feat: SMIME signed notification emails

- Add mail interceptor the signs outgoing email with SMIME
- Add lib and helpers to work with SMIME data
- New configuration params for setting up SMIME key and cert files

.gitignore

@@ -75,6 +75,8 @@ eslint-report.html
 /.rspec
 /plugins/*
 /.gitlab_pages_secret
+/.gitlab_smime_key
+/.gitlab_smime_cert
 package-lock.json
 /junit_*.xml
 /coverage-frontend/

changelogs/unreleased/feat-smime-signed-notification-emails.yml

+---
+title: Notification emails can be signed with SMIME
+merge_request: 30644
+author: Diego Louzán
+type: added

config/gitlab.yml.example

@@ -95,6 +95,15 @@ production: &base
     email_display_name: GitLab
     email_reply_to: noreply@example.com
     email_subject_suffix: ''
+    email_smime:
+      # Uncomment and set to true if you need to enable email S/MIME signing (default: false)
+      # enabled: false
+      # S/MIME private key file in PEM format, unencrypted
+      # Default is '.gitlab_smime_key' relative to Rails.root (i.e. root of the GitLab app).
+      # key_file: /home/git/gitlab/.gitlab_smime_key
+      # S/MIME public certificate key in PEM format, will be attached to signed messages
+      # Default is '.gitlab_smime_cert' relative to Rails.root (i.e. root of the GitLab app).
+      # cert_file: /home/git/gitlab/.gitlab_smime_cert
     # Email server smtp settings are in config/initializers/smtp_settings.rb.sample

config/initializers/1_settings.rb

 require_relative '../settings'
 require_relative '../object_store_settings'
+require_relative '../smime_signature_settings'
 # Default settings
 Settings['ldap'] ||= Settingslogic.new({})
@@ -171,6 +172,7 @@ Settings.gitlab['email_from'] ||= ENV['GITLAB_EMAIL_FROM'] || "gitlab@#{Settings
 Settings.gitlab['email_display_name'] ||= ENV['GITLAB_EMAIL_DISPLAY_NAME'] || 'GitLab'
 Settings.gitlab['email_reply_to'] ||= ENV['GITLAB_EMAIL_REPLY_TO'] || "noreply@#{Settings.gitlab.host}"
 Settings.gitlab['email_subject_suffix'] ||= ENV['GITLAB_EMAIL_SUBJECT_SUFFIX'] || ""
+Settings.gitlab['email_smime'] = SmimeSignatureSettings.parse(Settings.gitlab['email_smime'])
 Settings.gitlab['base_url']   ||= Settings.__send__(:build_base_gitlab_url)
 Settings.gitlab['url']        ||= Settings.__send__(:build_gitlab_url)
 Settings.gitlab['user']       ||= 'git'

config/initializers/action_mailer_hooks.rb

@@ -10,3 +10,8 @@ ActionMailer::Base.register_interceptors(
 )
 ActionMailer::Base.register_observer(::Gitlab::Email::Hook::DeliveryMetricsObserver)
+
+if Gitlab.config.gitlab.email_enabled && Gitlab.config.gitlab.email_smime.enabled
+  ActionMailer::Base.register_interceptor(::Gitlab::Email::Hook::SmimeSignatureInterceptor)
+  Gitlab::AppLogger.debug "S/MIME signing of outgoing emails enabled"
+end

config/smime_signature_settings.rb

+# Set default values for email_smime settings
+class SmimeSignatureSettings
+  def self.parse(email_smime)
+    email_smime ||= Settingslogic.new({})
+    email_smime['enabled'] = false unless email_smime['enabled']
+    email_smime['key_file'] ||= Rails.root.join('.gitlab_smime_key')
+    email_smime['cert_file'] ||= Rails.root.join('.gitlab_smime_cert')
+
+    email_smime
+  end
+end

doc/administration/index.md

@@ -64,6 +64,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 - [External Classification Policy Authorization](../user/admin_area/settings/external_authorization.md) **(PREMIUM ONLY)**
 - [Upload a license](../user/admin_area/license.md): Upload a license to unlock features that are in paid tiers of GitLab. **(STARTER ONLY)**
 - [Admin Area](../user/admin_area/index.md): for self-managed instance-wide configuration and maintenance.
+- [S/MIME Signing](smime_signing_email.md): how to sign all outgoing notification emails with S/MIME
 #### Customizing GitLab's appearance

