Zen Rails Security Checklist

Summary

This document provides a list of security measures to be implemented when developing a Ruby on Rails application. It is designed to serve as a quick reference and minimize vulnerabilities caused by developer forgetfulness.

This checklist is meant to be a community-driven resource. Your contributions are welcome!

Disclaimer: This document does not cover all possible security vulnerabilities. The authors do not take any legal responsibility for the accuracy or completeness of the information herein.

Supported Rails Versions

This document focuses on Rails 4 and 5. Vulnerabilities that were present in earlier versions and fixed in Rails 4 are not included.

The Checklist

Injection

Injection attacks are #1 at the OWASP Top10.

• Don’t use standard Ruby interpolation (#{foo}) to insert user inputted strings into ActiveRecord or raw SQL queries. Use the ? character, named bind variables or the ActiveRecord::Sanitization methods to sanitize user input used in DB queries. Mitigates SQL injection attacks.
• Don't pass user inputted strings to methods capable of evaluating code or running O.S. commands such as eval, system, syscall, %x(), and exec. Mitigates command injection attacks.

Resources:

• Ruby on Rails Security Guide - SQL Injection
• Rails SQL Injection Examples

Authentication (Devise)

Broken Authentication and Session Management are #2 at the OWASP Top 10.

• Enforce a minimum password length of 8 characters or more. Mitigates brute-force attacks.
• Consider validating passwords against:
  • Dictionary words. Since passwords have a minimum length requirement, the dictionary need only include words meeting that requirement.
  • A list of commonly used passwords such as these. The password_strength and StrongPassword gems provide such feature.
  • A leaked password database such as PasswordPing.
  • Context-specific words, such as the name of the application, the username, and derivatives thereof.
• Consider the pros and cons of enforcing password complexity rules such as mixtures of different character types. Most applications use it. However, the latest NIST Guidelines advise against it. An alternative is to increase the minimum length requirement and encourage the usage of passphrases. Devise only validates password length. Gems such as devise_security_extension, StrongPassword, devise_zxcvbn, and password_strength provide additional password validation capabilities, such as entropy calculation (based on password complexity). Validation may also be implemented with regex (code sample). Mitigate brute-force attacks.
• Lock the account after multiple failed login attempts. Use Devise's lockable module. Mitigates brute-force attacks.
• Require users to confirm their e-mail addresses on sign-up and when the e-mail address is changed. Use Devise's confirmable module and set config.reconfirmable = true in config/initializers/devise.rb. Mitigates the creation of bogus accounts with non-existing or third-party e-mails.
• Require users to input their old password on password change. Mitigates unauthorized password changes on session hijacking, CSRF or when a user forgets to log out and leaves the PC or mobile device unattended.
• Expire the session at log out and expire old sessions at every successful login. Devise does that by default. Also use Devise's timeoutable module to expire sessions after a period of inactivity (e.g., 30 minutes). Mitigates CSRF, session hijacking and session fixation attacks by reducing their time-frame.
• Notify user via email on password change. Set config.send_password_change_notification = true in config/initializers/devise.rb. Does not prevent an attacker from changing the victim's password, but warns the victim so he can contact the system administrator to revoke the attacker's access.
• Use generic error messages such as "Invalid email or password" instead of specifying which part (e-mail or password) is invalid. Devise does that by default. Mitigates user enumeration and brute-force attacks.
• Ensure all non-public controllers/actions require authentication. Add before_acion :authenticate_user! to ApplicationController and skip_before_action :authenticate_user! to publicly accessible controllers/actions. Avoid unauthorized access due to developer forgetfulness.
• Consider using the devise_security_extension gem, which provides additional security for Devise.
• Consider using two-factor authentication (2FA) as provided by Authy. See the devise-two-factor and authy-devise gems. Provides a highly effective extra layer of authentication security.
• Consider requiring authentication in config/routes.rb by putting non-public resources within a authenticate :user do block (see the Devise Wiki). Requiring authentication in both controllers and routes may not be DRY, but such redundancy provides additional security (see Defense in depth).
• Consider restricting administrator access by IP. If the client's IP is dynamic, restrict by IP block/ASN or by country via IP geolocation (country).

Sessions & Cookies

Broken Authentication and Session Management are #2 at the OWASP Top 10.

• Don't store data such as money/point balances or user privileges in a cookie or a CookieStore Session. Store it in the database instead. Mitigates replay attacks.
• Consider always using encrypted cookies. This is the default behavior in
  Rails 4 when secret_key_base is set. Strengthens cookie encryption and mitigates multiple attacks involving cookie tampering.
• Unless your JavaScript frontend needs to read cookies generated by the Rails server, set all cookies as httponly. Search the project for cookie accessors and add httponly: true. Example: cookies[:login] = {value: 'user', httponly: true}. Restricts cookie access to the Rails server. Mitigates attackers from using the victim's browser JavaScript to steal cookies after a successful XSS attack.

