Add support for Expect-CT HTTP header (#322)

* Update README with example usage for `Expect-CT`

* Add some tests for the expected API

* Add classes and loading to main entrypoints

* Add `ExpectCt` class

Adds the `ExpectCt` class and it's associated validation of the header
syntax

* Use full name of specification instead

* Use consistent double quotes

* Update README example to use correct naming for Expect-CT

* Set defaults for Expect-CT via helper

* Add missing comma in README example

* Add frozen string literal

README.md

@@ -19,6 +19,7 @@ The gem will automatically apply several headers that are related to security.
 - X-Permitted-Cross-Domain-Policies - [Restrict Adobe Flash Player's access to data](https://www.adobe.com/devnet/adobe-media-server/articles/cross-domain-xml-for-streaming.html)
 - Referrer-Policy - [Referrer Policy draft](https://w3c.github.io/webappsec-referrer-policy/)
 - Public Key Pinning - Pin certificate fingerprints in the browser to prevent man-in-the-middle attacks due to compromised Certificate Authorities. [Public Key Pinning Specification](https://tools.ietf.org/html/rfc7469)
+- Expect-CT - Only use certificates that are present in the certificate transparency logs. [Expect-CT draft specification](https://datatracker.ietf.org/doc/draft-stark-expect-ct/).
 - Clear-Site-Data - Clearing browser data for origin. [Clear-Site-Data specification](https://w3c.github.io/webappsec-clear-site-data/).
 
 It can also mark all http cookies with the Secure, HttpOnly and SameSite attributes (when configured to do so).
@@ -77,6 +78,11 @@ SecureHeaders::Configuration.default do |config|
     "storage",
     "executionContexts"
   ]
+  config.expect_certificate_transparency = {
+    enforce: false,
+    max_age: 1.day.to_i,
+    report_uri: "https://report-uri.io/example-ct"
+  }
   config.csp = {
     # "meta" values. these will shape the header, but the values are not included in the header.
     preserve_schemes: true, # default: false. Schemes are removed from host sources to save bytes and discourage mixed content.

lib/secure_headers.rb

@@ -12,6 +12,7 @@
 require "secure_headers/headers/x_permitted_cross_domain_policies"
 require "secure_headers/headers/referrer_policy"
 require "secure_headers/headers/clear_site_data"
+require "secure_headers/headers/expect_certificate_transparency"
 require "secure_headers/middleware"
 require "secure_headers/railtie"
 require "secure_headers/view_helper"
@@ -52,6 +53,7 @@ def opt_out?
   CSP = ContentSecurityPolicy
 
   ALL_HEADER_CLASSES = [
+    ExpectCertificateTransparency,
     ClearSiteData,
     ContentSecurityPolicyConfig,
     ContentSecurityPolicyReportOnlyConfig,

lib/secure_headers/configuration.rb

@@ -117,7 +117,7 @@ def deep_copy_if_hash(value)
 
     attr_writer :hsts, :x_frame_options, :x_content_type_options,
       :x_xss_protection, :x_download_options, :x_permitted_cross_domain_policies,
-      :referrer_policy, :clear_site_data
+      :referrer_policy, :clear_site_data, :expect_certificate_transparency
 
     attr_reader :cached_headers, :csp, :cookies, :csp_report_only, :hpkp, :hpkp_report_host
 
@@ -169,6 +169,7 @@ def dup
       copy.x_download_options = @x_download_options
       copy.x_permitted_cross_domain_policies = @x_permitted_cross_domain_policies
       copy.clear_site_data = @clear_site_data
+      copy.expect_certificate_transparency = @expect_certificate_transparency
       copy.referrer_policy = @referrer_policy
       copy.hpkp = @hpkp
       copy.hpkp_report_host = @hpkp_report_host
@@ -201,6 +202,7 @@ def validate_config!
       XDownloadOptions.validate_config!(@x_download_options)
       XPermittedCrossDomainPolicies.validate_config!(@x_permitted_cross_domain_policies)
       ClearSiteData.validate_config!(@clear_site_data)
+      ExpectCertificateTransparency.validate_config!(@expect_certificate_transparency)
       PublicKeyPins.validate_config!(@hpkp)
       Cookie.validate_config!(@cookies)
     end

