Bleichenbacher Attack in Hanami with Hmac Signatures

A Bleichenbacher attack exploits adaptive cryptographic padding errors to recover a secret key or forge valid signatures. In Hanami, when Hmac Signatures are used to protect API routes or webhook endpoints without strict verification, an attacker can send many carefully crafted requests and observe subtle differences in response behavior to iteratively recover the signing key or forge valid MACs.

Hanami commonly uses Hmac Signatures to ensure request integrity. A typical flow includes a timestamp and a signature header, e.g., X-API-Signature, derived with an algorithm such as HMAC-SHA256 over a canonical string of method, path, timestamp, and body. If the application reveals whether a signature is valid before checking the timestamp window or returns distinct errors for padding failures versus malformed input, it creates an oracle that a Bleichenbacher-style attack can leverage.

Consider a Hanami endpoint that verifies signatures like this:

module Web::Auth::SignatureVerification
  def self.verify(request_secret, received_signature, data_to_sign)
    expected = OpenSSL::HMAC.hexdigest('sha256', request_secret, data_to_sign)
    # Timing-sensitive comparison if done naively
    ActiveSupport::SecurityUtils.secure_compare(expected, received_signature)
  end
end

If secure_compare is not used and a simple string equality check is performed, an attacker can send modified signatures and measure response times to infer byte-by-byte correctness, effectively performing a Bleichenbacher attack. Even when using secure_compare, if the server discloses which part of the verification failed (e.g., timestamp out of window vs invalid signature), the attacker can still distinguish error paths and mount the oracle.

In a real scenario, an attacker might target a webhook URL protected only by Hmac Signatures where the timestamp check is lenient or the error messages differ. By observing whether a request is accepted, rejected, or rate-limited, the attacker iteratively adjusts the signature bytes. Over many requests, they can recover enough of the Hmac key material to forge valid requests, bypassing authentication entirely.

To map this to the 12 security checks run by middleBrick, such a flaw would likely surface in Authentication (weak oracle behavior), Input Validation (improper handling of malformed signatures), and possibly Data Exposure (if error messages leak verification details). middleBrick scans can detect inconsistent timing behavior and weak error handling patterns that facilitate adaptive padding oracle attacks.

Remediation focuses on ensuring signature verification is constant-time, fails closed on any error, and does not leak distinguishing information. Always use a constant-time comparison and validate all components (timestamp, nonce, body) before returning a result.

Below is a safe Hanami-style implementation using Hmac Signatures with OpenSSL and ActiveSupport’s secure utilities:

module Web::Auth::HmacSignature
  ALGORITHM = 'sha256'
  TIMESTAMP_TOLERANCE = 300 # 5 minutes in seconds

  def self.verify(request_secret, received_signature, http_method, path, timestamp, body)
    # 1) Normalize and build the string to sign exactly as the sender did
    data_to_sign = [http_method.upcase, path, timestamp, body].join("\n")

    # 2) Reject if timestamp is outside allowed window
    request_time = Time.at(timestamp.to_i)
    now = Time.now.utc
    if (now - request_time).abs > TIMESTAMP_TOLERANCE
      raise Errors::InvalidSignature, 'timestamp expired'
    end

    # 3) Compute expected signature using the shared secret
    expected = OpenSSL::HMAC.hexdigest(ALGORITHM, request_secret, data_to_sign)

    # 4) Constant-time comparison to prevent Bleichenbacher-style oracle
    unless ActiveSupport::SecurityUtils.secure_compare(expected, received_signature.to_s)
      raise Errors::InvalidSignature, 'invalid signature'
    end

    # If we reach here, signature and timestamp are valid
    true
  rescue => e
    # Log for monitoring but return a generic failure to the caller
    Hanami.logger.warn("Signature verification failed: #{e.message}")
    false
  end
end

Usage in a Hanami controller:

class Web::Controllers::Webhooks::Receive < Hanami::Action
  def call(params)
    request_secret = ENV.fetch('HMAC_SHARED_SECRET')
    received_signature = request.headers['X-API-Signature']
    http_method = request.request_method
    path = request.path
    timestamp = request.headers['X-API-Timestamp']
    body = request.body.read

    if Web::Auth::HmacSignature.verify(request_secret, received_signature, http_method, path, timestamp, body)
      # process the webhook
    else
      halt 401, { error: 'unauthorized' }.to_json
    end
  end
end

Additional hardening recommendations:

• Enforce a strict timestamp window and reject requests with timestamps too far in the past or future.
• Use a per-request nonce or include a digest of the body to prevent replay attacks.
• Ensure errors from the verification routine are uniform and do not indicate whether the timestamp, signature, or other fields were incorrect.
• Rotate the Hmac shared secret periodically and store it securely (e.g., environment variables or a secrets manager).

These steps mitigate Bleichenbacher-style adaptive oracle attacks by removing timing leaks and distinguishing error paths, making it infeasible to recover the Hmac key through repeated requests.