Monday, 2 November 2009

A Fiddler plug-in for inspecting WCF Binary encoded messages

If ever you're needing to debug the interaction between a Web Service and its clients, Microsoft’s Fiddler is the tool to use - this includes WCF Services so long as they're using a HTTP transport. The only thing Fiddler won't do is decode messages that are sent using WCF's proprietary Binary encoding - until today, that is: at lunch time, I took advantage of Fiddler's neat extensibility mechanism and created a rough-and-ready Inspector that will translate binary messages from gobbledegook to plain xml for your debugging pleasure.

You can download the plug-in and source from MSDN Code Gallery. To use it, just drop the plug-in  in the Inspectors folder of your Fiddler installation. Once you've reloaded Fiddler, switch to the Inspectors tab and look for WCF Binary.WCFBinaryFiddlerPlugin

Implementation Notes

  • There’s a very helpful page on the Fiddler site which tells you how to build Inspectors in .Net.
  • Fiddler gives each Inspector the raw bytes of each message, and it can do with it what it likes. Here’s how I decode a WCF Binary encoded message:
using System;
using System.Runtime.Serialization;
using System.ServiceModel.Channels;

...

private static readonly BufferManager _bufferManager = BufferManager.CreateBufferManager(int.MaxValue, int.MaxValue);

...

private string GetWcfBinaryMessageAsText(byte[] encodedMessage)
{
    var bindingElement = new BinaryMessageEncodingBindingElement();
    var factory = bindingElement.CreateMessageEncoderFactory();
    var message = factory.Encoder.ReadMessage(new ArraySegment<byte>(encodedMessage), _bufferManager);
    return message.ToString();
}

20 comments:

Brian Holyfield said...

I recently created a Burp plug-in for editing Binary XML data based on Richard Bergs decoder. You can find the plug-in and a Blog post explaining how it works here: http://www.gdssecurity.com/l/b/2009/11/19/wcf-binary-soap-plug-in-for-burp/

sundriedcoder said...

Awesome Sam!

Ashish Basran said...

Sam, This is great. Appreciate it!!!

Shaihan said...

doesn't work for me i get this error

System.Xml.XmlException: The input source is not correctly formatted.
at System.Xml.XmlExceptionHelper.ThrowXmlException(XmlDictionaryReader reader, String res, String arg1, String arg2, String arg3)
at System.Xml.XmlBufferReader.ReadValue(XmlBinaryNodeType nodeType, ValueHandle value)
at System.Xml.XmlBinaryReader.ReadNode()
at System.Xml.XmlBinaryReader.Read()
at System.Xml.XmlBaseReader.IsStartElement()
at System.Xml.XmlBaseReader.IsStartElement(XmlDictionaryString localName, XmlDictionaryString namespaceUri)
at System.ServiceModel.Channels.ReceivedMessage.ReadStartEnvelope(XmlDictionaryReader reader)
at System.ServiceModel.Channels.BufferedMessage..ctor(IBufferedMessageData messageData, RecycledMessageState recycledMessageState, Boolean[] understoodHeaders)
at System.ServiceModel.Channels.BinaryMessageEncoderFactory.BinaryMessageEncoder.ReadMessage(ArraySegment`1 buffer, BufferManager bufferManager, String contentType)
at System.ServiceModel.Channels.MessageEncoder.ReadMessage(ArraySegment`1 buffer, BufferManager bufferManager)
at BinaryMessageFiddlerExtension.BinaryInspector.GetWcfBinaryMessageAsText(Byte[] encodedMessage)
at BinaryMessageFiddlerExtension.BinaryInspector.UpdateView(Byte[] bytes)

Sam said...

Shaihan,
Have you checked that the message is actually encoded in WCF Binary form? This is the kind of stack trace you get when looking at a message encoded in some other form.

Anonymous said...

i am not sure. aren't the messages supposed to be sent in binary format in wcf by default? or do i have to explicitly instruct that? i am new into wcf. also i am new to fiddler as well. is there any way to check the message in xml format?

Shaihan said...

forgot to select the profile. the above comment is mine. thanks for the help

Sam said...

Messages are not always sent in Binary - it depends what binding you are using. If you are using HttpBinding, then the default is SOAP encoding using XML. To see whether the message is in XML format, just switch to the XML tab in the inspectors window in Fiddler

Anavar said...

I always had problems with binary coding, it is so complicated for me. Thanks for some advices about that you included in your post.

Anonymous said...

This doesnt work for me I get the following error:
System.Xml.XmlException: The prefix 'q' is not defined.
at System.Xml.XmlExceptionHelper.ThrowXmlException(XmlDictionaryReader reader, String res, String arg1, String arg2, String arg3)
at System.Xml.XmlBaseReader.LookupNamespace(PrefixHandleType prefix)
at System.Xml.XmlBinaryReader.ReadNode()
at System.Xml.XmlBinaryReader.Read()
at System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace)
at System.Xml.XmlDocument.Load(XmlReader reader)
at BinaryMessageFiddlerExtension.BinaryInspector.LoadMessageIntoDocument(Byte[] encodedMessage)
at BinaryMessageFiddlerExtension.BinaryInspector.GetWcfBinaryMessageAsText(Byte[] encodedMessage)
at BinaryMessageFiddlerExtension.BinaryInspector.UpdateView(Byte[] bytes)

anadrol said...

Thanks Sam.
Much appreciated, it's a great plugin.

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Aravind Pk said...

Will this work for fiddler version 2.3.2.4? its not working for me...

Samuel Jack said...

@Aravind, I haven't tried it, to be honest. Remember that the source is available if you want to find out why it's not working :-)

sumitg said...

Very nice!

Rob Siklos said...

This is amazing - thanks so much!

Just one thing - in the first sentence of your article, you mention that Fiddler is a Microsoft tool.  This is incorrect - it is written and distributed by Eric Lawrence.

Heidi said...

"Microsoft’s Fiddler"...lol. Nice plugin

Szuwi said...

It was created by someone from MS.

Tim H. said...

Where can I download this? It is no longer in the MS code gallery. This link is dead: http://code.msdn.microsoft.com/wcfbinaryinspector

Samuel Jack said...

Hi Tim. I've not done anything with this for a while. But it looks like https://github.com/waf/WCF-Binary-Message-Inspector is a good replacement.

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