lib/secure_headers/headers/expect_certificate_transparency.rb

@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+module SecureHeaders
+  class ExpectCertificateTransparencyConfigError < StandardError; end
+
+  class ExpectCertificateTransparency
+    HEADER_NAME = "Expect-CT".freeze
+    CONFIG_KEY  = :expect_certificate_transparency
+    INVALID_CONFIGURATION_ERROR = "config must be a hash.".freeze
+    INVALID_ENFORCE_VALUE_ERROR = "enforce must be a boolean".freeze
+    REQUIRED_MAX_AGE_ERROR      = "max-age is a required directive.".freeze
+    INVALID_MAX_AGE_ERROR       = "max-age must be a number.".freeze
+
+    class << self
+      # Public: Generate a Expect-CT header.
+      #
+      # Returns nil if not configured, returns header name and value if
+      # configured.
+      def make_header(config)
+        return if config.nil?
+
+        header = new(config)
+        [HEADER_NAME, header.value]
+      end
+
+      def validate_config!(config)
+        return if config.nil? || config == OPT_OUT
+        raise ExpectCertificateTransparencyConfigError.new(INVALID_CONFIGURATION_ERROR) unless config.is_a? Hash
+
+        unless [true, false, nil].include?(config[:enforce])
+          raise ExpectCertificateTransparencyConfigError.new(INVALID_ENFORCE_VALUE_ERROR)
+        end
+
+        if !config[:max_age]
+          raise ExpectCertificateTransparencyConfigError.new(REQUIRED_MAX_AGE_ERROR)
+        elsif config[:max_age].to_s !~ /\A\d+\z/
+          raise ExpectCertificateTransparencyConfigError.new(INVALID_MAX_AGE_ERROR)
+        end
+      end
+    end
+
+    def initialize(config)
+      @enforced   = config.fetch(:enforce, nil)
+      @max_age    = config.fetch(:max_age, nil)
+      @report_uri = config.fetch(:report_uri, nil)
+    end
+
+    def value
+      header_value = [
+        enforced_directive,
+        max_age_directive,
+        report_uri_directive
+      ].compact.join("; ").strip
+    end
+
+    def enforced_directive
+      # Unfortunately `if @enforced` isn't enough here in case someone
+      # passes in a random string so let's be specific with it to prevent
+      # accidental enforcement.
+      "enforce" if @enforced == true
+    end
+
+    def max_age_directive
+      "max-age=#{@max_age}" if @max_age
+    end
+
+    def report_uri_directive
+      "report-uri=\"#{@report_uri}\"" if @report_uri
+    end
+  end
+end

spec/lib/secure_headers/headers/expect_certificate_spec.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+require "spec_helper"
+
+module SecureHeaders
+  describe ExpectCertificateTransparency do
+    specify { expect(ExpectCertificateTransparency.new(max_age: 1234, enforce: true).value).to eq("enforce; max-age=1234") }
+    specify { expect(ExpectCertificateTransparency.new(max_age: 1234, enforce: false).value).to eq("max-age=1234") }
+    specify { expect(ExpectCertificateTransparency.new(max_age: 1234, enforce: "yolocopter").value).to eq("max-age=1234") }
+    specify { expect(ExpectCertificateTransparency.new(max_age: 1234, report_uri: "https://report-uri.io/expect-ct").value).to eq("max-age=1234; report-uri=\"https://report-uri.io/expect-ct\"") }
+    specify do
+      config = { enforce: true, max_age: 1234, report_uri: "https://report-uri.io/expect-ct" }
+      header_value = "enforce; max-age=1234; report-uri=\"https://report-uri.io/expect-ct\""
+      expect(ExpectCertificateTransparency.new(config).value).to eq(header_value)
+    end
+
+    context "with an invalid configuration" do
+      it "raises an exception when configuration isn't a hash" do
+        expect do
+          ExpectCertificateTransparency.validate_config!(%w(a))
+        end.to raise_error(ExpectCertificateTransparencyConfigError)
+      end
+
+      it "raises an exception when max-age is not provided" do
+        expect do
+          ExpectCertificateTransparency.validate_config!(foo: "bar")
+        end.to raise_error(ExpectCertificateTransparencyConfigError)
+      end
+
+      it "raises an exception with an invalid max-age" do
+        expect do
+          ExpectCertificateTransparency.validate_config!(max_age: "abc123")
+        end.to raise_error(ExpectCertificateTransparencyConfigError)
+      end
+
+      it "raises an exception with an invalid enforce value" do
+        expect do
+          ExpectCertificateTransparency.validate_config!(enforce: "brokenstring")
+        end.to raise_error(ExpectCertificateTransparencyConfigError)
+      end
+    end
+  end
+end

spec/spec_helper.rb

@@ -33,6 +33,7 @@ def expect_default_values(hash)
   expect(hash[SecureHeaders::XContentTypeOptions::HEADER_NAME]).to eq(SecureHeaders::XContentTypeOptions::DEFAULT_VALUE)
   expect(hash[SecureHeaders::XPermittedCrossDomainPolicies::HEADER_NAME]).to eq(SecureHeaders::XPermittedCrossDomainPolicies::DEFAULT_VALUE)
   expect(hash[SecureHeaders::ReferrerPolicy::HEADER_NAME]).to be_nil
+  expect(hash[SecureHeaders::ExpectCertificateTransparency::HEADER_NAME]).to be_nil
 end
 
 module SecureHeaders