doc/administration/smime_signing_email.md

+# Signing outgoing email with S/MIME
+
+Notification emails sent by Gitlab can be signed with S/MIME for improved
+security.
+
+> **Note:**
+Please be aware that S/MIME certificates and TLS/SSL certificates are not the
+same and are used for different purposes: TLS creates a secure channel, whereas
+S/MIME signs and/or encrypts the message itself
+
+## Enable S/MIME signing
+
+This setting must be explicitly enabled and a single pair of key and certificate
+files must be provided in `gitlab.rb` or `gitlab.yml` if you are using Omnibus
+GitLab or installed GitLab from source respectively:
+
+```yaml
+email_smime:
+  enabled: true
+  key_file: /etc/pki/smime/private/gitlab.key
+  cert_file: /etc/pki/smime/certs/gitlab.crt
+```
+
+- Both files must be provided PEM-encoded.
+- The key file must be unencrypted so that Gitlab can read it without user 
+  intervention.
+
+NOTE: **Note:** Be mindful of the access levels for your private keys and visibility to
+third parties.
+
+### How to convert S/MIME PKCS#12 / PFX format to PEM encoding
+
+Typically S/MIME certificates are handled in binary PKCS#12 format (`.pfx` or `.p12`
+extensions), which contain the following in a single encrypted file:
+
+- Server certificate
+- Intermediate certificates (if any)
+- Private key
+
+In order to export the required files in PEM encoding from the PKCS#12 file,
+the `openssl` command can be used:
+
+```bash
+#-- Extract private key in PEM encoding (no password, unencrypted)
+$ openssl pkcs12 -in gitlab.p12 -nocerts -nodes -out gitlab.key
+
+#-- Extract certificates in PEM encoding (full certs chain including CA)
+$ openssl pkcs12 -in gitlab.p12 -nokeys -out gitlab.crt
+```

doc/development/emails.md

@@ -5,6 +5,10 @@
 To view rendered emails "sent" in your development instance, visit
 [`/rails/letter_opener`](http://localhost:3000/rails/letter_opener).
+Please note that [S/MIME signed](../administration/smime_signing_email.md) emails
+[cannot be currently previewed](https://github.com/fgrehm/letter_opener_web/issues/96) with 
+`letter_opener`.
+
 ## Mailer previews
 Rails provides a way to preview our mailer templates in HTML and plaintext using

lib/gitlab/email/hook/smime_signature_interceptor.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Email
+    module Hook
+      class SmimeSignatureInterceptor
+        # Sign emails with SMIME if enabled
+        class << self
+          def delivering_email(message)
+            signed_message = Gitlab::Email::Smime::Signer.sign(
+              cert: certificate.cert,
+              key: certificate.key,
+              data: message.encoded)
+            signed_email = Mail.new(signed_message)
+
+            overwrite_body(message, signed_email)
+            overwrite_headers(message, signed_email)
+          end
+
+          private
+
+          def certificate
+            @certificate ||= Gitlab::Email::Smime::Certificate.from_files(key_path, cert_path)
+          end
+
+          def key_path
+            Gitlab.config.gitlab.email_smime.key_file
+          end
+
+          def cert_path
+            Gitlab.config.gitlab.email_smime.cert_file
+          end
+
+          def overwrite_body(message, signed_email)
+            # since this is a multipart email, assignment to nil is important,
+            # otherwise Message#body will add a new mail part
+            message.body = nil
+            message.body = signed_email.body.encoded
+          end
+
+          def overwrite_headers(message, signed_email)
+            message.content_disposition = signed_email.content_disposition
+            message.content_transfer_encoding = signed_email.content_transfer_encoding
+            message.content_type = signed_email.content_type
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/email/smime/certificate.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Email
+    module Smime
+      class Certificate
+        include OpenSSL
+
+        attr_reader :key, :cert
+
+        def key_string
+          @key.to_s
+        end
+
+        def cert_string
+          @cert.to_pem
+        end
+
+        def self.from_strings(key_string, cert_string)
+          key = PKey::RSA.new(key_string)
+          cert = X509::Certificate.new(cert_string)
+          new(key, cert)
+        end
+
+        def self.from_files(key_path, cert_path)
+          from_strings(File.read(key_path), File.read(cert_path))
+        end
+
+        def initialize(key, cert)
+          @key = key
+          @cert = cert
+        end
+      end
+    end
+  end
+end

