Prevent DTD download when parsing XML

Question

When using XmlDocument.Load , I am finding that if the document refers to a DTD, a connection is made to the provided URI. Is there any way to prevent this from happening?

Answer 1

Try something like this:

XmlDocument doc = new XmlDocument();
using (StringReader sr = new StringReader(xml))
  using (XmlReader reader = XmlReader.Create(sr, new XmlReaderSettings()))
  {
     doc.Load(reader);
  }

The thing to note here is that XmlReaderSettings has the ProhibitDtd property set to true by default.

Answer 2

This is actually a flaw in the XML specifications. The W3C is bemoaning that people all hit their servers like mad to load schemas billions of times. Unfortunately just about no standard XML library gets this right, they all hit the servers over and over again.

The problem with DTDs is particularly serious, because DTDs may include general entity declarations (for things like & -> &) which the XML file may actually rely upon. So if your parser chooses to forgo loading the DTD, and the XML makes use of general entity references, parsing may actually fail.

The only solution to this problem would be a transparent caching entity resolver, which would put the downloaded files into some archive in the library search path, so that this archive would be dynamically created and almost automatically bundled with any software distributions made. But even in the Java world there is not one decent such EntityResolver floating about, certainly not built-in to anything from apache foundation.

Answer 3

The document being loaded HAS a DTD.

With:

settings.ProhibitDtd = true;

I see the following exception:

Service cannot be started. System.Xml.XmlException: For security reasons DTD is prohibited in this XML document. To enable DTD processing set the ProhibitDtd property on XmlReaderSettings to false and pass the settings into XmlReader.Create method.

So, it looks like ProhibitDtd MUST be set to true in this instance.

It looked like ValidationType would do the trick, but with:

settings.ValidationType = ValidationType.None;

I'm still seeing a connection to the DTD uri.

Answer 4

After some more digging, maybe you should set the XmlResolver property of the XmlReaderSettings object to null.

'The XmlResolver is used to locate and open an XML instance document, or to locate and open any external resources referenced by the XML instance document. This can include entities, DTD, or schemas.'

So the code would look like this:

        XmlReaderSettings settings = new XmlReaderSettings();
        settings.XmlResolver = null;
        settings.DtdProcessing = DtdProcessing.Parse;
        XmlDocument doc = new XmlDocument();
        using (StringReader sr = new StringReader(xml))
            using (XmlReader reader = XmlReader.Create(sr, settings))
            {
                doc.Load(reader);
            }

Answer 5

Use an XMLReader to load the document and set the ValidationType property of the reader settings to None.