Buffer Overflow in Sinatra with Hmac Signatures

A buffer overflow in a Sinatra application that uses Hmac Signatures can occur when input validation is limited to signature verification alone and the application reads untrusted data into fixed-size buffers. In this context, the Hmac Signature is typically computed over selected headers or a request body to ensure integrity, but if the application uses a vulnerable Ruby C extension or native method to process the payload, an attacker can supply crafted data that overflows a stack or heap buffer before the signature verification step completes or while the application parses the request.

For example, consider a Sinatra endpoint that expects a JSON body and an X-API-Signature header containing an Hmac using SHA256. If the code deserializes the JSON with a library that internally uses native string operations without proper bounds checking, and the attacker sends an extremely long string in a field mapped to a fixed-size buffer, the overflow can corrupt memory. Even though the Hmac Signature validates that the request originates from a trusted source, the overflow may still be triggered during parsing or routing before the developer’s authorization logic runs. This means the integrity guarantee of the Hmac does not prevent memory corruption; it only ensures the request was not tampered with after generation.

In the context of middleBrick’s checks, this scenario maps to the BFLA/Privilege Escalation and Input Validation categories. The scanner tests unauthenticated endpoints and can detect indicators such as missing length checks on user-controlled fields, use of vulnerable C-based extensions, or patterns that historically correlate with buffer overflow conditions (e.g., sprintf, strcat) in Ruby C extensions. Because the attack surface is tested black-box, middleBrick can surface these risks without requiring credentials, while the Hmac Signature implementation remains intact as a control for integrity but not for memory safety.

Real-world references include patterns similar to CVE-2017-15468 (a vulnerability in Ruby 2.x where certain string operations could lead to memory corruption) and the broader OWASP API Top 10 category for Invalid Input Validation. Even when Hmac Signatures protect against tampering, they do not mitigate a buffer overflow introduced by unsafe processing of untrusted content, highlighting the need to combine cryptographic integrity checks with safe runtime practices and thorough input validation.

To remediate buffer overflow risks while preserving Hmac Signature integrity checks in Sinatra, focus on safe parsing, strict length validation, and avoiding unsafe native operations. Below are concrete, working examples that demonstrate a secure approach.

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

# Safely verify Hmac Signature for incoming requests
helpers do
  def verify_hmac_signature(request_body, signature_header, secret)
    expected = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), secret, request_body)
    # Use secure compare to avoid timing attacks
    ActiveSupport::SecurityUtils.secure_compare(expected, signature_header.to_s.strip)
  rescue
    false
  end
end

# Configure secret via environment (never hardcode in production)
SECRET = ENV['HMAC_SECRET']

before do
  content_type :json
  request_body = request.body.read
  request.body.rewind

  signature = request.env['HTTP_X_API_SIGNATURE']
  unless verify_hmac_signature(request_body, signature, SECRET)
    halt 401, { error: 'Invalid signature' }.to_json
  end

  # Safe parsing with explicit size limits
  parsed = JSON.parse(request_body, max_nesting: 5, create_additions: false)
  # Validate field lengths to prevent overflow in downstream processing
  if parsed['data'] && parsed['data']['content']&.bytesize > 1024
    halt 400, { error: 'Payload too large' }.to_json
  end

  # Store sanitized payload for routes
  @safe_payload = parsed
end

post '/submit' do
  # Use the pre-validated payload; no further parsing of untrusted strings with native methods
  content = @safe_payload.dig('data', 'content') || ''
  # Ensure content is treated as UTF-8 and avoid unsafe concatenation with native C strings
  sanitized = content.encode('UTF-8', invalid: :replace, undef: :replace, replace: '')
  { status: 'ok', received: sanitized.bytesize }.to_json
end

Key points in this example:

• The Hmac Signature is verified using OpenSSL with a secure compare to prevent timing attacks.
• JSON parsing includes options to limit nesting and disable unsafe object additions.
• Explicit length checks on user-controlled fields prevent excessively large inputs that could overflow buffers in native extensions.
• Encoding conversions are performed with safe options to avoid introducing malformed sequences that native code might mishandle.

If your Sinatra app relies on third-party gems, audit them for known buffer overflow issues and prefer pure-Ruby implementations where possible. middleBrick’s scans can validate that your Hmac implementation is present and that input validation rules are applied consistently across endpoints.