diff --git a/actionpack/lib/action_dispatch/constants.rb b/actionpack/lib/action_dispatch/constants.rb
index 2771b3adc59d9..f6a225de4cd2b 100644
--- a/actionpack/lib/action_dispatch/constants.rb
+++ b/actionpack/lib/action_dispatch/constants.rb
@@ -14,6 +14,7 @@ module Constants
       FEATURE_POLICY = "Feature-Policy"
       X_REQUEST_ID = "X-Request-Id"
       SERVER_TIMING = "Server-Timing"
+      STRICT_TRANSPORT_SECURITY = "Strict-Transport-Security"
     else
       VARY = "vary"
       CONTENT_ENCODING = "content-encoding"
@@ -23,6 +24,7 @@ module Constants
       FEATURE_POLICY = "feature-policy"
       X_REQUEST_ID = "x-request-id"
       SERVER_TIMING = "server-timing"
+      STRICT_TRANSPORT_SECURITY = "strict-transport-security"
     end
   end
 end
diff --git a/actionpack/lib/action_dispatch/middleware/ssl.rb b/actionpack/lib/action_dispatch/middleware/ssl.rb
index 9788aaadd9292..71c521841054f 100644
--- a/actionpack/lib/action_dispatch/middleware/ssl.rb
+++ b/actionpack/lib/action_dispatch/middleware/ssl.rb
@@ -88,7 +88,7 @@ def call(env)
 
     private
       def set_hsts_header!(headers)
-        headers["Strict-Transport-Security"] ||= @hsts_header
+        headers[Constants::STRICT_TRANSPORT_SECURITY] ||= @hsts_header
       end
 
       def normalize_hsts_options(options)
@@ -114,25 +114,33 @@ def build_hsts_header(hsts)
       end
 
       def flag_cookies_as_secure!(headers)
-        if cookies = headers["Set-Cookie"]
-          if cookies.is_a?(String)
-            cookies = cookies.split("\n")
-          end
+        cookies = headers[Rack::SET_COOKIE]
+        return unless cookies
 
-          headers["Set-Cookie"] = cookies.map { |cookie|
+        if Gem::Version.new(Rack::RELEASE) < Gem::Version.new("3")
+          cookies = cookies.split("\n")
+          headers[Rack::SET_COOKIE] = cookies.map { |cookie|
             if !/;\s*secure\s*(;|$)/i.match?(cookie)
               "#{cookie}; secure"
             else
               cookie
             end
           }.join("\n")
+        else
+          headers[Rack::SET_COOKIE] = Array(cookies).map do |cookie|
+            if !/;\s*secure\s*(;|$)/i.match?(cookie)
+              "#{cookie}; secure"
+            else
+              cookie
+            end
+          end
         end
       end
 
       def redirect_to_https(request)
         [ @redirect.fetch(:status, redirection_status(request)),
-          { "Content-Type" => "text/html; charset=utf-8",
-            "Location" => https_location_for(request) },
+          { Rack::CONTENT_TYPE => "text/html; charset=utf-8",
+            Constants::LOCATION => https_location_for(request) },
           (@redirect[:body] || []) ]
       end
 
diff --git a/actionpack/test/dispatch/ssl_test.rb b/actionpack/test/dispatch/ssl_test.rb
index 5353081fd2b97..a1da495481106 100644
--- a/actionpack/test/dispatch/ssl_test.rb
+++ b/actionpack/test/dispatch/ssl_test.rb
@@ -3,13 +3,16 @@
 require "abstract_unit"
 
 class SSLTest < ActionDispatch::IntegrationTest
-  HEADERS = { "Content-Type" => "text/html" }
-
   attr_accessor :app
 
   def build_app(headers: {}, ssl_options: {})
-    headers = HEADERS.merge(headers)
-    ActionDispatch::SSL.new lambda { |env| [200, headers, []] }, **ssl_options.reverse_merge(hsts: { subdomains: true })
+    app = lambda { |env| [200, headers, []] }
+    Rack::Lint.new(
+      ActionDispatch::SSL.new(
+        Rack::Lint.new(app),
+        **ssl_options.reverse_merge(hsts: { subdomains: true }),
+      )
+    )
   end
 end
 
@@ -128,9 +131,9 @@ def assert_hsts(expected, url: "https://example.org", hsts: { subdomains: true }
     self.app = build_app ssl_options: { hsts: hsts }, headers: headers
     get url
     if expected.nil?
-      assert_nil response.headers["Strict-Transport-Security"]
+      assert_nil response.headers["strict-transport-security"]
     else
-      assert_equal expected, response.headers["Strict-Transport-Security"]
+      assert_equal expected, response.headers["strict-transport-security"]
     end
   end
 
@@ -143,7 +146,8 @@ def assert_hsts(expected, url: "https://example.org", hsts: { subdomains: true }
   end
 
   test "defers to app-provided header" do
-    assert_hsts "app-provided", headers: { "Strict-Transport-Security" => "app-provided" }
+    headers = { ActionDispatch::Constants::STRICT_TRANSPORT_SECURITY => "app-provided" }
+    assert_hsts "app-provided", headers: headers
   end
 
   test "hsts: true enables default settings" do
@@ -180,82 +184,94 @@ def assert_hsts(expected, url: "https://example.org", hsts: { subdomains: true }
 end
 
 class SecureCookiesTest < SSLTest
-  DEFAULT = %(id=1; path=/\ntoken=abc; path=/; secure; HttpOnly)
-
-  def get(**options)
-    self.app = build_app(**options)
-    super "https://example.org"
-  end
-
-  def assert_cookies(*expected)
-    assert_equal expected, response.headers["Set-Cookie"].split("\n")
+  DEFAULT = if Gem::Version.new(Rack::RELEASE) < Gem::Version.new("3")
+    %(id=1; path=/\ntoken=abc; path=/; secure; HttpOnly)
+  else
+    ["id=1; path=/", "token=abc; path=/; secure; HttpOnly"]
   end
 
   def test_flag_cookies_as_secure
-    get headers: { "Set-Cookie" => DEFAULT }
+    get headers: { Rack::SET_COOKIE => DEFAULT }
     assert_cookies "id=1; path=/; secure", "token=abc; path=/; secure; HttpOnly"
   end
 
