Monday, 27 April 2015

Pwning a thin client in less than two minutes



Have you ever encountered a zero client or a thin client? It looks something like this...

If yes, keep reading below, if not, then if you encounter one, you know what you can do if you read below...

The model above is a T520, produced by HP - this model and other similar models are typically employed to support a medium/large VDI (Virtual Desktop Infrastructure) enterprise.

These clients run a Linux-based HP ThinPro OS by default and I had a chance to play with image version T6X44017 in particular, which is fun to play with it, since you can get a root shell in a very short time without knowing any password...

Normally, HP ThinPro OS interface is configured in a kiosk mode, as the concept of a thin/zero client is based on using a thick client to connect to another resource. For this purpose, a standard user does not need to authenticate to the thin client per se and would just need to perform a connection - e.g. VMware Horizon View. The user will eventually authenticate through the connection.

The point of this blog post is to demonstrate that a malicious actor can compromise such thin clients in a trivial and quick way provided physical access, a standard prerequisite in an attack against a kiosk.

During my testing, I have tried to harden as much as possible the thin client, with the following options:




I did not set the "Allow user to lock screen" to simulate a scenario where users can use any thin-clients (kiosk style). However, I have also noticed that the default password for the account "user" is "user", so if you find an environment where they enforce account lockout, you can try that password directly (it is very often unchanged...)

I also set a password for the administrator's view, so when a user attempts to switch to the admin view, a password would be required.


In this scenario, the standard user does not know this password and should only be able to use a single VMware Horizon View pre-configured connection, as shown below:


However, I have found out that unless there is user lockout enabled, then it is possible to get a root shell following the steps below:

- Select the connection profile, and edit the profile (if it auto-starts, then you should cancel the connection)

- A new window with a form is presented to you - fill the server field with dummy data and then click on the General options

- Perform the attack to "escape" from the ThinPro Control Center kiosk by entering under "Command Line Arguments": && xterm



- Click Ok to save the new "VMware Horizon View" profile

- Click on new "VMware Horizon View" profile and the connection will timeout/fail, as dummy data was entered. However, when you close/cancel the window, the xterm window will be spawn

- You have bash shell access to the HP Zero client

The user id is user, so no root yet.



However, if you check sudo -ll you see that by default, the account user can perform a lot of commands as root without the need to enter a password. The output of sudo is included in this pastebin and an exerpt below:

[SNIP]

Sudoers entry:
    RunAsUsers: root
    Commands:
NOPASSWD: /usr/bin/hpobl

[SNIP]

The most interesting command I have found is: /usr/bin/hpobl

This command allows access to the HP Easy Setup Wizard panel. Through this one, it is possible to change settings. However, in this scenario, you do not know any password, so you must find a way to get a root shell from this Wizard panel.


By going directly to the last step "Thank you" (all the previous steps can be ignored for the lolz), then click on the link - this will spawn Firefox to load the link you just clicked.



At this stage, you have launched HP Easy Setup Wizard as root and Firefox process is also launched as root. One elegant option to get a shell of Firefox is to set Firefox's external mail handler - Edit / Preferences / Applications / Mailto - and point it to /usr/bin/xterm



Then you just need to point a tab to: mailto:email@address.com and you will then be gratified with a root shell:



Here is a video of the entire attack which takes less than two minutes, in respect of the title of this blog post (in reality, the title was "...in less that one minute", but couldn't do it in that time frame lol...if you do it in less than one minute let me know...):



So what's the catch?

If you are performing a penetration testing against a VDI, look for quick wins as this one... if you are responsible for VDI, then consider that those machines can be compromised very easily - a soft key logger will be enough to get credentials to a Windows domain... also, check what configuration is enforced on the thin-client itself. It might even be more relaxed than the one considered in this scenario.

If you like kiosks and more in particular you like to break them, then you absolutely need to try: http://ikat.ha.cked.net/ . Greetz to Paul Craig, the "self-proclaimed" king of kiosks! ;-)

I haven't tried other HP Thin Pro images yet, but it might be possible that the attack shown in this blog also affects versions earlier than T6X44017.

If you find other ways to bypass HP ThinPro OS, please let me know.

4 comments:

  1. Gotta wonder if it calls Firefox with a full path.. if not, you can shave off some steps by tampering with your environment possibly...

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  2. &&xterm can be changed to &&firefox to shave off 20 seconds.

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    Replies
    1. you can directly do && /usr/bin/hpobl and then get to HP Easy Setup Wizard directly... ;-) I did that way in the video to show the priv escalation process ;-)

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