Xml External Entities in Grape with Hmac Signatures

XML External Entity (XXE) injection occurs when an XML parser processes external entity references within untrusted XML data. Grape is a REST-like API micro-framework for Ruby. When Grape endpoints accept XML payloads and parse them with libraries that are not hardened against XXE, an attacker can supply malicious XML that references internal resources, leading to data exposure or server-side request forgery. Adding Hmac Signatures for request integrity and authentication can inadvertently keep such endpoints permissive if signature verification is performed before or alongside unsafe XML parsing, giving the appearance of protection while the underlying parser remains exploitable.

Consider a Grape API that expects an XML body and validates an Hmac Signature to verify the request origin. If the server computes the Hmac over the raw request body and then parses the XML without disabling external entity processing, an attacker can bypass trust in the signature by sending a benign Hmac-signed payload that contains an external entity declaration. The signature will verify successfully because the Hmac covers what the client sent, but the parser may read local files or trigger SSRF via defined entities. This combination creates a scenario where authentication and integrity checks appear intact while the application processes untrusted XML in an unsafe manner.

For example, an attacker could provide an XML body with a DOCTYPE that declares an external file entity, and the parser expands it during processing. Even when Hmac Signatures confirm the request has not been tampered with in transit, the server-side behavior remains unsafe because the signature does not prevent malicious XML structure. Common vulnerable gems include older versions of Nokogiri when configured to load external DTDs or when default entity expansion is not explicitly disabled. Without explicit parser hardening, endpoints that consume XML and also use Hmac Signatures for authentication may still leak internal file contents or cause the server to make unintended network calls.

Using the OpenAPI/Swagger spec analysis capability of middleBrick, such unsafe patterns can be identified by correlating parameter definitions and request body schemas with the parsing implementation. middleBrick scans the unauthenticated attack surface and maps findings to frameworks like OWASP API Top 10, highlighting risks where trust in Hmac Signatures does not equate to safe data handling. The scanner does not fix the code but provides prioritized findings with remediation guidance, helping teams understand that signature validation must be paired with secure XML parsing practices.

To remediate XXE when Hmac Signatures are used in Grape, ensure XML parsing is hardened regardless of signature verification. Always validate the Hmac on the request body before routing, but treat all XML input as untrusted and disable external entities and DTDs in the parser. Below are concrete, working examples showing how to compute and verify Hmac Signatures in Grape while safely parsing XML.

Computing Hmac on the client

Generate an Hmac from the raw request body using a shared secret and include it in a header. This example uses Ruby's OpenSSL library to create a hex-encoded Hmac that the server can verify.

require 'openssl'
require 'base64'

shared_secret = 'your-secure-secret'
body = request_body.to_s
signature = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), shared_secret, body)
# Send 'signature' in a header, e.g., X-API-Signature

Verifying Hmac and safely parsing XML in Grape

In your Grape API, verify the Hmac before processing the body, then parse XML with external entities disabled. Use Nokogiri with options that prevent external loading and avoid expanding general entities.

require 'grape'
require 'nokogiri'
require 'openssl'

class SecureAPI < Grape::API
  helpers do
    def verified_body
      request.body.rewind
      raw = request.body.read
      provided = env['HTTP_X_API_SIGNATURE']
      secret = ENV['HMAC_SECRET']
      expected = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), secret, raw)
      halt 401, { error: 'invalid signature' }.to_json unless Rack::Utils.secure_compare(expected, provided)
      raw
    end
  end

  post '/submit' do
    raw = verified_body
    # Parse XML safely: do not load external DTDs or entities
    doc = Nokogiri::XML(raw, nil, &:noblanks)
    # Disable external entity processing explicitly
    doc.root&.remove_namespaces if doc.root
    # Further processing of sanitized XML...
    { status: 'ok' }
  end
end

Parser options to prevent XXE

When using Nokogiri, avoid default settings that may resolve external references. Prefer the options used above and, if using a SAX parser, implement a custom resolver that rejects external entities. For other XML libraries in the Ruby ecosystem, apply equivalent settings to block DOCTYPE and external entity resolution.

Complementary measures

• Keep dependencies updated; review Nokogiri and related gems for known XXE fixes.
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Use middleBrick’s CLI to scan your endpoints from the terminal with middlebrick scan <url> to detect risky XML handling patterns and see findings mapped to compliance frameworks like OWASP API Top 10.

• For continuous protection in development pipelines, consider the middleBrick GitHub Action to add API security checks to your CI/CD pipeline and fail builds if risk scores drop below your chosen threshold.

These steps ensure that Hmac Signatures provide integrity and authentication without leaving XML parsing vulnerable to injection.