Xml External Entities in Grape with Mongodb

An XML External Entity (XXE) injection in a Grape API can expose or manipulate data stored in Mongodb when the application parses untrusted XML input and uses that data to build database operations. XXE occurs when an attacker provides an XML payload that defines external entities, causing the parser to read local files, trigger SSRF to internal services, or consume excessive resources. In a Grape endpoint that receives XML, if the parser is configured to resolve external DTDs or entities, an attacker can leverage entities like &file; or &http; to read server-side files or make internal network requests. If the parsed data is later used to construct Mongodb queries—such as dynamic query filters, projection fields, or update operations—an attacker may influence what data is read or modified in the database.

For example, an endpoint that expects a JSON-like structure but receives XML can inadvertently allow entity expansion that injects unexpected keys into a Mongodb update operation. If the server uses a naive merge of XML-derived values into a Mongodb $set or $push operation, an attacker can manipulate fields that should be immutable, such as admin flags or record ownership identifiers. Because Grape does not enforce strict schema validation by default, malicious XML can map into Ruby hashes that are passed directly to the Mongodb driver, bypassing application-level guards.

Moreover, when combined with insecure error handling, XXE can leak stack traces or file paths in error responses, revealing details about the Mongodb deployment or the structure of stored documents. If the Mongodb instance is accessible only from the application layer, network-based entity expansions that reach internal endpoints may expose internal services through SSRF-like behavior. Even when Mongodb is not directly exploitable via injection, the combination of XXE and dynamic query construction can lead to unauthorized data access or manipulation, violating confidentiality and integrity objectives defined in frameworks like OWASP API Top 10.

Remediation focuses on preventing untrusted XML from reaching Mongodb operations and ensuring strict input validation. In Grape, configure the XML parser to disable external entity resolution and DTD loading. Avoid passing raw XML-derived hashes directly to Mongodb update or find operations; instead, validate and sanitize each field.

Secure Grape endpoint with sanitized input

class Api::V1::Users < Grape::API
  format :json

  desc 'Update user profile'
  params do
    requires :user_id, type: String, desc: 'User ID'
    requires :name, type: String, desc: 'Display name'
    optional :email, type: String, desc: 'Email address'
  end
  put ':user_id' do
    user_id = params[:user_id]
    # Strictly whitelist allowed fields for update
    allowed_updates = {}
    allowed_updates[:name] = params[:name] if params[:name].is_a?(String)
    allowed_updates[:email] = params[:email] if params[:email].is_a?(String)

    # Use sanitized update with $set only for allowed fields
    result = Mongo::Client.new(['localhost:27017'], database: 'mydb')[:users].update_one(
      { user_id: user_id },
      { '$set' => allowed_updates }
    )

    { updated: result.modified_count }
  end
end

Disable XML external general entities in Rack middleware

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

  def call(env)
    # Wrap the request body to disable external entities before Grape parses XML
    if env['CONTENT_TYPE']&.include?('xml')
      original_body = env['rack.input']
      secure_input = StringIO.new(original_body.read.gsub(/<!\[.*?\]\s*\[(.|\s)*?\]\s*>/, ''))
      env['rack.input'] = secure_input
      env['CONTENT_LENGTH'] = secure_input.string.bytesize.to_s
    end
    @app.call(env)
  end
end

# In config.ru
use XmlSanitizationMiddleware
run Api::V1::Base

Validate against a strict schema and reject unexpected elements

require 'nokogiri'
require 'json'

schema = {
  'name' => { type: String },
  'email' => { type: String }
}

xml = '<user><name>Alice</name><email>alice@example.com</email><admin>true</admin></user>'
doc = Nokogiri::XML(xml)
errors = []

schema.each do |field, rules|
  value = doc.at_xpath("//#{field}")&.content
  errors << "Missing #{field}" unless value
  errors << "Invalid type for #{field}" unless value.is_a?(rules[:type])
end
# Reject if unexpected elements exist
has_unexpected = doc.root.children.any? { |n| n.element? && !schema.key?(n.name) }
errors << 'Unexpected XML elements' if has_unexpected

if errors.empty?
  user_data = schema.transform_keys { |k| k.to_sym }.merge(
    name: doc.at_xpath('//name').content,
    email: doc.at_xpath('//email').content
  )
  # Safe to use user_data in Mongodb operations
else
  raise Grape::Exceptions::Validation, errors.join('; ')
end

By combining strict XML parsing rules, input whitelisting, and Mongodb update operations that only apply explicitly allowed fields, you reduce the risk that XXE-derived data can manipulate database behavior. This approach aligns with security practices referenced in findings from scans run by tools such as middleBrick, which highlight insecure XML parsing and improper input validation as common issues in API security assessments.