Put key in constant

app/controllers/omniauth_kerberos_spnego_controller.rb

+require 'omniauth/strategies/kerberos_spnego'
+
 class OmniauthKerberosSpnegoController < ApplicationController
   include KerberosSpnegoHelper
 
@@ -5,7 +7,7 @@ class OmniauthKerberosSpnegoController < ApplicationController
 
   def negotiate
     if spnego_provided? && (krb_principal = spnego_credentials!(spnego_token))
-      session[:kerberos_spnego_principal_name] = krb_principal
+      session[OmniAuth::Strategies::KerberosSpnego::SESSION_KEY] = krb_principal
       send_final_spnego_response
       redirect_to user_kerberos_spnego_omniauth_callback_path
       return

lib/omniauth/strategies/kerberos_spnego.rb

@@ -6,6 +6,8 @@ module OmniAuth
       include OmniAuth::Strategy
       include Gitlab::Routing.url_helpers
 
+      SESSION_KEY = :kerberos_spnego_principal_name
+
       option :name, 'kerberos_spnego'
 
       uid { username }
@@ -25,7 +27,7 @@ module OmniAuth
       def principal_name
         return @principal_name if defined?(@principal_name)
 
-        @principal_name = session.delete(:kerberos_spnego_principal_name)
+        @principal_name = session.delete(SESSION_KEY)
       end
 
       def request_phase