command. This provides feedback to the user about the invalid input, without exposing any sensitive information or system functionality.

Remember to keep the whitelist updated with only the necessary commands that your application needs to execute. Avoid adding commands that can modify system settings or access sensitive information.

Also, ensure that only authorized users have access to the functionality that allows executing system commands. Implement strong access controls and authentication mechanisms to prevent unauthorized access.

Keep the Ruby on Rails framework and other dependencies up to date to benefit from security patches and fixes. Regularly perform security testing, including penetration testing and code reviews, to identify and fix vulnerabilities in the application.

References​

Last updated on