H4F - invisible proxy... casper gem

Ruby is a great language for hackers and security researchers too. Of course you can build amazing web applications using Rails or Sinatra or even Padrino frameworks. You can also build great tools using sophisticated APIs that make very easy to craft HTTP requests, to intercept traffic, to run regular expression or even to build a transparent proxy in very few lines of code.

It all starts from a need…

As all stories go, I was in trouble with a web application during an assessment. It was consuming bandwidth and I was pretty sure it made also some ajax calls the developers refuse to admit.

Funny story, there are a lot of developers that are happy to hide some details in order to save some recommendiation, actually you just make my work a little harder and you make me upset.

So, there was me, my Safari browser and the target application. I was investigating WEBrick for my own interested, so when it was clear I needed a transparent proxy to gather all the traffic from Safari to webapp, I didn’t think about firing up another tool, but opened vim and cooked one in minutes.

A word about reusing

Before going further you may think I’m fool to start a transparent proxy from scratch instead of using one from Owasp Zap, or Paros.

You’re absolutely right. True to be told, I would have started suche kind of project anyway somewhere in the future since I need something very custom for my own usage.

I find that all proxies out there are memory consuming, most of them build in Java technology with an old looking GUI. Another point is that gathered links are hard to access to since there is no public API to call.

What I need is…

Here we are.

  1. I need a transparent proxy (for the moment), collecting outgoing links in order to show me what my browser is doing in every moment with a timestamp associated.

  2. Collected links must be accessible with an APIs and from the little binary tool.
  3. The proxy code must be organized in APIs so to be integrated in other security tools

casper rubygem

All the source code is on this repository on github.

Building a proxy in ruby is somewhat trivial, all you have to do is to extend the WEBrick::HTTPProxyServer

The config variable is the place when you put all the parameters you need to pass to HTTPProxyServer when you will call the super class constructor.

Obviously, the most important parameter is the one telling the proxy what it has to to. In this case we tell the proxy to call a log_requests routine to make all the business.

``` ruby casper class initializer def initialize(config={}) @req_count = 0 @hosts=[] config[:Port] ||= 8080 config[:AccessLog] = [] config[:ProxyContentHandler] = Proc.new do |req, res| log_requests(req, res) end

super(config) end

In the log\_requests there is no magic... we just dump the HTTP request with a
timestamp and we store the server into an hashmap so we can dump it when

``` ruby casper business logic module
  def log_requests(req, res)
    $stdout.puts "[#{Time.now}] #{req.request_line.chomp}\n"
    if @hosts.index(req.host).nil?
      @hosts << req.host

  def inc_req_count
    @req_count += 1

I put all the stuff in a private section to make sure you can’t override the default transparent behavior, in a first step.

Signal handling

Using casper is easy as well.

``` ruby the casper binary #!/usr/bin/env ruby require “casper”

server = Casper::Proxy.new trap(“INT”) { server.shutdown } trap(“INFO”) { server.info } trap(“USR1”) { server.dump } server.start

Just create an instance of the Casper::Proxy class and start it. You have your
proxy running.

The tricky part here is that I wanted to play with signals with the _trap_ ruby

As you may know, in a Unix environment there are a lot of signals you can send
to a process using the _kill_ system call. 

To learn more about the system call and the signal C library function:

$ man 2 kill $ man 3 signal

In the signal manpage there is also a list of signals you can send to a process
and even you can trap (most of them).

To be as compliant as possible to unix system programming default, I trapped
the SIGINT to the server shutdown call that is in the WEBrick::HTTPProxyServer
class and I trapped 2 custom signals to two different handlers.

SIGINFO will ask casper to print a one line statistic information row:

``` ruby casper info routine
def info
  $stdout.puts "[#{Time.now}] INFO #{@req_count} requests to #{@hosts.count} unique hosts"

SIGUSR1 will instead dump all visisted hosts observed by the proxy: ``` ruby casper dump routine def dump $stdout.puts “Hosts we communicate with “ if (@hosts.count == 0) $stdout.puts “None\n”

else @hosts.each do |h| $stdout.puts “ »> #{h}\n” end end end ```

And now?

Casper::Proxy class is 50 lines of code counting newlines and comments. With 50 lines of code, thanks to the great WEBrick API you can build your own transparent proxy.

However being transparent and gentle is not so funny is it?

Future enanchement will be:

  1. saving session to SQL. Using an ORM like ActiveModel or Datamapper will make casper to be decoupled from DBMS so we may want to write some custom addon to use the user preferred DBMS.
  2. we must implement a plugin system to make the user able to write some custom code and ask casper to execute it.
  3. we must write some injection routine to try to help penetration tester in analysis

(Updated: )

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