Resources:

• Ruby on Rails Security Guide - Sessions

Cross-Site Scripting (XSS)

XSS is #3 at the OWASP Top 10.

Handling User Input

• Always validate user input that may eventually be displayed to other users. Attempting to blacklist characters, strings or sanitize input tends to be ineffective (see examples of how to bypass such blacklists). A whitelisting approach is usually safer. Mitigates multiple XSS attacks.
• Consider using the loofah-activerecord gem to scrub your model attribute values. Mitigates multiple XSS attacks.
• If you must create links from user inputted URLs, be sure to validate them. If using regex, ensure that the string begins with the expected protocol(s), as in \Ahttps?. Mitigates XSS attacks such as entering javascript:dangerous_stuff()//http://www.some-legit-url.com as a website URL that is displayed as a link to other users (e.g., in a user profile page).
• When using regex for input validation, use \A and \z to match string beginning and end. Do not use ^ and $ as anchors. Mitigates XSS attacks that involve slipping JS code after line breaks, such as me@example.com\n<script>dangerous_stuff();</script>.
• Do not trust validations implemented at the client (frontend) as most implementations can be bypassed. Always (re)validate at the server.

Output Escaping & Sanitization

• Escape all HTML output. Rails does that by default, but calling html_safe or raw at the view suppresses escaping. Look for calls to these methods in the entire project, check if you are generating HTML from user-inputted strings and if those strings are effectively validated. Note that there are dozens of ways to evade validation. If possible, avoid calling html_safe and raw altogether. For custom scrubbing, see ActionView::Helpers::SanitizeHelper Mitigates XSS attacks.*
• Avoid sending user inputted strings in e-mails to other users. Attackers may enter a malicious URL in a free text field that is not intended to contain URLs and does not provide URL validation. Most e-mail clients display URLs as links. Mitigates XSS, phishing, malware infection and other attacks.

Resources:

• Ruby on Rails Security Guide - XSS
• OWASP XSS Filter Evasion Cheat Sheet
• OWASP Ruby on Rails Cheatsheet - Cross-site Scripting (XSS)
• Plataformatec Blog - The new HTML sanitizer in Rails 4.2

HTTP & TLS

• Force HTTPS over TLS (formerly known as SSL). Set config.force_ssl = true in config/environments/production.rb. May also be done in a TLS termination point such as a load balancer, Nginx or Passenger Standalone. Mitigates man-in-the-middle and other attacks.
• Use the SSL Server Test tool from Qualys SSL Lab to check the grade of your TLS certificate. Be sure to use the strongest (yet widely compatible) protocols and cipher suites, preferably with Ephemeral Diffie-Hellman support. Mitigates multiple SSL/TLS-related attacks such as BEAST and POODLE.
• Consider rate-limiting incoming HTTP requests, as implemented by the rack-attack and rack-throttle gems. Mitigates web scraping, HTTP floods, and other attacks.

Security-related headers

• Consider using the Secure Headers gem. Mitigates several attacks.
• Consider obfuscating the web server banner string. In other words, hide your web server name and version. Mitigates HTTP fingerprinting, making it harder for attackers to determine which exploits may work on your web server.

Authorization (Pundit)

• Implement authorization at the server. Hiding links/controls in the UI is not enough to protect resources against unauthorized access. Mitigates forced browsing attacks.
• Ensure all controllers/actions which require authorization call the authorize or policy_scope method (sample code). Mitigates forced browsing attacks due to developers forgetting to require authorization in some controller actions.
• When using DB records associated to users to populate select boxes, radio buttons or checkboxes, instead of querying by association (user.posts), consider using policy_scope. See additional details and sample code. Improves readability and maintainability of authorization policies.

Resources:

