Skip to main content

Pwning a thin client in less than one minute, again!


Back in 2015, I have published a blog post titled "Pwning a thin client in less two minutes" which attracted a lot of curiosity from the Internet and which was also featured in the HACKADAY blog.

Today, together with Vincent Hutsebaut (@vhutsebaut), we are releasing a further technique to pwn the same thin client and get a root shell without authentication, in less than one minute!

The attack detailed below is a typical kiosk attack which consists in a local privilege escalation which affects different versions of HP Thin Pro OS (HP ThinPro 4.4, HP ThinPro 5.0, HP ThinPro 5.1, HP ThinPro 5.2, HP ThinPro 5.2.1, HP ThinPro 6.0, HP ThinPro 6.1).

The vulnerability (CVE-2016-2246) has been patched by HP and a technical bulletin has been published. HP stated that they have fixed the issue before our report was sent to them and were on the way to publish a security bulletin when we contacted them.

Since the patch is out, let's dive into the vulnerability, which is detailed step by step below.

For those of you in rush, feel free to skip to the bottom of this post and watch the video.
As usual, if you enjoy it, the authors would love to see this article shared and retweeted ;-)

Description

In HP Thin Pro OS, the sudo configuration allows an unauthenticated user to abuse the keyboard layout tool to perform a privilege escalation attack and gain unauthorised root access on the machine.

The keyboard layout (located in "/usr/bin/hptc-keyboard-layout") runs as a privileged process and it is directly available to an unauthenticated user from the UI (user interface) of the HP Thin Pro Kiosk.

By abusing the available UI controls, an unauthenticated user can navigate on the file system and restore the original /etc/shadow file on the system, which will then allow to set a new admin password on the system.

Conditions

The following conditions are required:

- HP Thin OS Pro set in Kiosk mode;
- HP Thin OS Pro administrator password has already been set by an administrator;
- A malicious user has physical access to the Kiosk but does not have a user account and does not know the admin password.

Steps to reproduce (as a malicious user)

1) Click on the left side, "Control Panel" icon and then clicking the "Keyboard Layout" icon; ** note that the button and UI might be different from the OS version, but the keyboard layout tool is available to an unauthenticated user in Kiosk mode


Step 1 – Figure 1


2) Click on print icon, a "Print File" dialog prompt is provided to the user

Step 2 – Figure 1


3) Print File dialog allows to set an "output file" - by clicking on the "..." button to choose the folder

Step 3 – Figure 1


Step 3 – Figure 2


4) Navigate to /etc/ folder

Step 4 – Figure 1


5) Rename /etc/shadow into /etc/shadow-last-modified-by-admin

Step 5 – Figure 1


Step 5 – Figure 2


6) Rename /etc/shadow- into /etc/shadow

Step 6 – Figure 1


7) Click on the "Administrator/User Mode Switch"

Step 7 – Figure 1


8) Malicious user can set a new admin password and access the administrator mode of the kiosk

Step 8 – Figure 1



9) Launch an xterminal with root access

Step 9 – Figure 1


Step 9 – Figure 2


Further observations

The /etc/shadow- file remains as the original one even after that the admin password has been changed multiple times. In this example, passwd has already been set twice but the shadow- remains the one set originally in the OS (back in 2013), making the attack described possible.


In the sudoer configuration, it is possible to see the NOPASSWD tag set for the Keyboard Layout tool (usr/bin/hptc-keyboard-layout):


Below, a video showing the entire sequence:



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! ;-)

Comments

  1. Very nice !!!
    Well done !!

    ReplyDelete
  2. Very fricking nice - I had never thought of something like this!
    Thank god they patched it!

    ReplyDelete

Post a comment

Popular posts from this blog

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 c…

Alcatel Lucent Omnivista or: How I learned GIOP and gained Unauthenticated Remote Code Execution (CVE-2016-9796)

It is time for another advisory or better a blog post about Alcatel Lucent Omnivista and its vulnerabilities. Omnivista is a central management network tool and it is typically used in medium/large organisation with a complex VoIP/SIP infrastructure.



Interestingly enough, this software belongs to the niche of "undownloadable" software and it requires a license to work as well. My "luck" came during an engagement where it was already installed and this post documents one of the many 0days discovered during such audit.

The reasons why I wanted to dedicate a single blog post on this vulnerability are several.

First, remote code execution (RCE) is always a sweet bug to show. Second, I strongly believe that documenting vulnerabilities in applications using old protocols and standards, respectively GIOP and CORBA, can be beneficial for the infosec community, since no many examples of vulnerabilities in such applications are available or published on the Internet.

Actuall…

Trend Micro Threat Discovery Appliance - Session Generation Authentication Bypass (CVE-2016-8584)

In the last few months, I have been testing several Trend Micro products with Steven Seeley (@steventseeley). Together, we have found more than 200+ RCE (Remote Code Execution) vulnerabilities and for the first time we presented the outcome of our research at Hack In The Box 2017 Amsterdam in April.

The presentation is available as a PDF or as a Slideshare.


Since it was not possible to cover all discovered vulnerabilities with a single presentation, this blog post will cover and analyze a further vulnerability that did not make it to the slides, and which affects the Trend Micro Threat Discovery Appliance (TDA) product.

CVE-2016-8584 - TDA Session Generation Authentication Bypass

This was an interesting vulnerability, discovered after observing that two consecutive login attempts against the web interface returned the same session_id token. Following this observation, our inference was that time factor played a role. After further analysis and reversing of the TDA libraries, the sessio…