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Using Kali Linux to scan web applications on your local machine

The Kali Linux logo

Introduction

Having worked in the past for companies that allow long feedback and adaptation loops whilst developing and testing software, I'm no stranger to the frustration felt at having to make enormous changes due to requirements being updated at the last minute, or a feature that was written months ago getting flagged during the testing phase.

There have been a gazillion "Agile vs Waterfall" articles written by people that know far more about that sort of thing than I do, and so I'll stop short of evangelizing a particular model, but regardless of the underlying processes I still think there's a lot to be said for identifying issues as early as possible so that major headaches can be avoided later in the day.

As a web developer, it's my responsibility to write secure applications, and rather than leaving everything to the Pen Testers, it would be nice if I could catch some of the more obvious security issues myself whilst I'm developing my application. This certainly won't replace a proper penetration test, but I'm thinking that a move towards increased security awareness and software that is designed correctly from the ground up can't be a bad thing.

In this article, I'll show you how to use OWASP's ZAP (Zed Attack Proxy) (which comes bundled with Kali Linux) to test for some basic vulnerabilities in a web application that's being developed on your local machine.

You don't need to run Kali specifically for ZAP, as ZAP can be installed locally if necessary, but Kali contains so many useful tools that it's handy to know how to get it configured from VirtualBox. ZAP's just a good example to get started with really.

Disclaimer

This isn't a "how to hack" guide. It's aimed at web developers similar to myself that like to brush up on app security a little bit.

The described environment setup is one that I've personally found useful, so I thought I would share it with others.

Kali Linux contains some very powerful tools, and you shouldn't use any of them before you really understand what they do. You will need to research beyond this article. You should not use any of the tools or techniques described in this article against a system that you do not own, or for which you have not been given explicit permission to perform penetration tests. Everything that you see and read in this article is for informational purposes only, and I accept no responsibility or liability for any damage that may occur as a result of its use, either directly or indirectly.

Goals

By the end of this article, you should be able to:

  • Create a virtual machine (using VirtualBox) that will run a Kali Linux instance
  • Use OWASP ZAP to scan a web application that is running on localhost of the machine that hosts VirtualBox

Prerequisites

  • OSX (High Sierra)
  • a VirtualBox installation (I'm using version 5.2.8)
  • a web application that is running on your local development machine
  • a basic knowledge of web development and web security is recommended

The same commands will probably run on versions of OSX other than High Sierra, but I can't say for definite.

Get the Kali Linux image for VirtualBox

First of all, you'll need to grab yourself a copy of Kali Linux from the official downloads page.

The Kali Linux download page

Click on the "Kali Linux VirtualBox Images" tab, and be sure to download the image that's correct for your system.

If you're running this on High Sierra (as mentioned in the prerequisites) then you'll almost certainly require the "Kali-Linux VBox 64-Bit [OVA]" image. At the time of writing there's a torrent file and a direct download available too, but in either case it weights in at a fairly beefy 3.3gb, so plan your life accordingly.

Building the Virtual Machine

Once you have the Kali image (you'll need to extract the .ova file if it comes in an archive), it's time to spin it up as a virtual machine.

Start VirtualBox and choose File -> Import Appliance from the main menu bar.

When the "Appliance to import" dialog appears, choose the .ova file that we downloaded in the previous section and then click the "Continue" button.

The Appliance To Import dialog

Click the "Import" button when it appears.

It'll probably take a few seconds for the image to be imported, but when it's done you should see your machine appear on the left-hand side of the main VirtualBox Manager window.

In the main VirtualBox Manager window, select your virtual machine and then click the "Setting" icon.

