Insecure Deserialization in Grape with Hmac Signatures

Insecure deserialization in a Grape API that uses Hmac Signatures can occur when the server deserializes user-controlled data after validating an Hmac, but before or after signature verification is applied inconsistently. Grape, a REST-like API micro-framework for Ruby, often uses middleware to parse request bodies (e.g., JSON, XML, or MessagePack). If the application uses signed payloads via Hmac to ensure integrity, a mismatch in when and how the signature is checked relative to deserialization can expose the endpoint to tampering or injection of malicious objects.

Consider a scenario where the client sends an Hmac in a header (e.g., X-API-Signature) and the body contains serialized data. If the server computes the Hmac over the raw request body, validates the signature, and then deserializes the body using a dangerous method such as Marshal.load (for Ruby objects) or an XML parser with entity expansion enabled, an attacker can supply a serialized payload that executes code during deserialization. Even though the Hmac is verified, the signature may have been computed over a benign payload that the attacker can mutate if the server’s logic recomputes the signature after partial parsing or uses a per-request nonce or timestamp that is not covered by the Hmac.

For example, an XML External Entity (XXE) attack can occur if the deserialization library processes external entities. A crafted XML payload with external references can read server files or cause SSRF when combined with Hmac misuse. Similarly, Ruby’s Marshal format can instantiate arbitrary classes during deserialization, leading to Remote Code Execution (CVE-2013-0156 class). If the Hmac is computed only on a subset of the request (such as selected headers or a transformed body), an attacker might bypass integrity checks by altering non-covered parts that still affect deserialization behavior.

Insecure deserialization in this context is not about breaking the Hmac algorithm itself, but about the order and scope of integrity checks. The vulnerability surfaces when deserialization is performed on data that should remain immutable after signing, or when the signature does not cover all inputs that influence parsing. This can lead to privilege escalation, data tampering, or injection, depending on the types and filters used by the deserializer.

To detect this pattern, scans examine whether the API accepts serialized formats (e.g., XML, Marshal, or custom binary formats) and whether Hmac validation is applied consistently to the exact data that is deserialized. Findings will highlight the use of unsafe deserialization methods and incomplete signature coverage, providing remediation guidance to align verification with the full request payload.

Remediation focuses on ensuring the Hmac covers the exact data that is used during deserialization and avoiding dangerous deserialization methods. In Grape, you can implement a before filter that reads the raw request body, computes the Hmac using a strong algorithm like SHA-256, and compares it with the provided signature before any parsing occurs. This guarantees that the body cannot be altered without detection, and deserialization only happens after integrity is confirmed.

Use standard libraries for Hmac and prefer JSON over custom serialization formats. If you must accept XML or other formats, disable external entity processing and avoid Marshal for untrusted data. Below is a concrete example of Hmac verification in Grape before deserializing JSON safely.

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

class MyEndpoint < Grape::API
  before do
    provided = env['HTTP_X_API_SIGNATURE']
    raw_body = request.body.read
    # Compute Hmac using SHA256 with a server-side secret
    secret = ENV['API_HMAC_SECRET']
    computed = OpenSSL::HMAC.hexdigest('sha256', secret, raw_body)
    halt 401, { error: 'invalid signature' }.to_json unless secure_compare(provided, computed)
    # Rewind body so downstream parsing works
    request.body.rewind
  end

  helpers do
    def secure_compare(a, b)
      return false if a.nil? || b.nil?
      return false unless a.bytesize == b.bytesize
      l = a.unpack 'H*' * 2
      (l[0] ^ l[1]).hex == 0
    end
  end

  post '/data' do
    # Safe deserialization after Hmac validation
    params = JSON.parse(request.body.read)
    # Process params safely; avoid Marshal.load or YAML.safe_load with custom classes
    { received: params.slice('id', 'action') }
  end
end

If you use XML, ensure your parser is configured to prevent external entity expansion. For example, with Nokogiri, disable DTD and entity loading:

require 'nokogiri'

# Unsafe: avoid with untrusted sources
# doc = Nokogiri::XML(request.body.read)

# Safe configuration
parser = Nokogiri::XML::SAX::ParserStack.new
parser.options = Nokogiri::XML::ParseOptions::NOENT | Nokogiri::XML::ParseOptions::NONET
# Alternatively, use a DOM parser with entity expansion disabled
xml_content = request.body.read
doc = Nokogiri::XML(xml_content) do |config|
  config.options = Nokogiri::XML::ParseOptions::NONET | Nokogiri::XML::ParseOptions::NOENT
end

For object serialization, prefer JSON and schema validation (e.g., using dry-validation or strong parameters) instead of Marshal. If you must handle signed binary formats, ensure the Hmac is computed over the exact byte sequence that will be deserialized and that the deserialization library is hardened against injection. The middleBrick CLI can scan your Grape endpoints to verify that Hmac coverage aligns with deserialization boundaries and to flag unsafe parsing patterns.

Using the middleBrick dashboard, you can track these findings over time and, with the Pro plan, enable continuous monitoring so that future changes to signing or parsing logic are automatically assessed. The GitHub Action can fail builds if the risk score exceeds your defined threshold, preventing insecure deserialization patterns from reaching production.