lib/gitlab/email/smime/signer.rb

+# frozen_string_literal: true
+
+require 'openssl'
+
+module Gitlab
+  module Email
+    module Smime
+      # Tooling for signing and verifying data with SMIME
+      class Signer
+        include OpenSSL
+
+        def self.sign(cert:, key:, data:)
+          signed_data = PKCS7.sign(cert, key, data, nil, PKCS7::DETACHED)
+          PKCS7.write_smime(signed_data)
+        end
+
+        # return nil if data cannot be verified, otherwise the signed content data
+        def self.verify_signature(cert:, ca_cert: nil, signed_data:)
+          store = X509::Store.new
+          store.set_default_paths
+          store.add_cert(ca_cert) if ca_cert
+
+          signed_smime = PKCS7.read_smime(signed_data)
+          signed_smime if signed_smime.verify([cert], store)
+        end
+      end
+    end
+  end
+end

spec/config/smime_signature_settings_spec.rb

+require 'fast_spec_helper'
+
+describe SmimeSignatureSettings do
+  describe '.parse' do
+    let(:default_smime_key) { Rails.root.join('.gitlab_smime_key') }
+    let(:default_smime_cert) { Rails.root.join('.gitlab_smime_cert') }
+
+    it 'sets correct default values to disabled' do
+      parsed_settings = described_class.parse(nil)
+
+      expect(parsed_settings['enabled']).to be(false)
+      expect(parsed_settings['key_file']).to eq(default_smime_key)
+      expect(parsed_settings['cert_file']).to eq(default_smime_cert)
+    end
+
+    context 'when providing custom values' do
+      it 'sets correct default values to disabled' do
+        custom_settings = Settingslogic.new({})
+
+        parsed_settings = described_class.parse(custom_settings)
+
+        expect(parsed_settings['enabled']).to be(false)
+        expect(parsed_settings['key_file']).to eq(default_smime_key)
+        expect(parsed_settings['cert_file']).to eq(default_smime_cert)
+      end
+
+      it 'enables smime with default key and cert' do
+        custom_settings = Settingslogic.new({
+          'enabled' => true
+        })
+
+        parsed_settings = described_class.parse(custom_settings)
+
+        expect(parsed_settings['enabled']).to be(true)
+        expect(parsed_settings['key_file']).to eq(default_smime_key)
+        expect(parsed_settings['cert_file']).to eq(default_smime_cert)
+      end
+
+      it 'enables smime with custom key and cert' do
+        custom_key = '/custom/key'
+        custom_cert = '/custom/cert'
+        custom_settings = Settingslogic.new({
+          'enabled' => true,
+          'key_file' => custom_key,
+          'cert_file' => custom_cert
+        })
+
+        parsed_settings = described_class.parse(custom_settings)
+
+        expect(parsed_settings['enabled']).to be(true)
+        expect(parsed_settings['key_file']).to eq(custom_key)
+        expect(parsed_settings['cert_file']).to eq(custom_cert)
+      end
+    end
+  end
+end

spec/initializers/action_mailer_hooks_spec.rb

+require 'spec_helper'
+
+describe 'ActionMailer hooks' do
+  describe 'smime signature interceptor' do
+    before do
+      class_spy(ActionMailer::Base).as_stubbed_const
+    end
+
+    it 'is disabled by default' do
+      load Rails.root.join('config/initializers/action_mailer_hooks.rb')
+
+      expect(ActionMailer::Base).not_to(
+        have_received(:register_interceptor).with(Gitlab::Email::Hook::SmimeSignatureInterceptor))
+    end
+
+    describe 'interceptor testbed' do
+      where(:email_enabled, :email_smime_enabled, :smime_interceptor_enabled) do
+        [
+          [false, false, false],
+          [false, true,  false],
+          [true,  false, false],
+          [true,  true,  true]
+        ]
+      end
+
+      with_them do
+        before do
+          stub_config_setting(email_enabled: email_enabled)
+          stub_config_setting(email_smime: { enabled: email_smime_enabled })
+        end
+
+        it 'is enabled depending on settings' do
+          load Rails.root.join('config/initializers/action_mailer_hooks.rb')
+
+          if smime_interceptor_enabled
+            expect(ActionMailer::Base).to(
+              have_received(:register_interceptor).with(Gitlab::Email::Hook::SmimeSignatureInterceptor))
+          else
+            expect(ActionMailer::Base).not_to(
+              have_received(:register_interceptor).with(Gitlab::Email::Hook::SmimeSignatureInterceptor))
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/email/hook/disable_email_interceptor_spec.rb

