Only create :params, :cookies, :session Sexps once

lib/brakeman/util.rb

@@ -96,12 +96,17 @@ module Brakeman::Util
     nil
   end
 
+  #These are never modified
+  PARAMS_SEXP = Sexp.new(:params)
+  SESSION_SEXP = Sexp.new(:session)
+  COOKIES_SEXP = Sexp.new(:cookies)
+
   #Adds params, session, and cookies to environment
   #so they can be replaced by their respective Sexps.
   def set_env_defaults
-    @env[PARAMETERS] = Sexp.new(:params)
-    @env[SESSION] = Sexp.new(:session)
-    @env[COOKIES] = Sexp.new(:cookies)
+    @env[PARAMETERS] = PARAMS_SEXP
+    @env[SESSION] = SESSION_SEXP
+    @env[COOKIES] = COOKIES_SEXP
   end
 
   #Check if _exp_ represents a hash: s(:hash, {...})