• Pundit: Ensuring policies and scopes are used
• Pundit: Scopes

Files

File Uploads

• Avoid using user controlled filenames. If possible, assign "random" names to uploaded files when storing them in the OS. If not possible, whitelist acceptable characters. It is safer to deny uploads with invalid characters in the filenames than to attempt to sanitize them. Mitigates Directory Traversal Attacks such as attempting to overwrite system files by uploading files with names like ../../passwd.
• Avoid using libraries such as ImageMagick to process images and videos on your server. If possible, use an image/video processing service such as Transloadit, Cloudinary, or imgix. Mitigates multiple image/video processing related vulnerabilities such as these.
• Process uploaded files asynchronously. If not possible, implement per-client rate limiting. Mitigates DoS Attacks that involve overloading the server CPU by flooding it with uploads that require processing.
• Do not trust validations implemented at the client (frontend) as most implementations can be bypassed. Always (re)validate at the server.
• Validate files before processing. Mitigates DoS Attacks such as image bombs.
• Whitelist acceptable file extensions and acceptable Media Types (formerly known as MIME types). Validating file extensions without checking their media types is not enough as attackers may disguise malicious files by changing their extensions. Mitigates the upload of dangerous file formats such as shell or Ruby scripts.
• Limit file size. Mitigates against DoS attacks involving the upload of very large files.
• Consider uploading directly from the client (browser) to S3 or a similar cloud storage service. Mitigates multiple security issues by keeping uploaded files on a separate server than your Rails application.
• If allowing uploads of malware-prone files (e.g., exe, msi, zip, rar, pdf), scan them for viruses/malware. If possible, use a third party service to scan them outside your server. Mitigates server infection (mostly in Windows servers) and serving infected files to other users.
• If allowing upload of archives such as zip, rar, and gz, validate the target path, estimated unzip size and media types of compressed files before unzipping. Mitigates DoS attacks such as zip bombs, zipping malicious files in an attempt to bypass validations, and overwriting of system files such as /etc/passwd.

File Downloads

• Do not allow downloading of user-submitted filenames and paths. If not possible, use a whitelist of permitted filenames and paths. Mitigates the exploitation of directory traversal vulnerabilities to download sensitive files.

Resources:

• Ruby on Rails Security Guide - File Uploads and Downloads

Cross-Site Request Forgery (CSRF)

• Enforce CSRF protection by setting protect_from_forgery with: :exception in all controllers used by web views or in ApplicationController.
• Use HTTP verbs in a RESTful way. Do not use GET requests to alter the state of resources. Mitigates CSRF attacks.
• Up to Rails 4, there was a single CSRF token for all forms, actions, and methods. Rails 5 implements per-form CSRF tokens, which are only valid for a single form and action/method. Enable it by setting config.action_controller.per_form_csrf_tokens = true.

Resources:

• Ruby on Rails Security Guide - CSRF
• Big Binary Blog - Each form gets its own CSRF token in Rails 5

Sensitive Data Exposure

• If possible, avoid storing sensitive data such as credit cards, tax IDs and third-party authentication credentials in your application. If not possible, ensure that all sensitive data is encrypted at rest (in the DB) and in transit (use HTTPS over TLS). Mitigate theft/leakage of sensitive data.
• Do not log sensitive data such as passwords and credit card numbers. You may include parameters that hold sensitive data in config.filter_parameters at initializers/filter_parameter_logging.rb. For added security, consider converting filter_parameters into a whitelist. See sample code. Prevents plain-text storage of sensitive data in log files.
• HTML comments are viewable to clients and should not contain details that can be useful to attackers. Consider using server-side comments such as <%# This comment syntax with ERB %> instead of HTML comments. Avoids exposure of implementation details.
• Avoid exposing numerical/sequential record IDs in URLs, form HTML source and APIs. Consider using slugs (A.K.A. friendly IDs, vanity URLs) to identify records instead of numerical IDs, as implemented by the friendly_id gem. Additional benefits include SEO and better-looking URLs. Mitigates forced browsing attacks and exposure of metrics about your business, such as the number of registered users, number of products on stock, or number of receipts/purchases.
• Do not set config.consider_all_requests_local = true in the production environment. If you need to set config.consider_all_requests_local = true to use the better_errors gem, do it on config/environments/development.rb. Prevents leakage of exceptions and other information that should only be accessible to developers.
• Don't install development/test-related gems such as better_errors and web-console in the production environment. Place them within a group :development, :test do block in the Gemfile. Prevents leakage of exceptions and even REPL access if using better_errors + web-console.

Credentials & Secrets

• Do not commit sensitive data such as secret_key_base, DB, and API credentials to git repositories. Avoid storing credentials in the source code, use environment variables instead. If not possible, ensure all sensitive files such as /config/database.yml, config/secrets.yml (and possibly /db/seeds.rb if it is used to create seed users for production) are included in .gitignore. Mitigates credential leaks/theft.
• Use different secrets in the development and production environments. Mitigates credential leaks/theft.
• Use a secret_key_base with over 30 random characters. The rake secret command generates such strong keys. Strengthens cookie encryption, mitigating multiple cookie/session related attacks.

Routing, Template Selection, and Redirection

• Don't perform URL redirection based on user inputted strings. In other words, don't pass user input to redirect_to. If you have no choice, create a whitelist of acceptable redirect URLs or limit to only redirecting to paths within your domain (example code). Mitigates redirection to phishing and malware sites. Prevent attackers from providing URLs such as http://www.my-legit-rails-app.com/redirect?to=www.dangeroussite.com to victims.
• Do not use a user inputted string to determine the name of the template or view to be rendered. Prevents attackers from rendering arbitrary views such as admin-only pages.
• Avoid "catch-all" routes such as match ':controller(/:action(/:id(.:format)))' and make non-action controller methods private. Mitigates unintended access to controller methods.

