Be more specific with Nokogiri check

There are still strong reservations about using Nokogiri to parse
untrusted XML data.

http://www.wireharbor.com/hidden-security-risks-of-xml-parsing-xxe-attack/

It is also believed that many desktop operating systems are still
shipping out-of-date and vulnerable libxml2 libraries, which become
exposed via Nokogiri. For example:

http://stackoverflow.com/questions/18627075/nokogiri-1-6-0-still-pulls-in-wrong-version-of-libxml-on-os-x

While this isn't a problem for binary builds of Metasploit (Metasploit
Community, Express, or Pro) it can be a problem for development
versions or Kali's / Backtrack's version.

So, the compromise here is to allow for modules that don't directly
expose XML parsing. I can't say for sure that the various libxml2
vulnerabilities (current and future) aren't also exposed via
`Nokogiri::HTML` but I also can't come up with a reasonable demo.

Metasploit committers should still look at any module that relies on
Nokogiri very carefully, and suggest alternatives if there are any. But,
it's sometimes going to be required for complex HTML parsing.

tl;dr: Use REXML for XML parsing, and Nokogiri for HTML parsing if you
absolutely must.

tools/msftidy.rb

@@ -113,16 +113,32 @@ class Msftidy
     end
   end
 
+  # Updated this check to see if Nokogiri::XML.parse is being called
+  # specifically. The main reason for this concern is that some versions
+  # of libxml2 are still vulnerable to XXE attacks. REXML is safer (and
+  # slower) since it's pure ruby. Unfortunately, there is no pure Ruby
+  # HTML parser (except Hpricot which is abandonware) -- easy checks
+  # can avoid Nokogiri (most modules use regex anyway), but more complex
+  # checks tends to require Nokogiri for HTML element and value parsing.
   def check_nokogiri
-    msg = "Requiring Nokogiri in modules can be risky, use REXML instead."
+    msg = "Using Nokogiri in modules can be risky, use REXML instead."
     has_nokogiri = false
+    has_nokogiri_xml_parser = false
     @source.each_line do |line|
       if line =~ /^\s*(require|load)\s+['"]nokogiri['"]/
         has_nokogiri = true
         break
       end
     end
-    error(msg) if has_nokogiri
+    if has_nokogiri
+      @source.each_line do |line|
+        if line =~ /Nokogiri::XML.parse/ or line =~ /Nokogiri::XML::Reader/
+          has_nokogiri_xml_parser = true
+          break
+        end
+      end
+    end
+    error(msg) if has_nokogiri_xml_parser
   end
 
   def check_ref_identifiers