Re: Best XML library to validate XML from untrusted source

I don't see how XXE relates to doing XML validation that couldn't be addressed by limiting memory and CPU usage (which you would need to do either way), but you can nullify it by using XML::LibXML's load_ext_dtd and expand_entities options (as mentioned in the document you linked, under the heading "libxml2").

Rather than loading the entire document into memory, you'd want to use XML::LibXML's pull interface, XML::LibXML::Reader.

$ cat a.xml
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
   <!ELEMENT foo ANY >
   <!ENTITY xxe SYSTEM "file:///etc/passwd" >]><foo>&xxe;</foo>

$ perl -MXML::LibXML::Reader -e'
   my $reader = XML::LibXML::Reader->new(
      location        => $ARGV[0],
      load_ext_dtd    => 0,
      expand_entities => 0,
   );
   while ($reader->read) {
      printf("%d %d %s\n",
         $reader->depth,
         $reader->nodeType,
         $reader->name,
      );
   }
' a.xml
0 10 foo
0 1 foo
1 5 xxe
0 15 foo
[download]

$ cat bad.xml
<foo><bar></foo>

$ perl -MXML::LibXML::Reader -e'
   exit 1 if !eval {
      my $reader = XML::LibXML::Reader->new(
         location        => $ARGV[0],
         load_ext_dtd    => 0,
         expand_entities => 0,
      );
      1 while $reader->read;
      1
  };
' bad.xml

$ if [ $? -eq 0 ]; then echo "well-formed" ; else echo "error" ; fi
error
[download]

Comment on Re: Best XML library to validate XML from untrusted source Select or Download Code

Replies are listed 'Best First'.
Re^2: Best XML library to validate XML from untrusted source by Jenda (Abbot) on Oct 20, 2014 at 15:07 UTC
XML::LibXML::Reader is way too low-level and while the pull style tends to lead to a (very slightly) more readable code than ordinary, node-level push, it's still nothing I would dare to recommend ... to anyone. XML::Rules and XML::Twig give you the file in bite sized chunks which IMNSHO works much better than forcing a decomposition to individual atoms. Speaking of XML::Rules ... it's based on XML::Parser::Expat and allows setting its handlers so I think setting the Expat's ExternEnt to your handler should provide vsespb with the protection he's after. Jenda Enoch was right! Enjoy the last years of Rome.	[reply]
Re^3: Best XML library to validate XML from untrusted source by ikegami (Patriarch) on Oct 20, 2014 at 15:16 UTC
I have no idea why you wouldn't recommend `use XML::LibXML::Reader qw( ); my $reader = XML::LibXML::Reader->new( location => $file_or_url, load_ext_dtd => 0, expand_entities => 0, ); 1 while $reader->read;` [download] Wrapping this up just so you get something you can call higher-level simply is pure waste.	[reply] [d/l]
Re^4: Best XML library to validate XML from untrusted source by Jenda (Abbot) on Oct 20, 2014 at 20:15 UTC
Say, because it doesn't do anything? I mean, yes, it does some kind of basic format validation, but once you actually need to extract some data out of the file, things start getting complicated very quickly. Jenda Enoch was right! Enjoy the last years of Rome.	[reply]
Re^5: Best XML library to validate XML from untrusted source by ikegami (Patriarch) on Oct 21, 2014 at 17:17 UTC
Re^2: Best XML library to validate XML from untrusted source by vsespb (Chaplain) on Oct 23, 2014 at 12:14 UTC
Thank you for all your replies! Very useful. One note: I don't see how XXE relates to doing XML validation that couldn't be addressed by limiting memory and CPU usage (which you would need to do either way), XXE not just about DoS. For example I have API which accepts requests over XML. There is an API function: create object (with user supplied name). And another function: list all objects with its names. So attacker can create object with name equal to content of /etc/passwd and then list it, this way receive content of /etc/passwd. imho, pretty common case...	[reply]


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