Add option forbidden_inline_options

It is sometimes necessary to restrict the options that can be set
inline, ie. using the {::options ...} extensions.

By default, the template option is now forbidden to avoid possible
security problems. This addresses CVE-2020-14001.

lib/kramdown/options.rb

@@ -589,6 +589,16 @@ def self.simple_hash_validator(val, name)
       Used by: HTML converter
     EOF
 
+    define(:forbidden_inline_options, Object, %w[template], <<~EOF) do |val|
+      Defines the options that may not be set using the {::options} extension
+
+      Default: template
+      Used by: HTML converter
+    EOF
+      val.map! {|item| item.kind_of?(String) ? str_to_sym(item) : item }
+      simple_array_validator(val, :forbidden_inline_options)
+    end
+
   end
 
 end

lib/kramdown/parser/kramdown/extensions.rb

@@ -110,6 +110,12 @@ def handle_extension(name, opts, body, type, line_no = nil)
           opts.select do |k, v|
             k = k.to_sym
             if Kramdown::Options.defined?(k)
+              if @options[:forbidden_inline_options].include?(k) ||
+                  k == :forbidden_inline_options
+                warning("Option #{k} may not be set inline")
+                next false
+              end
+
               begin
                 val = Kramdown::Options.parse(k, v)
                 @options[k] = val

test/testcases/block/12_extension/options.text

@@ -19,3 +19,5 @@ some <span>*para*</span>
 Some text[^ab].
 
 [^ab]: Some text.
+
+{::options template="/etc/passwd" /}