We are doing security analysis of our code using veracode and its showing XXE flaw for below code, specifically where Deserialize() is invoked. How can we prevent serializer from accessing external entities. My attempt below to set XMLresolver to null for XMLReader is not working.
public static T DeserializeObject(string xml, string Namespace)
{
System.Xml.Serialization.XmlSerializer serializer = new System.Xml.Serialization.XmlSerializer(typeof(T), Namespace);
MemoryStream stream =
new MemoryStream(Encoding.Default.GetBytes(xml));
XmlReaderSettings settings = new XmlReaderSettings();
// allow entity parsing but do so more safely
settings.DtdProcessing = DtdProcessing.Ignore;
settings.XmlResolver = null;
using (XmlReader reader = XmlReader.Create(stream, settings))
{
return serializer.Deserialize(reader) as T;
}
}
Can anyone suggest what I might be missing or if there is something else to try.
I had the similar issue. You need to change xmlReader with xmlTextReader as you are reading from the string.
something like this -
public static T DeserializeObject(string xml, string Namespace)
{
System.Xml.Serialization.XmlSerializer serializer = new System.Xml.Serialization.XmlSerializer(typeof(T), Namespace);
//**** I don't think you need this block of code *********
//MemoryStream stream = new MemoryStream(Encoding.Default.GetBytes(xml));
//XmlReaderSettings settings = new XmlReaderSettings();
// allow entity parsing but do so more safely
//settings.DtdProcessing = DtdProcessing.Ignore;
//settings.XmlResolver = null;
//*********************************************
XmlTextReader reader = new XmlTextReader(xml)
{
XmlResolver = null
};
return serializer.Deserialize(reader) as T;
}
All the best!
Related
I have a project scanner complaining with a Warning about
XmlDataDocument serializedContent = new XmlDataDocument();
and
serializedContent.Load(objStream);
and giving this recommendation to use:
The best way to prevent XXE attacks is to disable XML entity
resolution by disabling inline DD setting DtdProcessing to
DtdProcessing.Prohibit or by disabling XML Entity resolution setting
the XmlReaderSettings.XmlResolver property to null:
XmlReaderSettings settings = new XmlReaderSettings () ;
settings.DtdProcessing = DtdProcessing. Prohibit;
settings.XmlResolver = null;
XmlReader reader = XmlReader.Create(stream, settings);
Here's the code that I have:
[Serializable]
...
XmlTextWriter objSerializer = new XmlSerializer(...);
MemoryStream objStream = new MemoryStream();
XmlTextWriter objwriter = null;
XmlDataDocument objSerializedContent = new XmlDataDocument(); // it complains here
objWriter = new XmlTextWriter (objStream, System.Text.Encoding.UTF8);
...
objSerializedContent.Load(objStream); // it complains here
How can I apply that scan recommendation using reader if I'm using XmlTextWriter & XmlDataDocument?
If you use
XmlReaderSettings settings = new XmlReaderSettings () ;
settings.DtdProcessing = DtdProcessing. Prohibit;
settings.XmlResolver = null;
and
using (XmlReader xr = XmlReader.Create(objStream, settings)) {
objSerializedContent.Load(xr);
}
instead of objSerializedContent.Load(objStream) your code uses an XmlReader over the MemoryStream where the XmlReader has the settings you want or need.
I already searched a lot and unable to find a solution and unable to determine the correct approach
I am serializing an object to xml string and deserializing it back to an object using c#. XML string after serialization adds a leading ?. When I dezerialize it back to the object I am getting an error There is an error in XML document (1, 1)
?<?xml version="1.0" encoding="utf-16"?>
Serialization code:
string xmlString = null;
MemoryStream memoryStream = new MemoryStream();
XmlSerializer xs = new XmlSerializer(typeof(T));
XmlSerializerNamespaces ns = new XmlSerializerNamespaces();
ns.Add("abc", "http://example.com/abc/");
XmlTextWriter xmlTextWriter = new XmlTextWriter(memoryStream,Encoding.Unicode);
xs.Serialize(xmlTextWriter, obj, ns);
memoryStream = (MemoryStream)xmlTextWriter.BaseStream;
xmlString = ConvertByteArrayToString(memoryStream.ToArray());
ConvertByteArrayToString:
UnicodeEncoding encoding = new UnicodeEncoding();
string constructedString = encoding.GetString(characters);
Deserialization Code:
XmlSerializer ser = new XmlSerializer(typeof(T));
StringReader stringReader = new StringReader(xml);
XmlTextReader xmlReader = new XmlTextReader(stringReader);
object obj = ser.Deserialize(xmlReader);
xmlReader.Close();
stringReader.Close();
return (T)obj;
I would like to know what I am doing wrong with encoding and I need a solution that works for most cases. Thanks
Use following function for serialization and Deserialization
public static string Serialize<T>(T dataToSerialize)
{
try
{
var stringwriter = new System.IO.StringWriter();
var serializer = new XmlSerializer(typeof(T));
serializer.Serialize(stringwriter, dataToSerialize);
return stringwriter.ToString();
}
catch
{
throw;
}
}
public static T Deserialize<T>(string xmlText)
{
try
{
var stringReader = new System.IO.StringReader(xmlText);
var serializer = new XmlSerializer(typeof(T));
return (T)serializer.Deserialize(stringReader);
}
catch
{
throw;
}
}
Your serialized XML contains a Unicode byte-order mark in the beginning, and this is where the deserializer fails.
