Bleichenbacher Attack in Hanami (Ruby)

The Bleichenbacher attack exploits flaws in RSA decryption padding used in TLS, particularly when implementations leak timing or error responses that distinguish between valid ciphertext and improperly decrypted data. In Hanami applications written in Ruby, this vulnerability surfaces when the framework handles TLS termination incorrectly or when developers expose internal decryption errors through HTTP responses without masking timing differences.

Hanami itself does not implement TLS, but Ruby applications often run behind reverse proxies or within server environments that may terminate TLS. If a Hanami application improperly handles TLS handshake failures or decrypts encrypted payloads using unsafe practices, it may inadvertently reveal distinguishable error patterns that can be leveraged by attackers to gradually recover plaintext.

For example, consider a Hanami endpoint that receives encrypted data via an API gateway using TLS 1.2, which supports RSA-PKCS#1 v1.5 encryption. If the gateway or server layer does not uniformly handle decryption errors — such as returning '400 Bad Request' for malformed padding versus timing out for valid ciphertext — this creates a timing channel. An attacker can perform a chosen-ciphertext attack by iterating over ciphertexts and observing whether decryption succeeds or fails, using statistical analysis to isolate valid plaintext blocks.

In practice, this could occur when a Ruby server using WEBrick or Puma terminates TLS and forwards decrypted data to a Hanami application. If the server logs detailed decryption errors and the application reflects them in responses, or if decryption time varies based on padding validity, the system becomes susceptible to Bleichenbacher-style attacks. This is especially relevant in API gateways or microservices that decrypt inbound traffic before passing it to Hanami routes.

While Hanami applications typically rely on external servers for TLS termination, developers must ensure that any layer handling RSA-based encryption does not expose distinguishable failure modes. Even if the Hanami code itself does not perform encryption, its integration with downstream systems or authentication providers that use RSA signatures or encrypted tokens can indirectly participate in attack surfaces if not hardened.

This scenario underscores the importance of secure TLS configuration across the entire stack, not just within the application framework. The vulnerability does not reside in Hanami’s Ruby code directly, but in how decryption is managed in the broader system context where Hanami operates.

To remediate Bleichenbacher-style vulnerabilities in a Hanami application, developers must ensure that decryption operations do not leak timing or error-based information, particularly when handling RSA-based encrypted inputs. This requires using constant-time operations and avoiding direct propagation of low-level cryptographic failures to end users.

In Ruby, this can be achieved by enforcing uniform error handling at the TLS termination layer or by using secure libraries that avoid exposing padding-related distinctions. For instance, when using OpenSSL directly in a Hanami app (e.g., for decrypting encrypted tokens), developers should not return different HTTP status codes based on cryptographic validation outcomes.

# Unsafe: Reveals decryption failure via HTTP status codes
def decrypt_token(ciphertext)
  plaintext = OpenSSL::Cipher::AES.new(256, :CBC).decrypt
  begin
    plaintext.update(ciphertext) + plaintext.final
  rescue OpenSSL::Cipher::CipherError
    render json: { error: 'Invalid padding' }, status: :bad_request
    return
  end
  plaintext
end

Instead, use a constant-time validation approach and always return generic failure responses:

# Safe: No timing or error leakage
def decrypt_token(ciphertext)
  begin
    # Simulate processing time regardless of outcome
    OpenSSL::Cipher::AES.new(256, :CBC).new
                .decrypt
                .update(ciphertext) + OpenSSL::Cipher::CBC.new.final
    rescue
    # Always return same response
  end
end

Additionally, ensure that TLS termination uses modern configurations that disable RSA-PKCS#1 v1.5 cipher suites or upgrade to TLS 1.3, which eliminates this attack vector entirely. In Hanami, this means configuring servers like Puma or nginx to enforce secure cipher suites and not expose decryption internals to application code.

Another mitigation is to avoid decrypting sensitive data in user-facing endpoints altogether. Instead, use signed tokens with asymmetric encryption (e.g., RSA-PSS or ECDSA) where padding oracles are not exploitable, or switch to authenticated encryption with associated data (AEAD) like AES-GCM, which does not suffer from padding vulnerabilities.

Finally, implement strict input validation and ensure that all encrypted payloads are processed through vetted cryptographic libraries that provide constant-time decryption. This reduces the attack surface and ensures that even if an attacker probes the API, they cannot distinguish between valid and invalid ciphertexts based on response behavior.