Buffer Overflow in Hanami with Bearer Tokens

A buffer overflow in a Hanami application that processes Bearer tokens can occur when token handling does not properly bound read or copy operations, for example when parsing headers or storing token material in fixed-size buffers. If the application reads the Authorization header into a fixed-length character array and does not validate token length before copying, an oversized token can overwrite adjacent stack memory. This can corrupt saved registers, overwrite a return address, or redirect execution flow. In a Hanami (Ruby) context, this typically surfaces through C extensions or native system calls that manipulate buffers directly, rather than through pure Ruby string operations which are generally bounds-checked.

Bearer tokens amplify risk when they are accepted from untrusted sources without length checks and then forwarded to lower-level components that lack safe abstractions. For instance, if a Hanami app accepts a token via request.env['HTTP_AUTHORIZATION'] and passes it to a native extension or a logging routine that uses fixed buffers, an attacker can supply a token crafted to exceed expected sizes. This can lead to arbitrary code execution, information disclosure, or denial of service. The vulnerability is not in Bearer tokens per se, but in how token data is handled when it moves into unsafe contexts.

An attacker may send a long, specially crafted Authorization header such as Authorization: Bearer AAAA... (many bytes) to probe for buffer overflow behavior. Because Hanami applications often integrate multiple layers (Rack, middleware, C extensions), an overflow in one layer can expose sensitive stack contents or subvert control flow. This maps to common weaknesses enumerated in the OWASP API Top 10 and can intersect with insecure deserialization or improper input validation checks. Detecting such issues requires testing the unauthenticated attack surface, for example by submitting oversized tokens and observing for crashes or unexpected behavior, which is what middleBrick’s unauthenticated scan performs across its 12 security checks.

Remediation focuses on validating and bounding token input before it reaches any buffer-using code. In Hanami, prefer Ruby-level string handling and avoid passing raw token data directly to C extensions or native system calls unless they explicitly enforce length limits. Use middleware to enforce token size policies and normalize Authorization header parsing.

# config/initializers/authorization_header.rb
# Reject tokens that exceed a safe length before they enter the app stack
module SafeBearerToken
  MAX_TOKEN_BYTES = 4096

  class TokenValidator
    def initialize(app)
      @app = app
    end

    def call(env)
      auth = env['HTTP_AUTHORIZATION']
      if auth&& auth.start_with?('Bearer ') && auth.bytesize > MAX_TOKEN_BYTES
        return [400, { 'Content-Type' => 'application/json' }, ['{"error":"token_too_large"}'].to_json]
      end
      @app.call(env)
    end
  end
end

# config/application.rb
config.middleware.use SafeBearerToken::TokenValidator

When integrating with external libraries or native extensions, explicitly check token length and avoid unsafe string operations. If you must pass tokens to a C extension, use String#freeze and explicit size checks, and prefer interfaces that accept length-validated pointers rather than relying on null-terminated buffers.

# Example of safe forwarding to an extension (pseudo-Ruby C extension interface)
raw_token = env['HTTP_AUTHORIZATION']&.sub('Bearer ', '')
if raw_token && raw_token.bytesize <= 4096
  # Native method that expects a pointer and length, not a null-terminated buffer
  NativeLib.process_token(raw_token, raw_token.bytesize)
else
  raise Errors::InvalidToken.new('Token size out of allowed bounds')
end

Additionally, ensure that any logs or error messages do not inadvertently echo the full token. Use middleBrick’s scan capabilities to verify that no unchecked inputs reach native code paths. The Pro plan’s continuous monitoring and GitHub Action integration can enforce token size policies in CI/CD, failing builds if risky patterns are detected.