Fix invalid forwarded host vulnerability

Prior to this commit, it was possible to pass an unvalidated host
through the `X-Forwarded-Host` header. If the value of the header
was prefixed with a invalid domain character (for example a `/`),
it was always accepted as the actual host of that request.

Since this host is used for all url helpers, an attacker could change
generated links and redirects. If the header is set to
`X-Forwarded-Host: //evil.hacker`, a redirect will be send to
`https:////evil.hacker/`. Browsers will ignore these four slashes
and redirect the user.

[CVE-2021-44528]

actionpack/lib/action_dispatch/middleware/host_authorization.rb

@@ -52,7 +52,7 @@ def sanitize_regexp(host)
 
         def sanitize_string(host)
           if host.start_with?(".")
-            /\A(.+\.)?#{Regexp.escape(host[1..-1])}\z/i
+            /\A([a-z0-9-]+\.)?#{Regexp.escape(host[1..-1])}\z/i
           else
             /\A#{Regexp.escape host}\z/i
           end
@@ -120,13 +120,9 @@ def call(env)
     end
 
     private
-      HOSTNAME = /[a-z0-9.-]+|\[[a-f0-9]*:[a-f0-9.:]+\]/i
-      VALID_ORIGIN_HOST = /\A(#{HOSTNAME})(?::\d+)?\z/
-      VALID_FORWARDED_HOST = /(?:\A|,[ ]?)(#{HOSTNAME})(?::\d+)?\z/
-
       def authorized?(request)
-        origin_host = request.get_header("HTTP_HOST")&.slice(VALID_ORIGIN_HOST, 1) || ""
-        forwarded_host = request.x_forwarded_host&.slice(VALID_FORWARDED_HOST, 1) || ""
+        origin_host = request.get_header("HTTP_HOST")
+        forwarded_host = request.x_forwarded_host&.split(/,\s?/)&.last
 
         @permissions.allows?(origin_host) && (forwarded_host.blank? || @permissions.allows?(forwarded_host))
       end

actionpack/test/dispatch/host_authorization_test.rb

@@ -167,6 +167,44 @@ class HostAuthorizationTest < ActionDispatch::IntegrationTest
     assert_match "Blocked host: 127.0.0.1", response.body
   end
 
+  test "blocks requests with spoofed relative X-FORWARDED-HOST" do
+    @app = ActionDispatch::HostAuthorization.new(App, ["www.example.com"])
+
+    get "/", env: {
+      "HTTP_X_FORWARDED_HOST" => "//randomhost.com",
+      "HOST" => "www.example.com",
+      "action_dispatch.show_detailed_exceptions" => true
+    }
+
+    assert_response :forbidden
+    assert_match "Blocked host: //randomhost.com", response.body
+  end
+
+  test "forwarded secondary hosts are allowed when permitted" do
+    @app = ActionDispatch::HostAuthorization.new(App, ".domain.com")
+
+    get "/", env: {
+      "HTTP_X_FORWARDED_HOST" => "example.com, my-sub.domain.com",
+      "HOST" => "domain.com",
+    }
+
+    assert_response :ok
+    assert_equal "Success", body
+  end
+
+  test "forwarded secondary hosts are blocked when mismatch" do
+    @app = ActionDispatch::HostAuthorization.new(App, "domain.com")
+
+    get "/", env: {
+      "HTTP_X_FORWARDED_HOST" => "domain.com, evil.com",
+      "HOST" => "domain.com",
+      "action_dispatch.show_detailed_exceptions" => true
+    }
+
+    assert_response :forbidden
+    assert_match "Blocked host: evil.com", response.body
+  end
+
   test "does not consider IP addresses in X-FORWARDED-HOST spoofed when disabled" do
     @app = ActionDispatch::HostAuthorization.new(App, nil)
 
@@ -205,18 +243,67 @@ class HostAuthorizationTest < ActionDispatch::IntegrationTest
     assert_match "Blocked host: sub.domain.com", response.body
   end
 
+  test "sub-sub domains should not be permitted" do
+    @app = ActionDispatch::HostAuthorization.new(App, ".domain.com")
+
+    get "/", env: {
+      "HOST" => "secondary.sub.domain.com",
+      "action_dispatch.show_detailed_exceptions" => true
+    }
+
+    assert_response :forbidden
+    assert_match "Blocked host: secondary.sub.domain.com", response.body
+  end
+
   test "forwarded hosts are allowed when permitted" do
     @app = ActionDispatch::HostAuthorization.new(App, ".domain.com")
 
     get "/", env: {
-      "HTTP_X_FORWARDED_HOST" => "sub.domain.com",
+      "HTTP_X_FORWARDED_HOST" => "my-sub.domain.com",
       "HOST" => "domain.com",
     }
 
     assert_response :ok
     assert_equal "Success", body
   end
 
+  test "lots of NG hosts" do
+    ng_hosts = [
+      "hacker%E3%80%82com",
+      "hacker%00.com",
+      "[email protected]",
+      "hacker.com/test/",
+      "hacker%252ecom",
+      ".hacker.com",
+      "/\/\/hacker.com/",
+      "/hacker.com",
+      "../hacker.com",
+      ".hacker.com",
+      "@hacker.com",
+      "hacker.com",
+      "hacker.com%[email protected]",
+      "hacker.com/.jpg",
+      "hacker.com\texample.com/",
+      "hacker.com/example.com",
+      "hacker.com\@example.com",
+      "hacker.com/example.com",
+      "hacker.com/"
+    ]
+
+    @app = ActionDispatch::HostAuthorization.new(App, "example.com")
+
+    ng_hosts.each do |host|
+      get "/", env: {
+        "HTTP_X_FORWARDED_HOST" => host,
+        "HOST" => "example.com",
+        "action_dispatch.show_detailed_exceptions" => true
+      }
+
+      assert_response :forbidden
+      assert_match "Blocked host: #{host}", response.body
+    end
+  end
+
   test "exclude matches allow any host" do
     @app = ActionDispatch::HostAuthorization.new(App, "only.com", exclude: ->(req) { req.path == "/foo" })