diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 462c8f3e7df..a36efcf8e2f 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -1,4 +1,7 @@
+require 'core_extensions/string/permit'
+
 class ApplicationController < ActionController::Base
+  String.include CoreExtensions::String::Permit
   include UserSessionContext
   include VerifyProfileConcern
   include LocaleHelper
diff --git a/app/controllers/sign_up/email_resend_controller.rb b/app/controllers/sign_up/email_resend_controller.rb
index 5a46b03636d..2ee962a5af0 100644
--- a/app/controllers/sign_up/email_resend_controller.rb
+++ b/app/controllers/sign_up/email_resend_controller.rb
@@ -1,7 +1,5 @@
 module SignUp
   class EmailResendController < ApplicationController
-    include UnconfirmedUserConcern
-
     def new
       @user = User.new
       @resend_email_confirmation_form = ResendEmailConfirmationForm.new
diff --git a/lib/core_extensions/string/permit.rb b/lib/core_extensions/string/permit.rb
new file mode 100644
index 00000000000..37fb341ebd4
--- /dev/null
+++ b/lib/core_extensions/string/permit.rb
@@ -0,0 +1,18 @@
+# When a controller uses Strong Parameters such as:
+# params.require(:user).permit(:email), the `user` param is assumed to
+# be a Hash, but it's easy for a pentester (for example) to set the
+# `user` param to a String instead, which by default would raise a 500
+# error because the String class doesn't respond to `permit`. To get
+# around that, we monkey patched the Ruby String class to raise an
+# instance of ActionController::ParameterMissing, which will return
+# a 400 error. 500 errors can potentially page people in the middle of
+# the night, whereas 400 errors don't.
+module CoreExtensions
+  module String
+    module Permit
+      def permit(*)
+        raise ActionController::ParameterMissing, '#permit called on String'
+      end
+    end
+  end
+end
diff --git a/spec/requests/invalid_sign_in_params_spec.rb b/spec/requests/invalid_sign_in_params_spec.rb
index c4721221143..b42a0196ea2 100644
--- a/spec/requests/invalid_sign_in_params_spec.rb
+++ b/spec/requests/invalid_sign_in_params_spec.rb
@@ -1,8 +1,12 @@
 require 'rails_helper'
 
 describe 'visiting sign in page with invalid user params' do
-  it 'does not raise an exception' do
-    get new_user_session_path, params: { user: 'test@test.com' }
+  it 'raises ActionController::ParameterMissing' do
+    params = { user: 'test@test.com' }
+    message_string = 'param is missing or the value is empty: #permit called on String'
+
+    expect { get new_user_session_path, params: params }.
+      to raise_error(ActionController::ParameterMissing, message_string)
   end
 end
 
diff --git a/spec/requests/params_is_string_instead_of_hash_spec.rb b/spec/requests/params_is_string_instead_of_hash_spec.rb
new file mode 100644
index 00000000000..8c265907ce8
--- /dev/null
+++ b/spec/requests/params_is_string_instead_of_hash_spec.rb
@@ -0,0 +1,30 @@
+require 'rails_helper'
+
+# When a controller uses Strong Parameters such as:
+# params.require(:user).permit(:email), the `user` param is assumed to
+# be a Hash, but it's easy for a pentester (for example) to set the
+# `user` param to a String instead, which by default would raise a 500
+# error because the String class doesn't respond to `permit`. To get
+# around that, we monkey patched the Ruby String class to raise an
+# instance of ActionController::ParameterMissing, which will return
+# a 400 error. 500 errors can potentially page people in the middle of
+# the night, whereas 400 errors don't.
+describe 'submitting email resend form with required param as String' do
+  it 'raises ActionController::ParameterMissing' do
+    params = { resend_email_confirmation_form: 'abcdef' }
+    message_string = 'param is missing or the value is empty: #permit called on String'
+
+    expect { post sign_up_create_email_resend_path, params: params }.
+      to raise_error(ActionController::ParameterMissing, message_string)
+  end
+end
+
+describe 'submitting email registration form with required param as String' do
+  it 'raises ActionController::ParameterMissing' do
+    params = { user: 'abcdef' }
+    message_string = 'param is missing or the value is empty: #permit called on String'
+
+    expect { post sign_up_register_path, params: params }.
+      to raise_error(ActionController::ParameterMissing, message_string)
+  end
+end