You should choose settings that are appropriate for your machine, but I've found the following settings to work best for me beyond the defaults:

System -> Motherboard:
    Base Memory = 12288mb

System -> Processor:
    CPUs = 2
    Execution Cap = 100%
    Enable PAE/NX = true

Display -> Screen:
    Video Memory = 128mb
    Enable 3D Acceleration = true

Network -> Adapter 1:
    Enable Network Adapter = true
    Attached to = NAT (or Bridged Adapter)

Ports -> USB:
    Enable USB Controller = true
    USB 1.1 (OHCI) Controller = true

It's worthwhile looking into "VirtualBox Guest Additions" if you haven't already. It's beyond the scope of this article, but it'll give you access to some useful enhancements such as clipboard sharing and full screen mode.

Running Kali Linux

When all your settings are complete, select your virtual machine in the main VirtualBox Manager window, and click on the "Start" icon.

You should be presented with the GRUB Loader screen:

The GRUB loader screen

Press Enter to select the default loader options.

After a few seconds, you should be presented with the login screen.

The default credentials for the root account in the Kali Linux VirtualBox images are as follows, but it's worthwhile to remember that this info can also be found by selecting the Virtual Machine in VirtualBox Manager and then looking in the pane on the right-hand side:

username: root
password: toor

Enter the root credentials at the login screen.

There are security issues associated with using the root account unnecessarily (especially with default credentials). You'll be fine to use root whilst you're getting to grips with Kali in the safety of your own development environment, but you should definitely learn about Unix security and how to create accounts that have different permissions if you don't know about this stuff already.

Accessing your Web Application from the guest machine

As mentioned in the prerequisites, you'll need a Web Application that's running on localhost (127.0.0.1) of the machine that hosts your virtual machine.

For the purposes of this article I have an application running at http://localhost:3000, but modify the following steps as necessary if your setup is different.

Ensure that your web application is running locally on localhost.

Open the default Firefox browser on your Kali virtual machine.

If you point the browser at your web application at the moment, you'll notice that it can't connect. This is because you're attempting to connect to localhost on the virtual machine (the guest) rather than localhost on your local machine (the host).

In order to proceed, we need to establish guest to host communication, which I ordinarily achieve by setting VirtualBox's network adapter to one of two different modes.

In VirtualBox Manager, select your virtual machine, navigate to Settings -> Network, and you'll see the different modes listed in the "Attached to" drop-down.

I'm not a networking expert, so be sure to investigate the other adapter modes that are available in VirtualBox just in case you can find something that works better for your purposes.

Using Bridged Adapter

Using VirtualBox's Bridged Adapter is by far the most straight forward method, but it won't work properly without being attached to a network. The guest will effectively be placed in the same network as the host, so you'll need some method of routing so that one device can see traffic from another. This has always been fine for my purposes, but it might not work so well if your machine is isolated.

The process is pretty straight forward:

On the host machine, run the following command to find the hostname:

hostname

You may need to use a different command of you're not using OSX.

On your guest machine, point the Firefox browser at your hostname, and add on any port that your web app us listening on.

For example: http://hackerbox-laptop:3000, where hackerbox-laptop is my hostname and 3000 is the port that my web application is using.

With Bridged Adapter you can use the ip that has been assigned to your host machine on your LAN rather than the hostname if you so choose. I prefer to use the hostname just in case the ip changes between sessions though.

Hopefully, you should see your web application in Firefox now.

Using NAT routing

If you set up your virtual machine to use NAT, then the process is slightly more complicated. One of the advantages to NAT, however, is that you don't have to rely on an external network. The guest becomes part of a separate subnet on which it can communicate with the host, with only access to the guest from outside the subnet being restricted. This might come in handy if you're out and about somewhere.

By default, VirtualBox assigns the internal ip address 10.0.2.2 to the host machine.

Confirm that this is the case by pointing Firefox (on the guest) at 10.0.2.2 (remember to add any port that your application might be listening on), and confirm that the web application (the one that should be running on the host) is accessible.

If all's well, you should be able to access your web application from the guest by pointing Firefox at 10.0.2.2

Intercepting traffic with OWASP ZAP

Now it's time to check that we can intercept the traffic that is sent to and from our web application.

