Bleichenbacher Attack in Sinatra with Basic Auth

A Bleichenbacher attack is a chosen-ciphertext attack against asymmetric encryption (most commonly RSA PKCS#1 v1.5 padding). While the attack targets encryption implementations, the risk surface in Sinatra applications using HTTP Basic Auth is expanded because authentication and session tokens can be exposed if encrypted data is mishandled. In this combination, a Sinatra app that accepts encrypted payloads (e.g., session tokens or API credentials) and relies on Basic Auth for identity can inadvertently expose behavior that allows an attacker to iteratively decrypt data by observing server responses to crafted ciphertexts.

Sinatra, being lightweight and flexible, does not enforce encryption for request handling by default. If endpoints that accept encrypted Basic Auth credentials (e.g., credentials encrypted client-side before being transmitted over HTTPS) do not use constant-time comparison and robust padding validation, they can become susceptible to timing-based Bleichenbacher-style oracle attacks. The attacker sends modified ciphertexts and measures response differentiations — such as 401 vs 400, or timing differences — to infer the plaintext. In this scenario, Basic Auth headers or cookies that carry encrypted values become the attack channel.

For example, an endpoint that decrypts an encrypted Basic Auth token and then performs a non-constant-time check can leak validity through response codes or latency. If the app decodes a Base64‑encoded Basic Auth credential and then decrypts it using RSA without proper padding verification, each request provides feedback that can be used to reconstruct the plaintext credential byte by byte. This is especially relevant when Sinatra applications use custom middleware to handle encrypted credentials rather than standard, well‑vetted authentication libraries.

Consider a Sinatra route that expects an encrypted Basic Auth header:

require 'sinatra'
require 'base64'
require 'openssl'

configure do
  set :bind, '0.0.0.0'
end

# Example vulnerable decryption endpoint
post '/login' do
  auth_header = request.env['HTTP_AUTHORIZATION'] || ''
  if auth_header.start_with?('Basic ')
    encoded = auth_header.split(' ').last
    encrypted_data = Base64.strict_decode64(encoded)
    private_key = OpenSSL::PKey::RSA.new(File.read('private_key.pem'))
    decrypted = private_key.private_decrypt(encrypted_data, OpenSSL::PKey::RSA::NO_PKCS1_PADDING)
    credentials = decrypted.split(':')
    if credentials[0] == 'admin' && credentials[1] == 'secret'
      status 200
      body 'Authenticated'
    else
      status 401
      body 'Invalid credentials'
    end
  else
    status 400
    body 'Missing Authorization header'
  end
end

In this example, private_key.private_decrypt with NO_PKCS1_PADDING is nonstandard and can introduce padding oracle behavior depending on OpenSSL’s internal handling. An attacker who can send encrypted payloads and observe whether the response is 401 (invalid padding or decryption failure) versus 200 (success) can mount a Bleichenbacher attack to recover the plaintext credentials. Even if HTTPS is used, the decryption oracle on the server creates a covert channel. The presence of Basic Auth amplifies the impact because successful decryption directly exposes username and password.

To detect this during a scan, middleBrick’s Encryption and Input Validation checks would flag missing constant-time comparisons and improper padding usage. The LLM/AI Security checks would further highlight risks if decrypted credentials are processed in ways that could leak via error messages or logs.

Remediation focuses on avoiding custom decryption of credentials in application code and relying on HTTPS with standard authentication mechanisms. If encrypted credentials must be used, ensure decryption uses constant-time operations and does not expose padding errors to the caller.

First, prefer standard HTTP authentication over HTTPS without custom encryption. Let the transport layer handle confidentiality and integrity:

require 'sinatra'
require 'base64'

configure do
  set :bind, '0.0.0.0'
end

# Secure approach: validate credentials over HTTPS without decrypting custom payloads
USERS = { 'admin' => '$2a$12$abc123...' } # store bcrypt hashes

post '/login' do
  auth_header = request.env['HTTP_AUTHORIZATION'] || ''
  if auth_header.start_with?('Basic ')
    encoded = auth_header.split(' ').last
    decoded = Base64.strict_decode64(encoded)
    username, password = decoded.split(':', 2)
    if username && USERS.key?(username) && BCrypt::Password.new(USERS[username]) == password
      status 200
      body 'Authenticated'
    else
      status 401
      body 'Invalid credentials'
    end
  else
    status 400
    body 'Missing Authorization header'
  end
end

This approach avoids custom encryption/decryption and leverages BCrypt for password hashing. The server does not perform RSA decryption, eliminating the padding oracle vector.

If you must work with encrypted Basic Auth values, use standard encryption schemes with constant-time comparison and avoid returning distinct error messages for padding versus authentication failures:

require 'sinatra'
require 'base64'
require 'openssl'
require 'active_support' # for secure_compare

configure do
  set :bind, '0.0.0.0'
end

# Example with constant-time comparison and generic failure response
post '/login' do
  auth_header = request.env['HTTP_AUTHORIZATION'] || ''
  if auth_header.start_with?('Basic ')
    encoded = auth_header.split(' ').last
    encrypted_data = Base64.strict_decode64(encoded)
    private_key = OpenSSL::PKey::RSA.new(File.read('private_key.pem'))
    # Use PKCS1 padding for standard decryption
    begin
      decrypted = private_key.private_decrypt(encrypted_data, OpenSSL::PKey::RSA::PKCS1_PADDING)
    rescue OpenSSL::PKey::RSAError
      status 401
      body 'Invalid credentials'
      halt
    end
    credentials = decrypted.split(':', 2)
    if credentials.length == 2 && ActiveSupport::SecurityUtils.secure_compare(credentials[0], 'admin') &&
       credentials[1] == 'secret' # In practice, compare password hashes
      status 200
      body 'Authenticated'
    else
      status 401
      body 'Invalid credentials'
    end
  else
    status 400
    body 'Missing Authorization header'
  end
end

Key remediation points:

• Use standard padding (PKCS1) instead of custom or no padding.
• Employ constant-time comparison for sensitive values.
• Return the same generic error message for decryption and authentication failures to eliminate oracle behavior.
• Avoid custom encryption for credentials; rely on HTTPS and standard auth mechanisms.

middleBrick’s Authentication and Encryption checks can verify that your Sinatra app does not expose padding-related timing differences and that error handling does not leak details. The GitHub Action can enforce these rules in CI/CD, while the CLI allows on-demand scanning from the terminal.