-  def test_flag_cookies_as_secure_with_single_cookie_in_array
-    get headers: { "Set-Cookie" => ["id=1"] }
-    assert_cookies "id=1; secure"
-  end
-
-  def test_flag_cookies_as_secure_with_multiple_cookies_in_array
-    get headers: { "Set-Cookie" => ["id=1", "problem=def"] }
-    assert_cookies "id=1; secure", "problem=def; secure"
-  end
-
   def test_flag_cookies_as_secure_at_end_of_line
-    get headers: { "Set-Cookie" => "problem=def; path=/; HttpOnly; secure" }
+    get headers: { Rack::SET_COOKIE => "problem=def; path=/; HttpOnly; secure" }
     assert_cookies "problem=def; path=/; HttpOnly; secure"
   end
 
   def test_flag_cookies_as_secure_with_more_spaces_before
-    get headers: { "Set-Cookie" => "problem=def; path=/; HttpOnly;  secure" }
+    get headers: { Rack::SET_COOKIE => "problem=def; path=/; HttpOnly;  secure" }
     assert_cookies "problem=def; path=/; HttpOnly;  secure"
   end
 
   def test_flag_cookies_as_secure_with_more_spaces_after
-    get headers: { "Set-Cookie" => "problem=def; path=/; secure;  HttpOnly" }
+    get headers: { Rack::SET_COOKIE => "problem=def; path=/; secure;  HttpOnly" }
     assert_cookies "problem=def; path=/; secure;  HttpOnly"
   end
 
   def test_flag_cookies_as_secure_with_has_not_spaces_before
-    get headers: { "Set-Cookie" => "problem=def; path=/;secure; HttpOnly" }
+    get headers: { Rack::SET_COOKIE => "problem=def; path=/;secure; HttpOnly" }
     assert_cookies "problem=def; path=/;secure; HttpOnly"
   end
 
   def test_flag_cookies_as_secure_with_has_not_spaces_after
-    get headers: { "Set-Cookie" => "problem=def; path=/; secure;HttpOnly" }
+    get headers: { Rack::SET_COOKIE => "problem=def; path=/; secure;HttpOnly" }
     assert_cookies "problem=def; path=/; secure;HttpOnly"
   end
 
   def test_flag_cookies_as_secure_with_ignore_case
-    get headers: { "Set-Cookie" => "problem=def; path=/; Secure; HttpOnly" }
+    get headers: { Rack::SET_COOKIE => "problem=def; path=/; Secure; HttpOnly" }
     assert_cookies "problem=def; path=/; Secure; HttpOnly"
   end
 
   def test_cookies_as_not_secure_with_secure_cookies_disabled
-    get headers: { "Set-Cookie" => DEFAULT }, ssl_options: { secure_cookies: false }
-    assert_cookies(*DEFAULT.split("\n"))
+    get headers: { Rack::SET_COOKIE => DEFAULT }, ssl_options: { secure_cookies: false }
+    assert_cookies("id=1; path=/", "token=abc; path=/; secure; HttpOnly")
   end
 
   def test_cookies_as_not_secure_with_exclude
     excluding = { exclude: -> request { /example/.match?(request.domain) } }
-    get headers: { "Set-Cookie" => DEFAULT }, ssl_options: { redirect: excluding }
+    get headers: { Rack::SET_COOKIE => DEFAULT }, ssl_options: { redirect: excluding }
 
-    assert_cookies(*DEFAULT.split("\n"))
+    assert_cookies("id=1; path=/", "token=abc; path=/; secure; HttpOnly")
     assert_response :ok
   end
 
   def test_no_cookies
     get
-    assert_nil response.headers["Set-Cookie"]
+    assert_nil response.headers[Rack::SET_COOKIE]
   end
 
   def test_keeps_original_headers_behavior
-    get headers: { "Connection" => "close" }
-    assert_equal "close", response.headers["Connection"]
+    get headers: { "connection" => "close" }
+    assert_equal "close", response.headers["connection"]
   end
+
+  # Array-based headers are only supported in Rack 3+
+  if Gem::Version.new(Rack::RELEASE) >= Gem::Version.new("3")
+    def test_flag_cookies_as_secure_with_single_cookie_in_array
+      get headers: { Rack::SET_COOKIE => ["id=1"] }
+      assert_cookies "id=1; secure"
+    end
+
+    def test_flag_cookies_as_secure_with_multiple_cookies_in_array
+      get headers: { Rack::SET_COOKIE => ["id=1", "problem=def"] }
+      assert_cookies "id=1; secure", "problem=def; secure"
+    end
+  end
+
+  private
+    def get(**options)
+      self.app = build_app(**options)
+      super "https://example.org"
+    end
+
+    def assert_cookies(*expected)
+      cookies = response.headers[Rack::SET_COOKIE]
+      if Gem::Version.new(Rack::RELEASE) < Gem::Version.new("3")
+        cookies = cookies.split("\n")
+      end
+      assert_equal expected, cookies
+    end
 end