@@ -13,9 +13,6 @@ describe Gitlab::Email::Hook::DisableEmailInterceptor do
   end
   after do
-    # Removing interceptor from the list because unregister_interceptor is
-    # implemented in later version of mail gem
-    # See: https://github.com/mikel/mail/pull/705
     Mail.unregister_interceptor(described_class)
   end

spec/lib/gitlab/email/hook/smime_signature_interceptor_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Email::Hook::SmimeSignatureInterceptor do
+  include SmimeHelper
+
+  # cert generation is an expensive operation and they are used read-only,
+  # so we share them as instance variables in all tests
+  before :context do
+    @root_ca = generate_root
+    @cert = generate_cert(root_ca: @root_ca)
+  end
+
+  let(:root_certificate) do
+    Gitlab::Email::Smime::Certificate.new(@root_ca[:key], @root_ca[:cert])
+  end
+
+  let(:certificate) do
+    Gitlab::Email::Smime::Certificate.new(@cert[:key], @cert[:cert])
+  end
+
+  let(:mail) do
+    ActionMailer::Base.mail(to: 'test@example.com', from: 'info@example.com', body: 'signed hello')
+  end
+
+  before do
+    allow(Gitlab::Email::Smime::Certificate).to receive_messages(from_files: certificate)
+
+    Mail.register_interceptor(described_class)
+    mail.deliver_now
+  end
+
+  after do
+    Mail.unregister_interceptor(described_class)
+  end
+
+  it 'signs the email appropriately with SMIME' do
+    expect(mail.header['To'].value).to eq('test@example.com')
+    expect(mail.header['From'].value).to eq('info@example.com')
+    expect(mail.header['Content-Type'].value).to match('multipart/signed').and match('protocol="application/x-pkcs7-signature"')
+
+    # verify signature and obtain pkcs7 encoded content
+    p7enc = Gitlab::Email::Smime::Signer.verify_signature(
+      cert: certificate.cert,
+      ca_cert: root_certificate.cert,
+      signed_data: mail.encoded)
+
+    # envelope in a Mail object and obtain the body
+    decoded_mail = Mail.new(p7enc.data)
+
+    expect(decoded_mail.body.encoded).to eq('signed hello')
+  end
+end

spec/lib/gitlab/email/smime/certificate_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Email::Smime::Certificate do
+  include SmimeHelper
+
+  # cert generation is an expensive operation and they are used read-only,
+  # so we share them as instance variables in all tests
+  before :context do
+    @root_ca = generate_root
+    @cert = generate_cert(root_ca: @root_ca)
+  end
+
+  describe 'testing environment setup' do
+    describe 'generate_root' do
+      subject { @root_ca }
+
+      it 'generates a root CA that expires a long way in the future' do
+        expect(subject[:cert].not_after).to be > 999.years.from_now
+      end
+    end
+
+    describe 'generate_cert' do
+      subject { @cert }
+
+      it 'generates a cert properly signed by the root CA' do
+        expect(subject[:cert].issuer).to eq(@root_ca[:cert].subject)
+      end
+
+      it 'generates a cert that expires soon' do
+        expect(subject[:cert].not_after).to be < 60.minutes.from_now
+      end
+
+      it 'generates a cert intended for email signing' do
+        expect(subject[:cert].extensions).to include(an_object_having_attributes(oid: 'extendedKeyUsage', value: match('E-mail Protection')))
+      end
+
+      context 'passing in INFINITE_EXPIRY' do
+        subject { generate_cert(root_ca: @root_ca, expires_in: SmimeHelper::INFINITE_EXPIRY) }
+
+        it 'generates a cert that expires a long way in the future' do
+          expect(subject[:cert].not_after).to be > 999.years.from_now
+        end
+      end
+    end
+  end
+
+  describe '.from_strings' do
+    it 'parses correctly a certificate and key' do
+      parsed_cert = described_class.from_strings(@cert[:key].to_s, @cert[:cert].to_pem)
+
+      common_cert_tests(parsed_cert, @cert, @root_ca)
+    end
+  end
+
+  describe '.from_files' do
+    it 'parses correctly a certificate and key' do
+      allow(File).to receive(:read).with('a_key').and_return(@cert[:key].to_s)
+      allow(File).to receive(:read).with('a_cert').and_return(@cert[:cert].to_pem)
+
+      parsed_cert = described_class.from_files('a_key', 'a_cert')
+
+      common_cert_tests(parsed_cert, @cert, @root_ca)
+    end
+  end
+
+  def common_cert_tests(parsed_cert, cert, root_ca)
+    expect(parsed_cert.cert).to be_a(OpenSSL::X509::Certificate)
+    expect(parsed_cert.cert.subject).to eq(cert[:cert].subject)
+    expect(parsed_cert.cert.issuer).to eq(root_ca[:cert].subject)
+    expect(parsed_cert.cert.not_before).to eq(cert[:cert].not_before)
+    expect(parsed_cert.cert.not_after).to eq(cert[:cert].not_after)
+    expect(parsed_cert.cert.extensions).to include(an_object_having_attributes(oid: 'extendedKeyUsage', value: match('E-mail Protection')))
+    expect(parsed_cert.key).to be_a(OpenSSL::PKey::RSA)
+  end
+end

