diff --git a/actionpack/lib/action_dispatch/constants.rb b/actionpack/lib/action_dispatch/constants.rb
index 971b42fd66b04..2771b3adc59d9 100644
--- a/actionpack/lib/action_dispatch/constants.rb
+++ b/actionpack/lib/action_dispatch/constants.rb
@@ -8,6 +8,8 @@ module Constants
     if Gem::Version.new(Rack::RELEASE) < Gem::Version.new("3")
       VARY = "Vary"
       CONTENT_ENCODING = "Content-Encoding"
+      CONTENT_SECURITY_POLICY = "Content-Security-Policy"
+      CONTENT_SECURITY_POLICY_REPORT_ONLY = "Content-Security-Policy-Report-Only"
       LOCATION = "Location"
       FEATURE_POLICY = "Feature-Policy"
       X_REQUEST_ID = "X-Request-Id"
@@ -15,6 +17,8 @@ module Constants
     else
       VARY = "vary"
       CONTENT_ENCODING = "content-encoding"
+      CONTENT_SECURITY_POLICY = "content-security-policy"
+      CONTENT_SECURITY_POLICY_REPORT_ONLY = "content-security-policy-report-only"
       LOCATION = "location"
       FEATURE_POLICY = "feature-policy"
       X_REQUEST_ID = "x-request-id"
diff --git a/actionpack/lib/action_dispatch/http/content_security_policy.rb b/actionpack/lib/action_dispatch/http/content_security_policy.rb
index 20eebb0f862bd..c0ea12beccc90 100644
--- a/actionpack/lib/action_dispatch/http/content_security_policy.rb
+++ b/actionpack/lib/action_dispatch/http/content_security_policy.rb
@@ -25,10 +25,6 @@ module ActionDispatch # :nodoc:
   #   end
   class ContentSecurityPolicy
     class Middleware
-      CONTENT_TYPE = "Content-Type"
-      POLICY = "Content-Security-Policy"
-      POLICY_REPORT_ONLY = "Content-Security-Policy-Report-Only"
-
       def initialize(app)
         @app = app
       end
@@ -57,14 +53,15 @@ def call(env)
       private
         def header_name(request)
           if request.content_security_policy_report_only
-            POLICY_REPORT_ONLY
+            ActionDispatch::Constants::CONTENT_SECURITY_POLICY_REPORT_ONLY
           else
-            POLICY
+            ActionDispatch::Constants::CONTENT_SECURITY_POLICY
           end
         end
 
         def policy_present?(headers)
-          headers[POLICY] || headers[POLICY_REPORT_ONLY]
+          headers[ActionDispatch::Constants::CONTENT_SECURITY_POLICY] ||
+            headers[ActionDispatch::Constants::CONTENT_SECURITY_POLICY_REPORT_ONLY]
         end
     end
 
diff --git a/actionpack/test/dispatch/content_security_policy_test.rb b/actionpack/test/dispatch/content_security_policy_test.rb
index 9af7da91f660e..aa8ee497cb4a7 100644
--- a/actionpack/test/dispatch/content_security_policy_test.rb
+++ b/actionpack/test/dispatch/content_security_policy_test.rb
@@ -302,6 +302,51 @@ def test_raises_runtime_error_when_unexpected_source
   end
 end
 
+class ContentSecurityPolicyMiddlewareTest < ActiveSupport::TestCase
+  def setup
+    @env = Rack::MockRequest.env_for("", {})
+    @env["action_dispatch.content_security_policy"] = ActionDispatch::ContentSecurityPolicy.new do |p|
+      p.default_src -> { :self  }
+    end
+    @env["action_dispatch.content_security_policy_nonce_generator"] = proc { "iyhD0Yc0W+c=" }
+    @env["action_dispatch.content_security_policy_report_only"] = false
+    @env["action_dispatch.show_exceptions"] = :none
+
+    @default_csp = "default-src 'self' https: http:"
+  end
+
+  def test_rack_lint
+    app = proc { [200, {}, []] }
+
+    assert_nothing_raised do
+      Rack::Lint.new(
+        ActionDispatch::ContentSecurityPolicy::Middleware.new(
+          Rack::Lint.new(app)
+        )
+      ).call(@env)
+    end
+  end
+
+  def test_does_not_override_app_content_security_policy
+    app = proc { [200, { ActionDispatch::Constants::CONTENT_SECURITY_POLICY => @default_csp }, []] }
+    _, headers, _ = Rack::Lint.new(
+      ActionDispatch::ContentSecurityPolicy::Middleware.new(Rack::Lint.new(app))
+    ).call(@env)
+
+    assert_equal @default_csp, headers[ActionDispatch::Constants::CONTENT_SECURITY_POLICY]
+  end
+
+  def test_does_not_override_app_content_security_policy_report_only
+    app = proc { [200, { ActionDispatch::Constants::CONTENT_SECURITY_POLICY_REPORT_ONLY => @default_csp }, []] }
+    @env["action_dispatch.content_security_policy_report_only"] = true
+    _, headers, _ = Rack::Lint.new(
+      ActionDispatch::ContentSecurityPolicy::Middleware.new(Rack::Lint.new(app))
+    ).call(@env)
+
+    assert_equal @default_csp, headers[ActionDispatch::Constants::CONTENT_SECURITY_POLICY_REPORT_ONLY]
+  end
+end
+
 class DefaultContentSecurityPolicyIntegrationTest < ActionDispatch::IntegrationTest
   class PolicyController < ActionController::Base
     def index
@@ -338,8 +383,10 @@ def call(env)
   end
 
   APP = build_app(ROUTES) do |middleware|
+    middleware.use Rack::Lint
     middleware.use PolicyConfigMiddleware
     middleware.use ActionDispatch::ContentSecurityPolicy::Middleware
+    middleware.use Rack::Lint
   end
 
   def app
@@ -488,8 +535,10 @@ def call(env)
   end
 
   APP = build_app(ROUTES) do |middleware|
+    middleware.use Rack::Lint
     middleware.use PolicyConfigMiddleware
     middleware.use ActionDispatch::ContentSecurityPolicy::Middleware
+    middleware.use Rack::Lint
   end
 
   def app
@@ -604,8 +653,10 @@ def call(env)
   end
 
   APP = build_app(ROUTES) do |middleware|
+    middleware.use Rack::Lint
     middleware.use PolicyConfigMiddleware
     middleware.use ActionDispatch::ContentSecurityPolicy::Middleware
+    middleware.use Rack::Lint
   end
 
   def app
@@ -660,8 +711,10 @@ def call(env)
   end
 
   APP = build_app(ROUTES) do |middleware|
+    middleware.use Rack::Lint
     middleware.use PolicyConfigMiddleware
     middleware.use ActionDispatch::ContentSecurityPolicy::Middleware
+    middleware.use Rack::Lint
   end
 
   def app
@@ -732,8 +785,10 @@ def call(env)
   end
 
   APP = build_app(ROUTES) do |middleware|
+    middleware.use Rack::Lint
     middleware.use PolicyConfigMiddleware
     middleware.use ActionDispatch::ContentSecurityPolicy::Middleware
+    middleware.use Rack::Lint
   end
 
   def app