Pentesting Webservices with Net.TCP Binding

Hi all,

Most of you that are  pentesters  may have already tested plenty of webservices using SOAP (Simple Object Access Protocol) for communication. Typically, such SOAP messages are transferred over HTTP (Hypertext Transfer Protocol) and are encapsulated in XML (Extensible Markup Language). Microsoft has developed different representations of this protocols to reduce the network load. As these representations/protocols aren’t really covered by typical tools out there, this post will show you some of them, and a proxy which can be used to simplify the testing.

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Python Library for De- and Encoding of WCF-Binary streams

In a .NET environment WCF services can use the proprietary WCF binary XML protocol described here. Microsoft uses this protocol to save some time parsing the transmitted XML data. If you have to (pen-) test such services, it would be nice to read (and modify) the communication between (for example) clients and servers. One possibility is Fiddler.

Fiddler’s strengths include its extensibility and its WCF binary plugins. Sadly, these plugins can only decode and display the binary content as XML text.

Our first tool of choice for webapp pentests (Burp Suite) has also a plugin feature, and one can also find plugins for decoding (and encoding XML back to) WCF binary streams. But all WCF binary plugins out there are based on the .NET library which means one either has to work on MS Windows or with Mono. Another disadvantage is the validation and auto-correction feature of such libraries… not very useful for penetration testing 😉

That’s why we decided to write a small python library according to Microsoft’s Open Specification which enables us to decode and encode WCF binary streams. The library has a rudimentary commandline interface for converting XML to WCF binary and vice versa, as well as a plugin for our python-to-Burp plugin (pyBurp).

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