spec/lib/gitlab/email/smime/signer_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Email::Smime::Signer do
+  include SmimeHelper
+
+  it 'signs data appropriately with SMIME' do
+    root_certificate = generate_root
+    certificate = generate_cert(root_ca: root_certificate)
+
+    signed_content = described_class.sign(
+      cert: certificate[:cert],
+      key: certificate[:key],
+      data: 'signed content')
+    expect(signed_content).not_to be_nil
+
+    p7enc = described_class.verify_signature(
+      cert: certificate[:cert],
+      ca_cert: root_certificate[:cert],
+      signed_data: signed_content)
+
+    expect(p7enc).not_to be_nil
+    expect(p7enc.data).to eq('signed content')
+  end
+end

spec/support/helpers/smime_helper.rb

+module SmimeHelper
+  include OpenSSL
+
+  INFINITE_EXPIRY = 1000.years
+  SHORT_EXPIRY = 30.minutes
+
+  def generate_root
+    issue(signed_by: nil, expires_in: INFINITE_EXPIRY, certificate_authority: true)
+  end
+
+  def generate_cert(root_ca:, expires_in: SHORT_EXPIRY)
+    issue(signed_by: root_ca, expires_in: expires_in, certificate_authority: false)
+  end
+
+  # returns a hash { key:, cert: } containing a generated key, cert pair
+  def issue(email_address: 'test@example.com', signed_by:, expires_in:, certificate_authority:)
+    key = OpenSSL::PKey::RSA.new(4096)
+    public_key = key.public_key
+
+    subject = if certificate_authority
+                X509::Name.parse("/CN=EU")
+              else
+                X509::Name.parse("/CN=#{email_address}")
+              end
+
+    cert = X509::Certificate.new
+    cert.subject = subject
+
+    cert.issuer = signed_by&.fetch(:cert, nil)&.subject || subject
+
+    cert.not_before = Time.now
+    cert.not_after = expires_in.from_now
+    cert.public_key = public_key
+    cert.serial = 0x0
+    cert.version = 2
+
+    extension_factory = X509::ExtensionFactory.new
+    if certificate_authority
+      extension_factory.subject_certificate = cert
+      extension_factory.issuer_certificate = cert
+      cert.add_extension(extension_factory.create_extension('subjectKeyIdentifier', 'hash'))
+      cert.add_extension(extension_factory.create_extension('basicConstraints', 'CA:TRUE', true))
+      cert.add_extension(extension_factory.create_extension('keyUsage', 'cRLSign,keyCertSign', true))
+    else
+      cert.add_extension(extension_factory.create_extension('subjectAltName', "email:#{email_address}", false))
+      cert.add_extension(extension_factory.create_extension('basicConstraints', 'CA:FALSE', true))
+      cert.add_extension(extension_factory.create_extension('keyUsage', 'digitalSignature,keyEncipherment', true))
+      cert.add_extension(extension_factory.create_extension('extendedKeyUsage', 'clientAuth,emailProtection', false))
+    end
+
+    cert.sign(signed_by&.fetch(:key, nil) || key, Digest::SHA256.new)
+
+    { key: key, cert: cert }
+  end
+end