diff --git a/app/controllers/auth/omniauth_callbacks_controller.rb b/app/controllers/auth/omniauth_callbacks_controller.rb
index f9cf6d655..b67823c7f 100644
--- a/app/controllers/auth/omniauth_callbacks_controller.rb
+++ b/app/controllers/auth/omniauth_callbacks_controller.rb
@@ -5,7 +5,7 @@ class Auth::OmniauthCallbacksController < Devise::OmniauthCallbacksController
 
   def self.provides_callback_for(provider)
     define_method provider do
-      @user = User.find_for_oauth(request.env['omniauth.auth'], current_user)
+      @user = User.find_for_omniauth(request.env['omniauth.auth'], current_user)
 
       if @user.persisted?
         LoginActivity.create(
diff --git a/app/models/concerns/omniauthable.rb b/app/models/concerns/omniauthable.rb
index a90d5d888..37d8efd48 100644
--- a/app/models/concerns/omniauthable.rb
+++ b/app/models/concerns/omniauthable.rb
@@ -19,17 +19,18 @@ module Omniauthable
   end
 
   class_methods do
-    def find_for_oauth(auth, signed_in_resource = nil)
+    def find_for_omniauth(auth, signed_in_resource = nil)
       # EOLE-SSO Patch
       auth.uid = (auth.uid[0][:uid] || auth.uid[0][:user]) if auth.uid.is_a? Hashie::Array
-      identity = Identity.find_for_oauth(auth)
+      identity = Identity.find_for_omniauth(auth)
 
       # If a signed_in_resource is provided it always overrides the existing user
       # to prevent the identity being locked with accidentally created accounts.
       # Note that this may leave zombie accounts (with no associated identity) which
       # can be cleaned up at a later date.
       user   = signed_in_resource || identity.user
-      user ||= create_for_oauth(auth)
+      user ||= reattach_for_auth(auth)
+      user ||= create_for_auth(auth)
 
       if identity.user.nil?
         identity.user = user
@@ -39,19 +40,35 @@ module Omniauthable
       user
     end
 
-    def create_for_oauth(auth)
-      # Check if the user exists with provided email. If no email was provided,
+    private
+
+    def reattach_for_auth(auth)
+      # If allowed, check if a user exists with the provided email address,
+      # and return it if they does not have an associated identity with the
+      # current authentication provider.
+
+      # This can be used to provide a choice of alternative auth providers
+      # or provide smooth gradual transition between multiple auth providers,
+      # but this is discouraged because any insecure provider will put *all*
+      # local users at risk, regardless of which provider they registered with.
+
+      return unless ENV['ALLOW_UNSAFE_AUTH_PROVIDER_REATTACH'] == 'true'
+
+      email, email_is_verified = email_from_auth(auth)
+      return unless email_is_verified
+
+      user = User.find_by(email: email)
+      return if user.nil? || Identity.exists?(provider: auth.provider, user_id: user.id)
+
+      user
+    end
+
+    def create_for_auth(auth)
+      # Create a user for the given auth params. If no email was provided,
       # we assign a temporary email and ask the user to verify it on
       # the next step via Auth::SetupController.show
 
-      strategy          = Devise.omniauth_configs[auth.provider.to_sym].strategy
-      assume_verified   = strategy&.security&.assume_email_is_verified
-      email_is_verified = auth.info.verified || auth.info.verified_email || auth.info.email_verified || assume_verified
-      email             = auth.info.verified_email || auth.info.email
-
-      user = User.find_by(email: email) if email_is_verified
-
-      return user unless user.nil?
+      email, email_is_verified = email_from_auth(auth)
 
       user = User.new(user_params_from_auth(email, auth))
 
@@ -61,7 +78,14 @@ module Omniauthable
       user
     end
 
-    private
+    def email_from_auth(auth)
+      strategy          = Devise.omniauth_configs[auth.provider.to_sym].strategy
+      assume_verified   = strategy&.security&.assume_email_is_verified
+      email_is_verified = auth.info.verified || auth.info.verified_email || auth.info.email_verified || assume_verified
+      email             = auth.info.verified_email || auth.info.email
+
+      [email, email_is_verified]
+    end
 
     def user_params_from_auth(email, auth)
       {
diff --git a/app/models/identity.rb b/app/models/identity.rb
index 869f06372..a396d7623 100644
--- a/app/models/identity.rb
+++ b/app/models/identity.rb
@@ -16,7 +16,7 @@ class Identity < ApplicationRecord
   validates :uid, presence: true, uniqueness: { scope: :provider }
   validates :provider, presence: true
 
-  def self.find_for_oauth(auth)
+  def self.find_for_omniauth(auth)
     find_or_create_by(uid: auth.uid, provider: auth.provider)
   end
 end
diff --git a/spec/models/identity_spec.rb b/spec/models/identity_spec.rb
index 689c9b797..081c254d8 100644
--- a/spec/models/identity_spec.rb
+++ b/spec/models/identity_spec.rb
@@ -1,16 +1,16 @@
 require 'rails_helper'
 
 RSpec.describe Identity, type: :model do
-  describe '.find_for_oauth' do
+  describe '.find_for_omniauth' do
     let(:auth) { Fabricate(:identity, user: Fabricate(:user)) }
 
     it 'calls .find_or_create_by' do
       expect(described_class).to receive(:find_or_create_by).with(uid: auth.uid, provider: auth.provider)
-      described_class.find_for_oauth(auth)
+      described_class.find_for_omniauth(auth)
     end
 
     it 'returns an instance of Identity' do
-      expect(described_class.find_for_oauth(auth)).to be_instance_of Identity
+      expect(described_class.find_for_omniauth(auth)).to be_instance_of Identity
     end
   end
 end