Develop Reference

XML XSD validation. When it occurs

I cannot understand when validation of XML occurs on Load or on Validate. Here is following code...
XmlDocument doc = null;
try
{
XmlReaderSettings settings = new XmlReaderSettings( );
settings.Schemas.Add("http://xxx/customs/DealFile/Common/ReleaseGoodsMessage",
ConfigurationManager.AppSettings.Get("Schemas"));
settings.ValidationType = ValidationType.Schema;
using (XmlReader reader = XmlReader.Create(path, settings)) {
doc = new XmlDocument( );
doc.Load(reader);
}
ValidationEventHandler eventHandler = new ValidationEventHandler(ValidationEventHandler);
doc.Validate(eventHandler);
}
catch(XmlSchemaException xmlErr)
{
// Do something
}
I expect a validation to occur on line doc.Validate(eventHandler);
However it always occurs on doc.Load(reader); . I've got an exception if something wrong with XML.
XMLHelpers.LoadXML(#"C:\work\Xml2Db\Xml2Db\Data\Tests\BadData\01.xml")
Exception thrown: 'System.Xml.Schema.XmlSchemaValidationException' in System.Xml.dll
xmlErr.Message
"The 'http://xxx/customs/DealFile/Common/ReleaseGoodsMessage:governmentProcedureType' element is invalid -
The value 'a' is invalid according to its datatype 'Int' - The string 'a' is not a valid Int32 value."
And this is the code from Microsoft's example https://learn.microsoft.com/en-us/dotnet/api/system.xml.xmldocument.validate?view=netcore-3.1
try
{
XmlReaderSettings settings = new XmlReaderSettings();
settings.Schemas.Add("http://www.contoso.com/books", "contosoBooks.xsd");
settings.ValidationType = ValidationType.Schema;
XmlReader reader = XmlReader.Create("contosoBooks.xml", settings);
XmlDocument document = new XmlDocument();
document.Load(reader);
ValidationEventHandler eventHandler = new ValidationEventHandler(ValidationEventHandler);
// the following call to Validate succeeds.
document.Validate(eventHandler);
...
It's actually the same.
But, pay attention on comment // the following call to Validate succeeds. . They also expect to get validation on the line document.Validate(eventHandler);
What's going on.

As your block of code sets up the settings object, it sets a schema and the Validator to use ValidationType.Schema (i.e.: use the schema).
When you setup the XmlReader, using your settings it's setup to validate according to the schema, too - which is causing your schema-based error/exception.
The call to document.Validate(eventHandler); is completely redundant, because it will succeed in all circumstances - because the xml has already been validated. The comment is correct "the following call to Validate succeeds" because the document has already been proved valid.

I suspect that you are failing to distinguish between XML that is well-formed and XML that is valid.
A well-formed XML document satisfies all of the rules of the XML specification. If it does not, you should get a well-formedness error from any XML parser.
If you also choose to
a) supply an XSD that describes you XML document and
b) tell your XML processor to validate against that XSD
then the XML processor will also check that the document satisfies the rules in the XML schema (an XML Schema is composed of one or more XSDs).
If you are still not sure, edit your question and supply the error message(s) that you are seeing. You don't need to include any confidential information - the error template is enough to tell which kind of error it is.

C# Load XML file and escape characters during runtime

I'm trying to load a remote XML file using C# but there are unescaped characters (e.g. '\', '&') that are preventing the file from being loaded by my program since I keep getting the error
"An error occurred while parsing EntityName"
Ideally I want to load the XML file and replace these characters during runtime so that the file is read properly.
String URLString = "REMOTE XML FILE PATH";
XmlTextReader reader = new XmlTextReader(URLString);
XmlDocument xmlDoc = new XmlDocument();
// This is where the error is occurring
xmlDoc.Load(URLString);
Any help would be greatly appreciated!

XDocument.Parse: Avoid replacing XXE references

I'm trying to protect against malicious XXE injections in the XMLs processed by my app. Therefore I'm using XDocument instead of XmlDocument.
The XML represents the payload of a web request so I call XDocument.Parse on its string content. However, I'm seeing the XXE references contained in the XML (&XXE) being replaced in the result with the actual value of ENTITY xxe.
Is it possible to parse the XML with XDocument without replacing &xxe ?
Thanks
EDIT:
I managed to avoid the replacement of xxes in the XML using XmlResolver=null for XDocument.Load

Instead of Parse try to use Load with a pre-configured reader:
var xdoc = XDocument.Load(new XmlTextReader(
new StringReader(xmlContent)) { EntityHandling = EntityHandling.ExpandCharEntities });
From MSDN:
When EntityHandling is set to ExpandCharEntities, the reader expands character entities and returns general entities as EntityReference nodes.

