Buffer Overflow in Hanami with Mutual Tls

A buffer overflow occurs when a program writes more data to a buffer than it can hold, corrupting adjacent memory. Hanami, a Ruby web framework, is typically resilient to classic C-style buffer overflows because Ruby manages memory and strings safely. However, the combination of Hanami and Mutual TLS (mTLS) can expose or amplify risks when unsafe native extensions, external libraries, or improperly handled request payloads interact with TLS termination and client certificate processing.

Mutual TLS requires both the client and server to present and validate certificates during the handshake. In Hanami, this usually occurs at the reverse proxy or load balancer (e.g., nginx, HAProxy) or via a Ruby TLS library like openssl or net/http) when terminating TLS inside the app. If Hanami or its dependencies process client certificate data or incoming request bodies into fixed-size C buffers—such as through native gems that parse TLS records or legacy C extensions—care must be taken to avoid overflows when handling large or malformed payloads.

For example, if a Hanami app uses an mTLS-enabled endpoint that accepts large JSON or form payloads and passes them to a native library for parsing (e.g., image processing, XML parsing, or a C-based crypto binding), an attacker could craft a request that exceeds expected buffer sizes. The TLS layer itself does not introduce the overflow, but mTLS increases the attack surface by ensuring that only authenticated clients can reach the vulnerable endpoint, making targeted exploitation more likely. Additionally, if certificate validation logic or session handling in Hanami or its dependencies uses fixed-size buffers for storing certificate metadata or headers, oversized certificates or headers could trigger overflow conditions.

Because middleBrick scans the unauthenticated attack surface, it can detect indicators such as unusual response patterns, missing rate limiting, or input validation gaps that may hint at unsafe handling of mTLS-bound requests. While middleBrick does not fix the issue, its findings can guide developers to review native dependencies and ensure that any code processing client certificates or request data avoids fixed-size buffers.

Concrete mitigations involve validating input sizes, avoiding C extensions that do not bounds-check writes, and ensuring that mTLS termination occurs in a component that does not pass raw request bodies to unsafe code. For Hanami, this often means keeping TLS termination at the proxy layer and strictly validating or sanitizing all incoming data before it reaches application logic.

To securely implement Mutual TLS in Hanami while avoiding buffer overflow risks, handle TLS termination at the infrastructure layer and keep application code focused on validated, bounded data processing. Below are concrete, realistic examples using Ruby’s openssl library and a Rack-compatible approach that can work with Hanami’s architecture.

1. mTLS with Ruby OpenSSL (server side)

This example shows a simple TCP server using OpenSSL that requires client certificates and validates them before processing request data. It avoids unsafe buffers by using Ruby strings and enforcing size limits on incoming payloads.

require 'openssl'
require 'socket'

ctx = OpenSSL::SSL::SSLContext.new
ctx.key = OpenSSL::PKey::RSA.generate(2048)
cert = OpenSSL::X509::Certificate.new(File.read('server.crt'))
ctx.cert = cert
ctx.extra_chain_cert = [OpenSSL::X509::Certificate.new(File.read('ca.crt'))]
ctx.verify_mode = OpenSSL::SSL::VERIFY_PEER
ctx.verify_callback = lambda do |preverify_ok, store_ctx|
  preverify_ok && store_ctx.error == 0
end

server = TCPServer.new('0.0.0.0', 8443)
ssl_server = OpenSSL::SSL::SSLServer.new(server, ctx)

loop do
  begin
    client = ssl_server.accept
    # Enforce a maximum request size to prevent overflow
    client.setsockopt(Socket::SOL_SOCKET, Socket::SO_RCVBUF, 16384)
    request_data = client.read_nonblock(16384) or next
    # Validate and parse request safely in Ruby (no fixed C buffers)
    if request_data.start_with?('POST /api')
      # Example: safe JSON parsing with size guard
      if request_data.bytesize <= 8192
        # Process request in Hanami-like routing pattern
        puts "Processing mTLS-secured request of size #{request_data.bytesize}"
      else
        client.write("HTTP/1.1 413 Payload Too Large\r\n\r\n")
      end
    end
    client.close
  rescue OpenSSL::SSL::SSLError => e
    puts "SSL error: #{e.message}"
  end
end

2. Hanami-like request handling with size validation

In a Hanami app, you can add a middleware layer to enforce payload size limits before requests reach your entities or use cases. This prevents large or malicious payloads from being processed in unsafe native code paths.

# config/initializers/middleware/buffer_guard.rb
class BufferGuard
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
    # Limit request body to 8 KB to mitigate overflow risks
    if request.content_length > 8192
      return [413, { 'Content-Type' => 'application/json' }, ['{"error":"payload_too_large"}']]
    end
    @app.call(env)
  end
end

# In your Hanami application configuration (e.g., config/application.rb)
require 'rack/buffer_guard'
use BufferGuard

3. mTLS with a reverse proxy (recommended for production)

For most Hanami deployments, terminate mTLS at a reverse proxy like nginx and proxy only validated, size-limited requests to the app. This keeps native code paths clean and avoids in-app TLS complexity.

# nginx configuration snippet
server {
    listen 443 ssl;
    ssl_certificate /etc/ssl/certs/server.crt;
    ssl_certificate_key /etc/ssl/private/server.key;
    ssl_client_certificate /etc/ssl/certs/ca.crt;
    ssl_verify_client on;

    location /api/ {
        # Limit client body size to prevent overflow
        client_max_body_size 8k;
        proxy_pass http://localhost:2300;
        proxy_set_header X-SSL-Client-Cert $ssl_client_escaped_cert;
    }
}

These examples emphasize safe handling of client data and certificates, avoiding fixed buffers, and using size guards—key practices to reduce overflow risks when combining Hanami with Mutual TLS.