11 February 2013

Jar full of cookies

Few posts back I've been giving tips about how to organize web fuzzing - you remember that part, color highlights, marking stuff for later. But one person (I think that was my only semi-active reader) asked me: "But those request are gonna expire, session will die". That is true - very often you no longer can reuse that request, unless of course you are planning to copy and paste all the cookies from more recent one. There, however is a faster method.

Set things up

Burp Suite has this nifty feature called Jar Cookie - basically Burp has ability to parse every Set-Cookie header and store cookies in a database. Good thing is that other tools are able to use the same jar. While issuing a request Burp will replace every matching cookie header with the most recent value obtained from the jar.
In the Options/Sessions tab you have the ability to set which tool traffic should be monitored to update a jar. To configure what tool should use the cookie Jar you have to edit default session handling rule - take a look at scope tab. Now, before you start fuzzing (or just playing with some stored requests) you only have to login to application through proxy and newest cookies will be placed in a jar.

How about magic trick

This is just the beginning - cookie jar/session management options are even richer. In Options/Sessions tab you can set a lot of possible actions. First - macros. You can set up automatic sequences of request, retrieve some parameters like anti-CSRF token or simply log you automatically to the application. In session handling rules you can configure some behaviours making use of previously set up macros (but not only). For example in Intruder before every request you may want to issue different request to obtain a valid anti-CSRF token and then use it while issuing one with tampered parameters. Of course details will differ between applications you are testing, but I encourage you to try it yourself. Remember - what sometimes seems to be overly complicated can in fact save you a lot of manual and mindless cop-and-paste job.

As always some additional information can be find at BurpSuite Blog.

6 February 2013

JSON Decoder

Long time no see. Usually people start such notes with oh-so-cliche quote from Mark Twain, but I've already did that on numerous occasions, so no. Anyway, despite the hidden motto of this blog ("no promises, it will be released when it's done") I wrote something. Finally, yesterday I've overcome my pathological laziness and finished version 1 of very small Burp Extensions - JSON Decoder. Code itself is not very impressing, nor is the functionality, but it's a start - now, knowing the basics I can move to more impressive stuff.

The Extension

Since version 1.5.01 Burp Suite Pro comes with new API for writing extensions. No longer you need to write them in Java, bundle into JAR and are forced to do some mojo magic to make them run. New API also gives you access to much more of the Burp internals. I'm not going to give you a tutorial how to write them, but I encourage you to read some of official tutorials on PortSwigger blog. If I see correctly there are eleven tutorials covering quite wide selection of topics.

So, what is my extension doing? Not that much (at least in this version) - it's just an additional tab with pretty printed JSON packet. I have other plans for that but I need to find time (and I've started flying BMS 4.32 again, so no rest for the wicked). I have some others extensions as a work in progress, but they are not in the ready-to-show state.


Debugging burp extension is a bit like "Why? Because Fuck You, that's why" experience. You have made a typo, mixed expected type or declared too many parameters in function definition? All you get is JavaRuntimeException. You think that you won't made those mistakes? Let me show you what kind of mistakes I did while coding this extension.

Typos - I've spend 30 minutes failing to spot the difference between CreateTxtEditor() and createTxtEditor(). While writing an extension make sure that every API function follows CamelCase conventions (it can be tricky, because python names are usually flat). For example you can convert byte[] data variable in two ways - burp.helpers.byteToString(data) or data.tostring().

Difference between Java.String and byte[] - some functions accept byte[], some String - always check which type function expects and what it returns. It will save you time spent inserting countless println() lines.

Given the low complexity of my code I was able to use oldest, print everything technique of debugging, but if you are writing something more complex please read this blog entry.

Bit more about Burp stuff

If you are a new to Burp I can recommend a book written by my friend - grab it here. You can read it yourself or give to that new Junior Pentester that just joined.