Resources:

• OWASP Ruby on Rails Cheatsheet - Redirects and Forwards (URL validation)

Third-party Software

• Apply the latest security patches in the OS frequently. Pay special attention to internet-facing services such as application servers (Passenger, Puma, Unicorn), web servers (Nginx, Apache, Passenger Standalone) and SSH servers.
• Update Ruby frequently.
• Watch out for security vulnerabilities in your gems. Run bundler-audit frequently or use a service like Appcanary.

Security Tools

• Run Brakeman before each deploy. If using an automated code review tool like Code Climate, enable the Brakeman engine.
• Consider using a continuous security service such as Detectify.
• Consider using a Web Application Firewall (WAF) such as NAXSI for Nginx, ModSecurity for Apache and Nginx. Mitigates XSS, SQL Injection, DoS, and many other attacks.

Others

• Use strong parameters in the controllers. This is the default behavior as of Rails 4. Mitigates mass assignment attacks such as overwriting the role attribute of the User model for privilege escalation purposes.
• Implement Captcha or Negative Captcha on publicly exposed forms. reCAPTCHA is a great option, and there is a gem that facilitates Rails integration. Other options are the rucaptcha and negative-captcha gems. Mitigates automated SPAM (spambots).

Details and Code Samples

Password validation regex

We may implement password strength validation in Devise by adding the following code to the User model.

validate :password_strength

private

def password_strength
  minimum_length = 8
  # Regex matches at least one lower case letter, one uppercase, and one digit
  complexity_regex = /\A(?=.*[a-z])(?=.*[A-Z])(?=.*[0-9])/
  # When a user is updated but not its password, the password param is nil
    if password.present? && password.length < minimum_length || !password.match(complexity_regex)
    errors.add :password, 'must be 8 or more characters long, including 
                             at least one lowercase letter, one uppercase
                             letter, and one digit.'
  end
end

Pundit: ensure all actions are authorized

Add the following to app/controllers/application_controller.rb

after_action :verify_authorized, except: :index, unless: :devise_controller?
after_action :verify_policy_scoped, only: :index, unless: :devise_controller?

Add the following to controllers that do not require authorization. You may create a concern for DRY purposes.

after_action_skip :verify_authorized
after_action_skip :verify_policy_scoped

Pundit: only display appropriate records in select boxes

Think of a blog-like news site where users with editor role have access to specific news categories, and admin users have access to all categories. The User and the Category models have an HMT relationship. When creating a blog post, there is a select box for choosing a category. We want editors only to see their associated categories in the select box, but admins must see all categories. We could populate that select box with user.categories. However, we would have to associate all admin users with all categories (and update these associations every time a new category is created). A better approach is to use Pundit Scopes to determine which categories are visible to each user role and use the policy_scope method when populating the select box.

# app/views/posts/_form.html.erb
f.collection_select :category_id, policy_scope(Category), :id, :name

Convert filter_parameters into a whitelist

Developers may forget to add one or more parameters that contain sensitive data to filter_parameters. Whitelists are usually safer than blacklists as they do not generate security vulnerabilities in case of developer forgetfulness. The following code converts filter_parameters into a whitelist.

# config/initializers/filter_parameter_logging.rb
if Rails.env.production?
  # Parameters whose values are allowed to appear in the production logs:
  WHITELISTED_KEYS = %w(foo bar baz)
  
  # (^|_)ids? matches the following parameter names: id, *_id, *_ids
  WHITELISTED_KEYS_MATCHER = /((^|_)ids?|#{WHITELISTED_KEYS.join('|')})/.freeze
  SANITIZED_VALUE = '[FILTERED]'.freeze
  
  Rails.application.config.filter_parameters << lambda do |key, value|
    unless key.match(WHITELISTED_KEYS_MATCHER)
      value.replace(SANITIZED_VALUE)
    end
  end
else
  # Keep the default blacklist approach in the development environment
  Rails.application.config.filter_parameters += [:password]
end

Authors

• Bruno Facca - LinkedIn - Email: bruno at facca dot info

Contributing

Contributions are welcome. If you would like to correct an error or add new items to the checklist, feel free to create an issue and/or a PR. If you are interested in contributing regularly, drop me a line at the above e-mail to become a collaborator.

References and Further Reading

• Ruby on Rails Security Guide
• The Rails 4 Way by Obie Fernandez, Chapter 15
• OWASP Top Ten
• SitePoint: Common Rails Security Pitfalls and Their Solutions
• Rails Security Audit by Hardhat
• Rails Security Checklist by Eliot Sykes

License

Released under the MIT License.