Check and handle when a tainted variable is used

lib/liquid/errors.rb

@@ -54,5 +54,6 @@ module Liquid
   class StandardError < Error; end
   class SyntaxError < Error; end
   class StackLevelError < Error; end
+  class TaintedError < Error; end
   class MemoryError < Error; end
 end

lib/liquid/template.rb

@@ -60,6 +60,12 @@ module Liquid
       # :strict will enforce correct syntax.
       attr_writer :error_mode
 
+      # Sets how strict the taint checker should be.
+      # :lax ignores the taint flag completely (like previous liquid versions)
+      # :warn adds a warning, but does not interrupt the rendering
+      # :error raises an error when tainted output is used
+      attr_writer :taint_mode
+
       def file_system
         @@file_system
       end
@@ -80,6 +86,10 @@ module Liquid
         @error_mode || :lax
       end
 
+      def taint_mode
+        @taint_mode || :lax
+      end
+
       # Pass a module with filter methods which should be available
       # to all liquid views. Good for registering the standard library
       def register_filter(mod)

lib/liquid/variable.rb

@@ -94,6 +94,16 @@ module Liquid
         end
         filterargs << keyword_args unless keyword_args.empty?
         output = context.invoke(filter[0], output, *filterargs)
+      end.tap do |obj|
+        if obj.tainted?
+          case Template.taint_mode
+          when :warn
+            @warnings ||= []
+            @warnings << "variable '#{@name}' is tainted and was not escaped"
+          when :error
+            raise TaintedError, "Error - variable '#{@name}' is tainted and was not escaped"
+          end
+        end
       end
     end
   end