To remove the BOM you need to create a different version of encoding suppressing BOM instead of using default Encoding.Unicode:
new XmlTextWriter(memoryStream, new UnicodeEncoding(false, false))
Here the second false prevents BOM being prepended to the string.
I'm working on an integration with a third party application that sends us an XML message. Their XML looks something like this:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE theirObj SYSTEM "theirDTD-2.0.dtd">
<theirObj>
<properties>
<datasource>ThirdParty</datasource>
<datetime>2009-03-05T14:45:39</datetime>
</properties>
<data>
...
</data>
</theirObj>
I'm trying to deserialize it using the XmlSerializer:
public theirObj Deserialize(string message) {
if( string.IsNullOrWhiteSpace( message ) ) {
throw new ArgumentNullException( "message" );
}
XmlSerializer xmlSerializer = new XmlSerializer( typeof(theirObj ) );
TextReader textReader = new StringReader( message );
using (XmlReader xmlReader = new XmlTextReader( textReader )) {
object deserializedObject = xmlSerializer.Deserialize( xmlReader );
theirObj ent = deserializedObject as theirObj ;
if (ent == null) {
throw new InvalidCastException("Unable to cast deserialized object to an theirObj object. {0}".FormatInvariant( deserializedObject));
}
return ent;
}
}
}
I generated the objects using xsd.exe.
If I remove the <!DOCTYPE> tag then it deserializes fine.
Is there a way to get XmlSerializer to ignore the <!DOCTYPE> tag?
I know I could strip it out before passing it the XmlSerializer, but I'd rather not go to that level of XML manipulation if I don't have to.
Instead of using XmlTextReader, call XmlReader.Create and pass it an XmlReaderSettings object with DtdProcessing set to Ignore:
TextReader textReader = new StringReader( message );
var settings = new XmlReaderSettings { DtdProcessing = DtdProcessing.Ignore };
using (XmlReader xmlReader = XmlReader.Create(textReader, settings))
Note: The DtdProcessing property was added in .NET 4.0. In .NET 3.5, you can instead set ProhibitDtd to false and XmlResolver to null:
var settings = new XmlReaderSettings { ProhibitDtd = false, XmlResolver = null };
DOCTYPE has no built-in XmlSerlization attributes. In fact, this is because the XML Serialization is element based rather than document based. I think you can use the following approach to skip DOCTYPE in your serialization:
public static String Serialize(object obj)
{
StringBuilder builder = new StringBuilder();
XmlSerializer serializer = new XmlSerializer(typeof(theirObj));
using (XmlWriter writer = XmlWriter.Create(builder, new XmlWriterSettings() { OmitXmlDeclaration = true }))
xmlSerializer.Serialize(writer, obj);
return builder.ToString();
}
Then, you inject it back once the document has been deserialized.
You can just remove the doctype
TextReader textReader = new StringReader( message );
XmlDocument XDoc = new XmlDocument();
XDoc.Load(textReader);
XmlDocumentType XDType = XDoc.DocumentType;
XDoc.RemoveChild(XDType);
using (XmlReader xmlReader = new XmlTextReader(XDoc)) {
object deserializedObject = xmlSerializer.Deserialize( xmlReader );
theirObj ent = deserializedObject as theirObj ;
if (ent == null) {
throw new InvalidCastException("Unable to cast deserialized object to an theirObj object. {0}".FormatInvariant( deserializedObject));
}
return ent;
}
I am using this method to transform an object to XML:
protected XmlDocument SerializeAnObject(object obj)
{
XmlDocument doc = new XmlDocument();
DataContractSerializer serializer = new DataContractSerializer(obj.GetType());
MemoryStream stream = new MemoryStream();
try
{
serializer.WriteObject(stream, obj);
stream.Position = 0;
doc.Load(stream);
return doc;
}
finally
{
stream.Close();
stream.Dispose();
}
}
Eventually I get something like:
<CaCT>
<CTC i:nil="true" xmlns="http://schemas.datacontract.org/2004/07/a.b.BusinessEntities.InnerEntities" />
<CTDescr xmlns="http://schemas.datacontract.org/2004/07/a.b.BusinessEntities.InnerEntities">blabla</CTDescr>
<CaId>464</CaId>
</CaCT>
How can I get rid of the i:nil="true" and the xmlns="http://schemas.datacontract.org/2004/07/a.b.BusinessEntities.InnerEntities"?
Personally I've always found that hand-written XML serialization with LINQ to XML works well. It's as flexible as you want, you can make it backward and forward compatible in whatever way you want, and obviously you don't end up with any extra namespaces or attributes that you don't want.
