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Proxy Chaining

The issue:
While doing one assessment, we faced one issue of our ZAP proxy throwing response ‘401 Unauthorized’ while we were trying to fuzz one application. The application was using NTLM authentication, where the client needs to send the domain name, username and user-password’s hash combination to the server, in order to entertain the requests. NTLM is windows challenge/ response authentication protocol. For more info on NTLM working: http://msdn.microsoft.com/en-us/library/windows/desktop/aa378749%28v=vs.85%29.aspx .
So, we were not able to fuzz the parameters as it was sending back ‘401 Unauthorized’ response, don’t know for what reason despite us providing the windows credentials to ZAP [Fig-1].


 Fig-1

So, we had no other option except trying other similar web proxies. We tried WebScarab and provided Windows authentication by going Tools-> Credentials [Fig-2]
                                                                                   Fig-2
When we again try to capture the request response it was working fine. Then we wanted to fuzz the parameters in one of the URLs. We need to right click on any of the requests on Summary tab and select Use as a fuzz template to send it to Fuzz tab. To our bad luck again, though Webscarab was working fine with windows authentication, in Fuzzing tab, the parameters were not appearing [Fig-3].
                                                                                Fig-3

Now the scenarios were:
·         ZAP was sending unauthorized error while fuzzing, even we had supplied windows credentials
·         Webscarab was throwing issues as the parameters , to be fuzzed, were not being displayed
A bizarre situation indeed!
Proxy chaining:
So, we are going to use something already available in the tools and we can use it effectively to overcome some of the limitations of individual tools. The proxy chain works as follows:
·         The client (browser) will forward the request to Proxy1 (ZAP)
·         Proxy1 will in turn forward it to Proxy2 (Webscarab)
·         Proxy2 will pass it to the server
So, the requirements for performing this action are:
·         The client (browser) will run on the same port as Proxy1 (ZAP), say 8880
·         We shall use Proxy chaining option in Prox1 (ZAP) to forward this request to Proxy2 (Webscarab)
·         We shall configure the same port no. (say 8008) in Proxy1 (ZAP), on which Proxy2 (Webscarab) is running
·         The Proxy2 (Webscarab) will automatically forward the request to the server

Now, the overall scenarios are similar to Fig-4:


                                                                     Fig-4
Setting proxy chain option in ZAP:
Go to Tools-> Options-> Connection-> Use an outgoing proxy server and specify the address as 127.0.0.1 and port as 8008 [Fig-5].

                                                                        Fig-5
Port 8008 is where the next proxy (Webscarab runs). We need not specify it in Webscarab as it’s by default runs at this port.
Now, we all set. Try to access the url in the browser. The browser will forward the request to ZAP on port 8880, ZAP will in turn pass it to Webscarab which is running at port 8008. Webscarab passes this to the server. Chain is complete.
Let’s see how this responds, when we again try to fuzz it in ZAP. Nice, we can now successfully fuzz the parameters, without getting the ‘401 Unauthorized’ response! We now get 200 OK responses [Fig-6].
Conclusion:
So, we saw how we can use proxy chaining to use features of both the proxies, subsequently solving individual tool issues. 



Fig-6

Reference: http://resources.infosecinstitute.com/chaining-web-proxies-to-overcome-limitations/


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