Credential Stuffing in Hanami with Mutual Tls

Credential stuffing relies on automated requests using lists of breached username and password pairs. In Hanami, a Ruby web framework that encourages explicit routing and controller actions, the presence of mutual TLS (mTLS) does not by itself prevent application-layer attacks. Mutual TLS authenticates the client to the server using a client certificate, but it does not validate the identity of the human user behind the request. If mTLS is used to allow access to the login or session creation endpoint, an attacker can still automate submissions against that endpoint as long as they possess a valid client certificate.

When mTLS is implemented in Hanami, the typical setup involves verifying the client certificate in a Rack middleware or during the TLS handshake at the load balancer or reverse proxy. Hanami itself may see the request as authenticated at the transport layer, which can lead to a false sense of security. For example, if the login route does not enforce additional user authentication beyond the client certificate, an attacker can iterate through credential pairs while presenting a valid certificate. The application may log successful TLS handshakes but still process invalid user credentials, effectively bypassing protections that would otherwise block automated login attempts.

Another subtlety involves rate limiting and session management. Mutual TLS can be tied to a client identity, but if Hanami does not correlate the certificate with a user context, protections such as account lockouts or progressive delays may not apply per user. An attacker can rotate client certificates or use a small set of certificates shared across many automated requests, while the server treats each certificate as a distinct client. This can allow credential stuffing attempts to fly under rate-limiting rules that are defined per certificate rather than per user account. The interplay between mTLS at the edge and application-level controls in Hanami must be carefully designed so that one layer does not create a gap the other layer fails to close.

From a scanning perspective, middleBrick checks whether authentication mechanisms are properly enforced on endpoints, including how credentials and TLS client identity are used together. Without correlating mTLS identities with application users, findings related to weak or missing controls on login flows can still surface. The scanner tests unauthenticated attack surfaces, and when mTLS is present but insufficiently coupled to user authentication, it may still identify risky paths where credential stuffing could be attempted.

Real-world attack patterns such as those listed in the OWASP API Top 10, including weak authentication and insufficient rate limiting, remain relevant. For instance, CVE-2023-30547-type issues can manifest when access controls rely on transport-layer identity without validating the actual user. In Hanami, this might mean that a session is established after verifying a client certificate while skipping proper password checks. Security testing should therefore verify that both mTLS and user authentication are required and that protections like rate limiting are applied at the user level, not only at the connection level.

Remediation focuses on ensuring that mutual TLS is treated as one factor in a multi-factor context and that Hanami enforces user authentication independently. The following examples show how to configure a Hanami application to require both a valid client certificate and explicit user credentials, and how to apply per-user rate limiting to mitigate credential stuffing.

1) Enforce user login in addition to mTLS in a Hanami controller:

module Web::Controllers::Session
  class Create
    include Web::Action

    before :authenticate_client!

    def call(params)
      # mTLS client identity is available from the request env
      client_dn = request.env['SSL_CLIENT_S_DN']
      # Require explicit user credentials
      user = UserRepository.new.find_by(email: params[:email])
      if user && LoginAuthorizer.new(user).authenticate(params[:password])
        # Establish session only if both mTLS and password are valid
        session[:user_id] = user.id
        session[:mtls_dn] = client_dn
        redirect_to routes.dashboard_path
      else
        self.status = 401
        render 'session/new'
      end
    end

    private

    def authenticate_client!
      # Reject requests without a valid client certificate
      unless request.env['SSL_CLIENT_VERIFY'] == 'SUCCESS'
        self.status = 403
        halt 'Client certificate required'
      end
    end
  end
end

2) Apply per-user rate limiting to defend against credential stuffing:

module Web::Controllers::Session
  class Create
    include Web::Action

    RATE_LIMIT = 5   # attempts
    WINDOW     = 60  # seconds

    def call(params)
      user = UserRepository.new.find_by(email: params[:email])
      key = "rate_limit:user:#{user.id}"
      current = redis.incr(key)
      redis.expire(key, WINDOW) unless current == 1

      if current > RATE_LIMIT
        self.status = 429
        return { error: 'Too many attempts, try again later' }
      end

      # proceed with authentication as shown above
    end
  end
end

3) Use Rack middleware to validate client certificates and map them to application users when appropriate, while logging mismatches for monitoring:

# config/initializers/mtls_user_mapping.rb
require 'openssl'

class MtlsUserMapping
  def initialize(app)
    @app = app
  end

  def call(env)
    if env['SSL_CLIENT_VERIFY'] == 'SUCCESS'
      cert = OpenSSL::X509::Certificate.new(env['SSL_CLIENT_CERT'])
      subject = cert.subject.to_s
      # Example mapping logic: extract user identifier from certificate subject
      user_email = extract_email_from_subject(subject)
      if user_email
        env['warden'].set_user(UserRepository.new.find_by(email: user_email))
      end
    end
    @app.call(env)
  end

  private

  def extract_email_from_subject(subject)
    # Simplified extraction; tailor to your certificate policy
    subject.match(/emailAddress=([^,]+)/)&.captures&.first
  end
end

# In your Hanami configuration
# config.middleware.use MtlsUserMapping

These examples demonstrate that mutual TLS should complement, not replace, user authentication and rate limiting in Hanami. By tying client identity to user accounts and enforcing limits per user, you reduce the surface for automated credential submission. Security testing tools like middleBrick can validate that these controls are effective and that no gaps remain between mTLS and application-layer protections.