Fix for a security issue in the REXML library from the ruby-core list:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/18414
https://bugs.gentoo.org/show_bug.cgi?id=236060

Index: lib/rexml/document.rb
===================================================================
--- lib/rexml/document.rb	(revision 18834)
+++ lib/rexml/document.rb	(working copy)
@@ -32,6 +32,7 @@
 	  # @param context if supplied, contains the context of the document;
 	  # this should be a Hash.
 		def initialize( source = nil, context = {} )
+      @entity_expansion_count = 0
 			super()
 			@context = context
 			return if source.nil?
@@ -200,6 +201,27 @@
 			Parsers::StreamParser.new( source, listener ).parse
 		end
 
+    @@entity_expansion_limit = 10_000
+
+    # Set the entity expansion limit. By defualt the limit is set to 10000.
+    def Document::entity_expansion_limit=( val )
+      @@entity_expansion_limit = val
+    end
+
+    # Get the entity expansion limit. By defualt the limit is set to 10000.
+    def Document::entity_expansion_limit
+      return @@entity_expansion_limit
+    end
+
+    attr_reader :entity_expansion_count
+    
+    def record_entity_expansion
+      @entity_expansion_count += 1
+      if @entity_expansion_count > @@entity_expansion_limit
+        raise "number of entity expansions exceeded, processing aborted."
+      end
+    end
+
 		private
 		def build( source )
       Parsers::TreeParser.new( source, self ).parse
Index: lib/rexml/entity.rb
===================================================================
--- lib/rexml/entity.rb	(revision 18834)
+++ lib/rexml/entity.rb	(working copy)
@@ -73,6 +73,7 @@
 		# all entities -- both %ent; and &ent; entities.  This differs from
 		# +value()+ in that +value+ only replaces %ent; entities.
 		def unnormalized
+      document.record_entity_expansion unless document.nil?
 			v = value()
 			return nil if v.nil?
 			@unnormalized = Text::unnormalize(v, parent)
Index: test/rexml/test_document.rb
===================================================================
--- test/rexml/test_document.rb	(revision 0)
+++ test/rexml/test_document.rb	(revision 0)
@@ -0,0 +1,42 @@
+require "rexml/document"
+require "test/unit"
+
+class REXML::TestDocument < Test::Unit::TestCase
+  def test_new
+    doc = REXML::Document.new(<<EOF)
+<?xml version="1.0" encoding="UTF-8"?>
+<message>Hello world!</message>
+EOF
+    assert_equal("Hello world!", doc.root.children.first.value)
+  end
+
+  XML_WITH_NESTED_ENTITY = <<EOF
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE member [
+  <!ENTITY a "&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;">
+  <!ENTITY b "&c;&c;&c;&c;&c;&c;&c;&c;&c;&c;">
+  <!ENTITY c "&d;&d;&d;&d;&d;&d;&d;&d;&d;&d;">
+  <!ENTITY d "&e;&e;&e;&e;&e;&e;&e;&e;&e;&e;">
+  <!ENTITY e "&f;&f;&f;&f;&f;&f;&f;&f;&f;&f;">
+  <!ENTITY f "&g;&g;&g;&g;&g;&g;&g;&g;&g;&g;">
+  <!ENTITY g "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx">
+]>
+<member>
+&a;
+</member>
+EOF
+
+  def test_entity_expansion_limit
+    doc = REXML::Document.new(XML_WITH_NESTED_ENTITY)
+    assert_raise(RuntimeError) do
+      doc.root.children.first.value
+    end
+    REXML::Document.entity_expansion_limit = 100
+    assert_equal(100, REXML::Document.entity_expansion_limit)
+    doc = REXML::Document.new(XML_WITH_NESTED_ENTITY)
+    assert_raise(RuntimeError) do
+      doc.root.children.first.value
+    end
+    assert_equal(101, doc.entity_expansion_count)
+  end
+end