Pentesting Fast Infoset Based Web Applications With Burp

Tuesday, October 10, 2017 At 7:45AM

If you run into a .NET application you sometimes end up with some not very well known protocols like WCF Binary protocol or, in a recent case, a Fast Infoset binary encoding – a binary encoding of the XML Infoset and an alternative to the usual text-based XML Infoset encoding. We will briefly describe the Fast Infoset format and present a Burp plugin, which facilitates pentesting web applications using this XML representation.

Fast Infoset is a lossless compression format for XML-based data. The format is mostly utilised in web applications that transfer a large amount of data between a client and a server; usually a thick client processing data offline and exchanging data infrequently with a server. You can identify that Fast Infoset is involved when an HTTP request uses a Content-Type of application/fastinfoset.

An example request may look like this:

If you decompress the body with gzip, it is a little bit more readable.

From an attacker’s perspective, the main problem with this encoding format is that you can’t easily edit requests or responses on-the-fly like you would with text-based message bodies.Since the encoding relies on the previous and following strings, if you try to tamper with the data, the server will throw an exception saying that the data which you just have sent it is not properly encoded.

Some quick research revealed a few public repositories implementing Fast Infoset decoding but only one was working properly (written by Lu Jun). However, this plugin does not support editing and re-encoding decoded Fast Infoset data, only viewing it.

We decided it would be a worthwhile effort to develop a fully working Burp plugin for decoding and encoding Fast Infoset based requests. You can find a compiled JAR and the corresponding source code in the following Github repository:
https://github.com/GDSSecurity/FastInfoset-Burp-Plugin

Once you load the plugin via Burp extender, you can easily view decoded Fast Infoset requests and responses, and tamper with them in Burp Proxy and Repeater.

Gotham Digital Science, LLC (GDS), is a wholly owned subsidiary of Aon, PLC

This material has been prepared for informational purposes only and should not be relied on for any other purpose. You should consult with your own professional advisors or IT specialists before implementing any recommendation or following the guidance provided herein. Further, the information provided and the statements expressed are not intended to address the circumstances of any particular individual or entity. Although we endeavor to provide accurate and timely information and use sources that we consider reliable, there can be no guarantee that such information is accurate as of the date it is received or that it will continue to be accurate in the future. The examples provided in this article are not based upon an actual Stroz/Aon client, but was provided for illustrative purposes only.

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