Beast Attack in Hanami with Mutual Tls

The BEAST (Browser Exploit Against SSL/TLS) attack targets predictable initialization vectors (IVs) used in block ciphers such as TLS 1.0 and TLS 1.1 with CBC suites. In Hanami, when Mutual TLS is enabled but the application or the underlying server configuration still negotiates TLS 1.0 or TLS 1.1 with CBC cipher suites, the client ciphertext becomes partially predictable across requests. This creates a scenario where an attacker on the network can iteratively guess plaintext blocks by observing how each request changes the IV, especially when the attacker can inject or influence part of the request and observe the size or timing of responses.

Mutual TLS changes the trust boundary but does not change the cipher behavior. If Hanami terminates TLS with a server or reverse proxy that prefers CBC suites and TLS 1.0/1.1, the IV predictability problem persists even when client certificates are required. The attacker can perform a chosen-plaintext scenario by submitting known content and observing resulting ciphertexts or error patterns, gradually revealing session tokens or authentication cookies carried in headers or cookies. Because Mutual TLS binds the client identity to the TLS layer, an attacker in possession of a valid client certificate can still leverage BEAST to decrypt or forge application-layer data if the protocol does not protect IVs properly.

Hanami applications that rely on outdated web servers or Ruby TLS stacks may inadvertently offer CBC-based ciphers. For example, a server configured with SSLCipherSuite HIGH:!aNULL:!MD5:!3DES might still include TLS_RSA_WITH_AES_256_CBC_SHA. When this cipher is used, the BEAST attack becomes feasible regardless of the strong authentication provided by Mutual TLS. The presence of client certificates does not mitigate IV predictability; it only confirms the identity of the attacker-controlled client in scenarios where the server accepts maliciously crafted ciphertexts.

To determine whether your Hanami deployment is at risk, you can use middleBrick to scan the endpoint and review protocol and cipher suite findings. middleBrick runs 12 security checks in parallel, including Encryption and Input Validation, and maps findings to real CVEs and frameworks such as OWASP API Top 10. Its unauthenticated, black-box scanning completes in 5–15 seconds, providing a protocol and cipher suite breakdown without requiring credentials or agents.

Remediation focuses on disabling weak protocols and CBC cipher suites, and enforcing modern AEAD ciphers. In Hanami, you typically configure TLS at the web server or reverse proxy (e.g., Puma, NGINX, or a load balancer), and ensure the Ruby process does not negotiate TLS 1.0 or TLS 1.1. Below are concrete, realistic configuration snippets for common stacks.

Ruby SSL Context with Modern Ciphers

When terminating TLS directly in Ruby using OpenSSL::SSL::SSLContext, explicitly set protocols and ciphers to avoid CBC and legacy protocols:

require 'openssl'

ctx = OpenSSL::SSL::SSLContext.new
ctx.min_version = :TLS1_2
ctx.ciphers = [ 'ECDHE-ECDSA-AES256-GCM-SHA384', 'ECDHE-RSA-AES256-GCM-SHA384',
                'ECDHE-ECDSA-CHACHA20-POLY1305', 'ECDHE-RSA-CHACHA20-POLY1305' ]
ctx.verify_mode = OpenSSL::SSL::VERIFY_PEER
ctx.extra_chain_cert = your_intermediate_cert
ctx.cert = your_server_cert
ctx.key = your_private_key

server = TCPServer.new('0.0.0.0', 443)
ssl_server = OpenSSL::SSL::SSLServer.new(server, ctx)

This configuration disables TLS 1.0 and TLS 1.1 and restricts ciphers to GCM and ChaCha20-Poly1305 AEAD suites, which are not vulnerable to BEAST. Mutual TLS is preserved by setting verify_mode to VERIFY_PEER and providing CA certificates for client validation.

NGINX Reverse Proxy with Mutual Tls

If you front Hanami with NGINX, enforce modern protocols and ciphers in the server block and require client certificates:

server {
    listen 443 ssl;
    server_name api.example.com;

    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305';
    ssl_prefer_server_ciphers on;

    ssl_certificate /etc/ssl/certs/server.crt;
    ssl_certificate_key /etc/ssl/private/server.key;

    # Mutual TLS
    ssl_client_certificate /etc/ssl/certs/ca.pem;
    verify_client on;

    location / {
        proxy_pass http://localhost:23000;
        proxy_set_header X-SSL-Client-Cert $ssl_client_escaped_cert;
        proxy_set_header X-SSL-Client-Issuer $ssl_client_issuer;
    }
}

With these settings, NGINX will reject TLS 1.0/1.1 and CBC suites, while requiring valid client certificates. The Hanami app then receives the client certificate details via headers for additional authorization checks if needed.

For deployment workflows, the middleBrick CLI can validate your configurations by scanning the live endpoint. Use middlebrick scan <url> to obtain an encrypted-grade protocol analysis and cipher suite report. If you integrate middleBrick into CI/CD with the GitHub Action, you can fail builds automatically when weak protocols or vulnerable ciphers are detected. The Pro plan adds continuous monitoring so future configuration changes are caught before deployment.