As its name suggests, ZAP (Zed Attack Proxy) can be used to analyse responses and even make sneaky modifications to requests on their way back to the web server.

Before we can do this though, we'll need to set things up so that Firefox will route its traffic through ZAP.

In Kali, navigate to Applications -> Web Application Analysis, and then select owasp-zap.

After a few seconds, ZAP should start, and you'll be given the option to persist the session.

It doesn't really matter which option you choose for our purposes here, so just click Start for now.

In ZAP, navigate to tools -> options -> Local Proxies

Ensure that the Local Proxy address is set to localhost, and that this port is 8080

ZAP options

Click the Ok button.

Next we need to configure Firefox.

You'll need to perform a similar configuration should you choose to use a different browser.

In Firefox, navigate to Preferences -> Advanced -> Network -> Settings...

Firefox options

As can be seen in the screenshot above, select Manual proxy configuration. Set HTTP Proxy to localhost, and set the Port to 8080

If localhost appears in the No Proxy for box then remove it.

Click the Ok button.

Now, let's see if we can grab some traffic from our web app.

Point Firefox at your web application (this process might differ slightly depending upon the network settings you used in the previous steps), and you should see some traffic appear in ZAP. Have a good click around in your application so that you can get a feel for what ZAP is doing.

You should see something similar to the screenshot below:

Traffic intercepted by the ZAP proxy

These are all the http requests and responses that occur when the web browser communicates with your web application.

If you select one of these entries, you can view the request and response information in the top-right pane (see the Request and Response tabs).

Inspecting an http request in ZAP
Inspecting an http response in ZAP

ZAP can help you perform many types of penetration test on your applications, but you'll need to read the documentation as the full capabilities of this tool extend way beyond the scope of my article. Right now though, I'm going to show you how to run a basic vulnerability scan on a specific part of your application.

You may have already noticed the scary looking Attack button in the Quick Start pane. If you type the url to your web application in the text field and then click the Attack button, ZAP will test as many pages as it can find in your app.

Using the ZAP auto scanner

This is all very well if your application is small, or if you really want to test every single area all at once, but it's quite often useful to be able to test a specific request for vulnerabilities.

To test a specific request, navigate to the relevant area in Firefox, and then find the corresponding request in the ZAP history tab, as we saw previously.

Right-click on the relevant request in the ZAP history tab.

The ZAP context menu

When the context menu appears, choose Attack and then Active Scan...

Running an active scan in ZAP

A great number of http requests should fly past in the Active Scan pane while ZAP tests the selected part of your application for common vulnerabilities such as directory traversal, XSS and SQL Injection to name just a few.

The exact purpose of each request is beyond the scope of this article, but if you examine each of the requests and responses as described above, you should be able to get an idea of what a basic scan entails.

When the scan is complete, you can click on the Alerts tab to learn more about the vulnerabilities that have been detected (if any).

Summary

In this article we've seen how to scan a local web application using Kali Linux and OWASP's Zed Attack Proxy, running inside VirtualBox.

I've found this setup to be really useful in helping to expose and understand vulnerabilities whilst an application is still in the development stages, and hopefully you will too.

I haven't even scratched the surface on what any of the tools can really do, and I encourage you to learn as much as you possibly can about the Kali suite (and the many other tools and platforms that are available). You should also do your best to learn about the tasks that penetration testing tools perform beneath the surface rather than relying purely on automation.

I hope that someone finds this helpful :)

If you liked that article, then why not try:

Creating a bootable USB from an ISO image in OSX (High Sierra)

I have a fairly old desktop machine that I've been planning to rejuvenate with a Linux Mint installation, so in preparation I decided to create a bootable USB using one of the official ISO images. I encountered a couple of gotchas when I tried this on OSX High Sierra, so I thought I'd document my findings.

  • owasp
  • zap
  • kali
  • linux
  • localhost
  • development
  • virtualbox
  • vbox