Buffer Overflow in Grape with Hmac Signatures

A buffer overflow in a Grape API that uses Hmac Signatures typically arises when input validation is limited to authentication structure and does not extend to the content of the request body or query parameters. An attacker can send a crafted payload that is accepted as a valid Hmac Signature but triggers excessive memory use while being processed by the server-side verification logic. Because Hmac verification relies on computing a hash over the raw request data, a very large or maliciously formed payload can cause the runtime to allocate large buffers or iterate over input in an unsafe way before the signature check fails.

In Grape, this can manifest when a developer parses the request body (e.g., JSON or multipart) and then computes an Hmac over the raw or partially parsed data using a shared secret. If the parsing logic does not enforce size limits or does not reject oversized parts before verification, an attacker can exploit the path where the signature appears valid but the payload is intended to exhaust resources. Even when the signature itself is verified safely, the surrounding code that reads streams or concatenates payloads can introduce unchecked buffers. The risk is higher when the Hmac verification is performed eagerly, before any length checks, because the server spends CPU and memory on data that should be rejected earlier.

Consider a scenario where a client sends a POST with an X-API-Signature header and a large JSON payload. If the Grape app uses a before filter that reads the entire request body to compute the Hmac and then parses JSON without a size cap, an oversized body can lead to memory pressure or a crash. The vulnerability is not in the Hmac algorithm itself, but in how the data feeding the verification is handled. Proper remediation requires validating input length and structure before computing the Hmac, and enforcing strict limits on payload size and part counts.

To mitigate buffer overflow risks while using Hmac Signatures in Grape, enforce input size limits before reading the request body, and avoid materializing large payloads in memory during signature verification. Use streaming or chunked processing where possible, and validate Content-Length and individual part sizes before performing cryptographic operations. Below are concrete, safe patterns for a Grape API that uses Hmac Signatures.

• Set a maximum request size in the middleware or route to reject oversized requests early:

class MaxSizeValidator
  def initialize(app, max_size = 10 * 1024 * 1024) # 10 MB
    @app = app
    @max_size = max_size
  end

  def call(env)
    content_length = env['CONTENT_LENGTH'].to_i
    if content_length > @max_size
      [413, { 'Content-Type' => 'application/json' }, [{ error: 'request_entity_too_large' }.to_json]]
    else
      @app.call(env)
    end
  end
end

use MaxSizeValidator

• Verify Hmac using a constant-time comparison and avoid processing large payloads unnecessarily. Parse JSON only after size checks and use strict parameter whitelisting:

require 'json'
require 'openssl'

class SecureEndpoint < Grape::API
  before { validate_hmac(request.env) }

  helpers do
    def validate_hmac(env)
      signature = env['HTTP_X_API_SIGNATURE']
      return error!('missing_signature', 401) unless signature

      raw_body = env['rack.input'].read
      # reject if body unexpectedly large after earlier guard
      if raw_body.bytesize > 10 * 1024 * 1024
        error!('request_entity_too_large', 413)
      end

      expected = OpenSSL::HMAC.hexdigest('sha256', ENV['SHARED_SECRET'], raw_body)
      unless secure_compare(signature, expected)
        error!('invalid_signature', 401)
      end

      # re-initialize input for downstream parsing
      env['rack.input'].rewind
    end

    def secure_compare(a, b)
      return false unless a.bytesize == b.bytesize
      l = a.unpack 'C*'
      r = b.unpack 'C*'
      res = 0
      l.each_with_index { |x, i| res |= x ^ r[i] }
      res == 0
    end
  end

  params do
    requires :user_id, type: Integer
    requires :action, type: String, values: %w[create update]
  end
  post '/resource' do
    # Safe to parse JSON here after Hmac and size checks
    payload = JSON.parse(request.body.read)
    # business logic
    { status: 'ok' }
  end
end

• For multipart uploads, limit part sizes and count, and validate before Hmac computation:

params do
  requires :file, type: File, desc: 'a file upload'
  requires :metadata, type: String, desc: 'json metadata'
end
post '/upload' do
  # Reject parts that exceed a per-part limit, e.g., 5 MB
  file = params[:file]
  metadata = params[:metadata]

  if file[:tempfile].size > 5 * 1024 * 1024
    error!('file_too_large', 413)
  end

  # Compute Hmac over a canonical representation if needed
  data_to_sign = { file_original_filename: file[:filename], metadata: metadata }.to_json
  signature = OpenSSL::HMAC.hexdigest('sha256', ENV['SHARED_SECRET'], data_to_sign)
  halt 401, { error: 'invalid_signature' }.to_json unless secure_compare(request.env['HTTP_X_API_SIGNATURE'], signature)

  { status: 'uploaded' }
end