Use the following example to stop resolving XXE (schemas and DTD).
Dim objXmlReader As System.Xml.XmlTextReader = Nothing
objXmlReader = New System.Xml.XmlTextReader(_patternFilePath)
objXmlReader.XmlResolver = Nothing
patternDocument = XDocument.Load(objXmlReader)

XslCompiledTransform cant transform with XmlTestReader created from string

I have a problem with XslCompiledTransform class.
If I tried to run this code:
string pathToXsltFile, pathToInputFile, pathToOutputFile;
XsltSettings xsltSettings = new XsltSettings(true, true);
XslCompiledTransform myXslTransform = new XslCompiledTransform();
XmlTextReader reader = new XmlTextReader(pathToFile);
myXslTransform.Load(reader, xsltSettings, new XmlUrlResolver());
myXslTransform.Transform(pathToInputFile, pathToOutputFile);
It works fine.
But if I want to create XmlTextReader from a string (text):
MemoryStream mStrm = new System.IO.MemoryStream(Encoding.UTF8.GetBytes(text));
XmlTextReader xmlReader = new XmlTextReader(mStrm);
mStrm.Position = 0;
And try to run:
myXslTransform.Load(xmlReader, xsltSettings, new XmlUrlResolver());
myXslTransform.Transform(pathToInputFile, pathToOutputFile);
I get a Exception:
"this operation is not supported for a relative uri"
For some reasons I don't want to create temporaty file and create XmlTextReader from path to this file.
Edit:
Full exception message:
"An error occurred while loading document ''.
See InnerException for a complete description of the error."
InnerException.Message:
"This operation is not supported for a relative URI."
Stack trace:
at System.Xml.Xsl.Runtime.XmlQueryContext.GetDataSource(String uriRelative, String uriBase)
at <xsl:template match=\"gmgml:FeatureCollection\">(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator {urn:schemas-microsoft-com:xslt-debug}current)
at <xsl:apply-templates>(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime, XPathNavigator )
at Root(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
at Execute(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlSequenceWriter results)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer, Boolean closeWriter)
at System.Xml.Xsl.XmlILCommand.Execute(XmlReader contextDocument, XmlResolver dataSources, XsltArgumentList argumentList, Stream results)
at System.Xml.Xsl.XslCompiledTransform.Transform(String inputUri, String resultsFile)
at MyNamespace.ApplyXslTransformation1(String input, String output, String xsltFileName)
the statement causing the exception:
myXslTransform.Transform(pathToInputFile, pathToOutputFile);
About the document function I will have to ask tommorrow. I've get the xslt file from the other person.
When I've created the XmlTextReader file from the path to the xslt file everytning was fine. I've also try to use:
myXslTransform.Load(pathToXsltFile, xsltSettings, new XmlUrlResolver());
myXslTransform.Transform(pathToInputFile, pathToOutputFile);
And it was also fine.
Now i get the encrypted xslt. I've decrypt it and I want to create XmlTextReader from the decrypted string. Besause of the security reason i don't wont to create temporaty xslt decrypted file.

I think we need to see the XSLT and any calls to the document function it does. In general you need to be aware that the document function has a second argument that can serve as a base URI to resolve URIs resulting from the first argument. Without the second argument being passed in as in e.g. <xsl:value-of select="document('foo.xml')"/> the stylesheet code itself provides the base URI. If you load the stylesheet code from a string that mechanism might not resolve URIs the same way as it happens with a stylesheet loaded from the file system or a HTTP URI. The solution to that problem depends on the location of the resource you want to load and how that relates to the main input file. If you want to load foo.xml from the same location as the main input document then doing document('foo.xml', /) instead of document('foo.xml') should work.

I think this is caused by your manual setting of the memory stream's position to 0; you're confusing the XmlTextReader.
I tried the above and it works fine for me when I comment that line out.
Is there a particular reason you are setting it to 0?

Assuming this question is about using XslCompiledTransform in a .Net Core application, I found the answer to "This operation is not supported for a relative URI." at the site https://github.com/dotnet/corefx/issues/31390
The relevant answer (by vcsjones commented on Jul 26, 2018) is:
"I believe you are running in to a known compatibility change. .NET Core does not allow resolving external URIs for XML by default and is documented here.
As the documentation says, the old behavior can be restored, if you so choose, by putting
AppContext.SetSwitch("Switch.System.Xml.AllowDefaultResolver", true);
In your application. Try placing that at the top of your example program."
When I added
AppContext.SetSwitch("Switch.System.Xml.AllowDefaultResolver", true);
as the top line of
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
in startup, the error "This operation is not supported for a relative URI" went away. At that moment, a new error occurred calling Load with a XmlReader relating to finding the other files referenced by the XSL file. When I then instead passed the file path to the xsl in Load, it all worked as expected.
var resolver = new XmlUrlResolver {Credentials = CredentialCache.DefaultCredentials};
var transform = new XslCompiledTransform();
transform.Load(XslPath, new XsltSettings(true, true), resolver);
var settings = new XmlWriterSettings {OmitXmlDeclaration = true};
using (var results = new StringWriter())
using(var writer = XmlWriter.Create(results, settings))
{
using (var reader = XmlReader.Create(new StringReader(document)))
{
transform.Transform(reader, writer);
}
return results.ToString();
}
I add this in hope helps someone else trying to debug why XslCompiledTransform thows "This operation is not supported for a relative URI." in .net core.

Prevent DTD download when parsing XML

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?

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);
}

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.

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.

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.

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