diff --git a/Gemfile b/Gemfile
index 6ae9086a5415cd90a41ca9f7ac0dfdb0548bd3a0..8a30a819660d562dca91c071c3b1709e59f16607 100644
--- a/Gemfile
+++ b/Gemfile
@@ -349,3 +349,6 @@ gem 'health_check', '~> 2.1.0'
 # System information
 gem 'vmstat', '~> 2.1.0'
 gem 'sys-filesystem', '~> 1.1.6'
+
+# Secure headers for Content Security Policy
+gem 'secure_headers', '~> 3.3'
diff --git a/Gemfile.lock b/Gemfile.lock
index 9ec5fe1282064ee93244392a8b1aad51bfab47fd..f3f4d895ae4a9b2707a4bc4dfbcfc9823e3e365d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -645,6 +645,8 @@ GEM
     sdoc (0.3.20)
       json (>= 1.1.3)
       rdoc (~> 3.10)
+    secure_headers (3.3.2)
+      useragent
     seed-fu (2.3.6)
       activerecord (>= 3.1)
       activesupport (>= 3.1)
@@ -767,6 +769,7 @@ GEM
       get_process_mem (~> 0)
       unicorn (>= 4, < 6)
     uniform_notifier (1.9.0)
+    useragent (0.16.7)
     uuid (2.3.8)
       macaddr (~> 1.0)
     version_sorter (2.0.0)
@@ -944,6 +947,7 @@ DEPENDENCIES
   sass-rails (~> 5.0.0)
   scss_lint (~> 0.47.0)
   sdoc (~> 0.3.20)
+  secure_headers (~> 3.3)
   seed-fu (~> 2.3.5)
   select2-rails (~> 3.5.9)
   sentry-raven (~> 1.1.0)
diff --git a/config/initializers/secure_headers.rb b/config/initializers/secure_headers.rb
new file mode 100644
index 0000000000000000000000000000000000000000..9fd24a667cc2a79ecaf5559df04091104682d9b5
--- /dev/null
+++ b/config/initializers/secure_headers.rb
@@ -0,0 +1,109 @@
+# CSP headers have to have single quotes, so failures relating to quotes
+# inside Ruby string arrays are irrelevant.
+# rubocop:disable Lint/PercentStringArray
+require 'gitlab/current_settings'
+include Gitlab::CurrentSettings
+
+# If Sentry is enabled and the Rails app is running in production mode,
+# this will construct the Report URI for Sentry.
+if Rails.env.production? && current_application_settings.sentry_enabled
+  uri = URI.parse(current_application_settings.sentry_dsn)
+  CSP_REPORT_URI = "#{uri.scheme}://#{uri.host}/api#{uri.path}/csp-report/?sentry_key=#{uri.user}"
+else
+  CSP_REPORT_URI = ''
+end
+
+# Content Security Policy Headers
+# For more information on CSP see:
+# - https://gitlab.com/gitlab-org/gitlab-ce/issues/18231
+# - https://developer.mozilla.org/en-US/docs/Web/Security/CSP/CSP_policy_directives
+SecureHeaders::Configuration.default do |config|
+  # Mark all cookies as "Secure", "HttpOnly", and "SameSite=Strict".
+  config.cookies = {
+    secure: true,
+    httponly: true,
+    samesite: {
+      strict: true 
+    }
+  }
+  config.x_content_type_options = "nosniff"
+  config.x_xss_protection = "1; mode=block"
+  config.x_download_options = "noopen"
+  config.x_permitted_cross_domain_policies = "none"
+  config.referrer_policy = "origin-when-cross-origin"
+  config.csp = {
+    # "Meta" values.
+    report_only: true,
+    preserve_schemes: true,
+
+    # "Directive" values.
+    # Default source allows nothing, more permissive values are set per-policy.
+    default_src: %w('none'),
+    # (Deprecated) Don't allow iframes.
+    frame_src: %w('none'),
+    # Only allow XMLHTTPRequests from the GitLab instance itself.
+    connect_src: %w('self'),
+    # Only load local fonts.
+    font_src: %w('self'),
+    # Load local images, any external image available over HTTPS.
+    img_src: %w(* 'self' data:),
+    # Audio and video can't be played on GitLab currently, so it's disabled.
+    media_src: %w('none'),
+    # Don't allow <object>, <embed>, or <applet> elements.
+    object_src: %w('none'),
+    # Allow local scripts and inline scripts.
+    script_src: %w('unsafe-inline' 'unsafe-eval' 'self'),
+    # Allow local stylesheets and inline styles.
+    style_src: %w('unsafe-inline' 'self'),
+    # The URIs that a user agent may use as the document base URL.
+    base_uri: %w('self'),
+    # Only allow local iframes and service workers
+    child_src: %w('self'),
+    # Only submit form information to the GitLab instance.
+    form_action: %w('self'),
+    # Disallow any parents from embedding a page in an iframe.
+    frame_ancestors: %w('none'),
+    # Don't allow any plugins (Flash, Shockwave, etc.)
+    plugin_types: %w(),
+    # Blocks all mixed (HTTP) content.
+    block_all_mixed_content: true,
+    # Upgrades insecure requests to HTTPS when possible.
+    upgrade_insecure_requests: true
+  }
+
+  # Reports are sent to Sentry if it's enabled.
+  if current_application_settings.sentry_enabled
+    config.csp[:report_uri] = %W(#{CSP_REPORT_URI})
+  end
+
+  # Allow Bootstrap Linter in development mode.
+  if Rails.env.development?
+    config.csp[:script_src] << "maxcdn.bootstrapcdn.com"
+  end
+
+  # reCAPTCHA
+  if current_application_settings.recaptcha_enabled
+    config.csp[:script_src] << "https://www.google.com/recaptcha/"
+    config.csp[:script_src] << "https://www.gstatic.com/recaptcha/"
+    config.csp[:frame_src] << "https://www.google.com/recaptcha/"
+    config.x_frame_options = "SAMEORIGIN"
+  end
+
+  # Gravatar
+  if current_application_settings.gravatar_enabled?
+    config.csp[:img_src] << "www.gravatar.com"
+    config.csp[:img_src] << "secure.gravatar.com"
+    config.csp[:img_src] << Gitlab.config.gravatar.host
+  end
+
+  # Piwik
+  if Gitlab.config.extra.has_key?('piwik_url') && Gitlab.config.extra.has_key?('piwik_site_id')
+    config.csp[:script_src] << Gitlab.config.extra.piwik_url
+    config.csp[:img_src] << Gitlab.config.extra.piwik_url
+  end
+
+  # Google Analytics
+  if Gitlab.config.extra.has_key?('google_analytics_id')
+    config.csp[:script_src] << "https://www.google-analytics.com"
+  end
+end