Crlf Injection in Rails with Firestore

Crlf Injection occurs when an attacker can inject a CRLF sequence (\r\n) into a header or field that is later reflected in a response. In a Rails application that interacts with Google Cloud Firestore, this typically arises when user-controlled input is used to construct Firestore document IDs, map parameters, or values that later appear in HTTP headers or logs. While Firestore itself does not interpret CRLF sequences as control characters, the surrounding Rails stack does: the framework and underlying Rack server use headers and cookies extensively, and reflected data can reach HTTP responses where injected \r\n can enable header manipulation or response splitting.

Consider a scenario where a Rails controller reads a document ID from a request parameter and uses it to fetch a document from Firestore. If the ID contains \r\n sequences and is later echoed into a header (for example, a custom X-Document-ID header in the response), an attacker can inject additional headers or split the response. This is a classic response-splitting scenario mapped to the BFLA/Privilege Escalation category in middleBrick’s checks. Even if Firestore stores the ID as-is, the Rails layer that outputs it into headers becomes the attack surface. The same applies when Firestore metadata or document fields are used to build cookies or cache-control headers without sanitization.

Because middleBrick tests unauthenticated attack surfaces and runs checks in parallel — including BFLA/Privilege Escalation and Input Validation — it can flag cases where CRLF sequences appear in data that eventually reaches headers. The scanner does not assume Firestore is immune; it examines how Rails processes and reflects any user-influenced values, including those sourced from Firestore documents or IDs. Understanding this chain helps developers apply context-specific fixes rather than relying on a one-size-fits-all input filter.

To mitigate Crlf Injection when using Firestore in Rails, treat any user-influenced data that may reach headers or cookies as untrusted. This includes document IDs, map keys used for parameterization, and field values that you reflect into headers or logs. Use strict allowlists for document IDs and encode/sanitize any data that is echoed into headers. Below are concrete patterns and Firestore code examples for Rails that reduce risk.

1. Validate and sanitize document IDs before using them in Firestore operations

Instead of directly using request parameters as Firestore document IDs, validate them against a strict pattern. For identifiers that should be alphanumeric with optional hyphens, use a regex allowlist and reject any input containing \r, \n, or other control characters.

# app/services/document_lookup.rb
class DocumentLookup
  SAFE_ID = /\A[a-zA-Z0-9\-_]{1,100}\z/

  def self.find(user_supplied_id)
    # Reject unsafe IDs early
    return nil unless user_supplied_id&.match?(SAFE_ID)

    # Safe to use with Firestore client
    firestore_doc = Firestore::Document.new("projects/PROJECT_ID/databases/(default)/documents/items/#{user_supplied_id}")
    # Perform Firestore operations via the official client
    # For example, using the google-cloud-firestore gem:
    # require "google/cloud/firestore"
    # db = Google::Cloud::Firestore.new
    # doc_ref = db.doc("items/#{user_supplied_id}")
    # doc_ref.get
  rescue => e
    Rails.logger.warn("Invalid document ID or Firestore error: #{e.message}")
    nil
  end
end

2. Encode reflected values before setting headers

If you must reflect a Firestore-derived value into a header, ensure it is sanitized and does not contain line breaks. Use strict encoding for header-safe strings and avoid concatenating raw document fields into headers.

# app/controllers/documents_controller.rb
class DocumentsController < ApplicationController
  def show
    # Assume params[:id] is user-supplied
    safe_id = DocumentLookup.find(params[:id])
    if safe_id
      doc_data = fetch_firestore_data(safe_id)
      # Encode to prevent CRLF injection into headers
      reflected_value = CGI.escape(doc_data[:display_name].to_s).gsub(/\+/, ' ') # simple sanitization
      response.headers['X-Document-Name'] = "Document-#{reflected_value}"
      render json: doc_data
    else
      render json: { error: 'Invalid document ID' }, status: :bad_request
    end
  end

  private

  def fetch_firestore_data(doc_id)
    # Placeholder for actual Firestore fetch logic
    { display_name: doc_id, content: 'sample' }
  end
end

3. Avoid using Firestore metadata in cookie or cache-control values

When setting cookies or cache headers, do not directly use document fields that may contain newline characters. Instead, use Rails helpers that enforce safe values or transform the data to a safe representation.

# app/controllers/concerns/header_sanitization.rb
module HeaderSanitization
  def set_safe_header(key, value)
    # Remove or replace CRLF and other control characters
    sanitized = value.to_s.delete("\r\n")
    response.headers[key] = sanitized
  end
end

# Usage in a controller
class ReportsController < ApplicationController
  include HeaderSanitization

  def download
    report_name = fetch_firestore_report_name(params[:report_id])
    set_safe_header('X-Report-Name', report_name)
    # Proceed with response logic
  end

  private

  def fetch_firestore_report_name(report_id)
    # Firestore fetch logic here
    'Monthly-Report-2025'
  end
end

These patterns align with the Input Validation and BFLA/Privilege Escalation checks that middleBrick runs in parallel. By validating IDs at the boundary and encoding outputs, you reduce the risk of CRLF Injection while preserving legitimate Firestore functionality. Remember that middleBrick detects and reports such issues — including mappings to frameworks like OWASP API Top 10 — but it does not fix or block; it provides findings with remediation guidance to guide your hardening efforts.