Obviously it becomes more complicated the more complicated your classes are, but I've found it works very well for simple classes. It's at least an alternative to consider.
protected string SerializeAnObject(object obj)
{
XmlSerializerNamespaces xmlNamespaces = new XmlSerializerNamespaces();
xmlNamespaces.Add("", "");
XmlWriterSettings writerSettings = new XmlWriterSettings();
writerSettings.OmitXmlDeclaration = true;
XmlSerializer serializer = new XmlSerializer(obj.GetType());
using (MemoryStream ms = new MemoryStream())
{
using (XmlWriter stream = XmlWriter.Create(ms, writerSettings))
{
serializer.Serialize(stream, obj, xmlNamespaces);
return Encoding.UTF8.GetString(ms.ToArray());
}
}
}
I have to create an XML file with all the elements prefixed, like this:
<ps:Request num="123" xmlns:ps="www.ladieda.com">
<ps:ClientId>5566</ps:ClientId>
<ps:Request>
When i serialize my object, c# is smart and does this:
<Request num="123" xmlns="www.ladieda.com">
<ClientId>5566</ClientId>
<Request>
That is good, because the ps: is not necessary.
But is there a way to force C# to serialize all the prefixes?
My serialize code is this (for incoming object pObject):
String XmlizedString = null;
MemoryStream memoryStream = new MemoryStream();
XmlSerializer xs = new XmlSerializer(pObject.GetType());
XmlTextWriter xmlTextWriter = new XmlTextWriter(memoryStream, Encoding.UTF8);
xs.Serialize(xmlTextWriter, pObject);
memoryStream = (MemoryStream)xmlTextWriter.BaseStream;
XmlizedString = UTF8ByteArrayToString(memoryStream.ToArray());
return XmlizedString;
private String UTF8ByteArrayToString(Byte[] characters)
{
UTF8Encoding encoding = new UTF8Encoding();
String constructedString = encoding.GetString(characters);
return (constructedString);
}
First of all, if the consumer of your string were processing XML, then they wouldn't care about the prefix, since it doesn't matter (to XML). Perhaps they don't understand XML, and think they're processing a string (which might need to have the string "ps:" on every element).
Second of all, you should change your code a bit:
XmlSerializer xs = new XmlSerializer(pObject.GetType());
using (MemoryStream memoryStream = new MemoryStream())
{
XmlWriterSettings settings = new XmlWriterSettings()
{
Encoding = Encoding.UTF8
};
using (XmlWriter writer = XmlWriter.Create(memoryStream, settings))
{
xs.Serialize(writer, pObject);
}
return Encoding.UTF8.GetString(memoryStream.ToArray());
}
This will properly dispose of the stream and XmlWriter if an exception is thrown, stops using the deprecated XmlTextWriter class, and yet still returns a string containing XML written for UTF-8.
Finally, to control the prefix, see "How to: Qualify XML Element and XML Attribute Names":
XmlSerializerNamespaces myNamespaces = new XmlSerializerNamespaces();
myNamespaces.Add("ps", "www.ladieda.com");
XmlSerializer xs = new XmlSerializer(pObject.GetType());
using (MemoryStream memoryStream = new MemoryStream())
{
XmlWriterSettings settings = new XmlWriterSettings()
{
Encoding = Encoding.UTF8
};
using (XmlWriter writer = XmlWriter.Create(memoryStream, settings))
{
xs.Serialize(writer, pObject, myNamespaces);
}
return Encoding.UTF8.GetString(memoryStream.ToArray());
}
Also check out XmlNamespaceDeclarationsAttribute. Caveat: when deserializing it will only give you namespaces defined by that element, it won't have namespaces defined in parent elements. If you don't have a consistent root type then use the XmlSerializer.Serialize() overload from #John Saunders.
http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlnamespacedeclarationsattribute.aspx
In another question #John Saunders suggests using this attribute in regards to controlling xmlns in WSDL: Namespace Prefixes in Wsdl (.net)
From MSDN Sample:
// C#
using System;
using System.IO;
using System.Xml.Serialization;
[XmlRoot("select")]
public class Select {
[XmlAttribute] public string xpath;
[XmlNamespaceDeclarations] public XmlSerializerNamespaces xmlns;
}
public class Test {
public static void Main(string[] args) {
Select mySelect = new Select();
mySelect.xpath = "myNS:ref/#common:y";
mySelect.xmlns = new XmlSerializerNamespaces();
mySelect.xmlns.Add("MyNS", "myNS.tempuri.org");
mySelect.xmlns.Add("common", "common.tempuri.org");
XmlSerializer ser = new XmlSerializer(typeof(Select));
ser.Serialize(Console.Out, mySelect);
}
}
// Output:
// <?xml version="1.0" encoding="IBM437"?>
// <select xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmln:xsi="http://www.w3.org/2001/XMLSchema-instance"
// xmlns:common="common.tempuri.org" xmlns:MyNS="myNS.tempuri.org" xpath="myNS:ref/#common:y" />