Thursday, November 19, 2009 At 7:38PM
Update 2010: With the official release of Burp Suite v1.3, both plug-ins discussed in this post can be used with either Pro or Free versions of Burp.
If you run into a Silverlight application that consumes WCF, there’s a good chance it will use Binary XML Message Encoding to send data between the Silverlight client and the WCF endpoint. These messages usually include a Content-Type: application/soap+msbin1 header to indicate that they are using Microsoft’s .NET Binary Format for SOAP (NBFS). From an attack perspective, the main problem with this encoding format is that you can’t simply edit requests or responses on-the-fly like you would with text-based SOAP messages, since the recipient of the message expects the data to be properly encoded (otherwise it will throw an exception) and, as such, will throw an exception if it’s not.
My initial research into what security tools support NBFS didn’t turn up much. The only option I found were two WCF Binary Inspectors for Fiddler (one here written by Richard Berg, and another here written by Samuel Jack). Both of these inspectors are essentially plug-ins for Fiddler that add support to view NBFS encoded data. Originally these both looked like the solution I was after, however upon further analysis I realized that while those plug-ins let you VIEW encoded messages, they don’t let you EDIT them. I decided it would be a worthwhile effort to try and leverage the plug-in architecture of Burp Suite (through use of the BurpExtender interface) to write a NBFS plug-in for Burp.
The Solution (sort of)
Not wanting to re-invent the wheel, I figured I would leverage the work that had already been done with Fiddler by calling into one of the existing Fiddler libraries from Burp. I chose to use Richard Berg’s code since it looks like it can be ported entirely to Java down the road if needed (it doesn’t rely on WCF’s built-in decoder). Luckily for me, his code also had all of the methods needed to both encode and decode message data.
The way the plug-in works is pretty simple…when a request comes in, the processProxyMessage method of BurpExtender is used to check whether the requests should be decoded and, if so, passes the request data to the C# library. The C# library decodes the message and returns the plain-text version back to Burp. As requests exit Burp, the processHttpMessage method of BurpExtender is used to determine whether the request needs to be re-encoded and, if so, calls into the C# library again.
There are a couple of interesting points to note here:
- The
processHttpMessageof BurpExtender is currently only supported in the Professional version of Burp Suite. It is my understanding that this method will be supported in the Free version starting with the next release (v1.3) but for now only licensed users of Burp pro have access to this extender method. - Both the
processProxyMessageANDprocessHttpMessagemethods of BurpExtender alway fire BEFORE a response can be edited by the user. Unfortunately this precludes the Plug-in from being able to re-encode RESPONSE messages should the user want to edit one.
What this means is that you’ll need to resort to the proxy chaining as a workaround for this if you use the Burp Free Edition (explained in more detail below). Additionally, even if you use Burp Professional Edition, you’ll need to use this workaround if you want to edit RESPONSE data (REQUEST data can be edited on the fly with a single instance of Burp Professional).
Plug-In Versions
There are two version of the Burp plug-in available:
Burp Professional Edition Plug-in: Allows binary requests to be edited on the fly. This version does not support editing of response data. Pro users can use the Free Edition Plug-in with Burp Professional for editing response data.
Burp Universal Plug-in: The Universal Plug-in works with both Free and Professional Editions of Burp and supports editing of binary REQUESTS and RESPONSES. The caveat to using this version of the plug-in is that you’ll need to chain two burp instances together as outlined in the diagram below for the plugin to work properly.
The purpose of chaining two proxies together is as follows:
- The first instance handles decoding requests, intercepting (and editing) requests, and re-encoding edited responses. Set this instance to intercept REQUESTS only (not responses) and to use the 2nd proxy as the next hop.
- The second instance handles re-encoding edited requests, decoding responses, and intercepting (and editing) responses. Set this instance to intercept RESPONSES only (not requests).
Each proxy will add or remove a custom header (X-WCF-Proxy: must-encode) to edited requests/responses which they use to notify each other of whether re-encoding of a message is necessary. This custom header is removed when read by the plug-in, so it shouldn’t ever get disclosed to the target system.
Albeit it slightly crude, I didn’t see much in the way of a better work around (I am certainly open to suggestions if anyone has any). It should be noted that this workaround is ONLY necessary if you are using the Free Edition (1.2.x) of Burp Suite OR if you want to want to edit WCF binary response content using Burp Professional Edition. Editing WCF binary request data is supported with a single instance of the Burp Professional Plug-In.
Next Steps
These plug-ins were created as a proof of concept for the talk at OWASP AppSec DC 2009. Looking forward, the C# decoding library should easily port to pure Java since it doesn’t make use of the native WCF decoding classes. This would not only eliminate cross-language calls but would also make the plug-in platform independent (since the implementation would be in pure Java). The drawback to this approach, of course, is that we would be using a home grown decoder for a proprietary Microsoft protocol that could change down the road.
In any case, hopefully the plug-ins will be useful in the short term until more security tools include native support for NBFS messages. You can find both versions of the plug-in available for free on our tools page.
Author: Brian Holyfield
©Aon plc 2023
