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Remote command execution

Need​

Prevention of unauthorized remote command execution

Context​

  • Usage of Ruby for building dynamic and object-oriented applications
  • Usage of the system library for interacting with the operating system and managing system resources.

Description​

Non compliant code​

def execute_command(user_input)
  command = "ls -l #{user_input}"
  system(command)
end

The above code is a simple Ruby function that uses the system command to execute an OS command. The command that is executed is "ls -l" followed by a user input. This user input is not sanitized or validated in any way before being used in the command. This opens up a serious security vulnerability as it allows for remote command execution.

The system function in Ruby executes the given command in a subshell, and the command is subject to shell expansion. This means that if the user input includes shell meta-characters, they will be interpreted by the shell. For example, if the user input is "; rm -rf /", the command that will be executed is "ls -l ; rm -rf /". This is actually two commands - the first is "ls -l", and the second is "rm -rf /", which will delete all files in the filesystem.

This is a serious security vulnerability, as it allows an attacker to execute any command they want on the server, with the permissions of the user running the Ruby script. This could lead to data loss, data theft, or a complete system compromise.

Steps​

  • Validate and sanitize user input before using it to build OS commands.
  • Use parameterized queries or prepared statements to build OS commands.
  • Avoid using user input directly in OS commands.
  • Implement input validation to ensure that user input meets the expected format and length.
  • Implement output encoding to prevent any special characters from being interpreted as commands.
  • Implement least privilege principle by running the application with limited permissions.
  • Regularly update and patch the underlying operating system and dependencies to mitigate any known vulnerabilities.

Compliant code​

def execute_command(user_input)
  # Validate and sanitize user_input
  user_input = sanitize(user_input)
  command = "ls -l #{user_input}"
  system(command)
end

def sanitize(input)
  # Remove any potentially harmful characters
  sanitized_input = input.gsub(/[^0-9A-Za-z \\-]/, '')
  return sanitized_input
end

The original code was vulnerable to remote command execution because it was directly using user input to build an OS command. This could allow an attacker to manipulate the input and execute arbitrary commands on the system.

The fixed code includes a new method sanitize(input), which is used to sanitize the user input before it is used to build the OS command. This method uses a regular expression to remove any characters from the input that are not alphanumeric, spaces, or hyphens. This effectively neutralizes any special elements that could modify the OS command.

In addition to this, it's recommended to implement further security measures such as:

  • Using parameterized queries or prepared statements to build OS commands.
  • Avoiding the use of user input directly in OS commands wherever possible.
  • Implementing input validation to ensure that user input meets the expected format and length.
  • Implementing output encoding to prevent any special characters from being interpreted as commands.
  • Running the application with limited permissions (least privilege principle).
  • Regularly updating and patching the underlying operating system and dependencies to mitigate any known vulnerabilities.

References​

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