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…

Microsoft .NET MVC ReDoS (Denial of Service) Vulnerability - CVE-2015-2526 (MS15-101)

Microsoft released a security bulletin (MS15-101) describing a .NET MVC Denial of Service vulnerability (CVE-2015-2526) that I reported back in April. This blog post analyses the vulnerability in details, starting from the theory and then providing a PoC exploit against a MVC web application developed with Visual Studio 2013.
For those of you who want to see the bug, you can directly skip to the last part of this post or watch the video directly... ;-)

A bit of theory

The .NET framework (4.5 tested version) uses backtracking regular expression matcher when performing a match against an expression. Backtracking is based on the NFA (non-deterministic finite automata) algorithm engine which is designed to validate all input states. By providing an “evil” regex expression – an expression for which the engine can be forced to calculate an exponential number of states - it is possible to force the engine to calculate an exponential number of states, leading to a condition defined such as “ca…

UXSS in McAfee Endpoint Security, www.mcafee.com and some extra goodies...

During the HITB2017AMS talk given in Amsterdam with @Steventseeley, I promised that I would have disclosed vulnerabilities affecting a security vendor product other than Trend Micro.

For those who have come to my blog for the first time and are looking at "insecurities" of security vendors, you might be interested as well on how we found 200+ remote code execution vulnerabilities in Trend Micro software...

But this blog post is dedicated to two McAfee products instead: McAfee Endpoint Security and SiteAdvisor Enterprise (now part of McAfee Endpoint Security). For simplicity, I will just refer to McAfee Endpoint Security for the rest of this post.

First let's demonstrate a particular type of XSS, a UXSS, considering that fact that it only affects the McAfee Endpoint Security plugin and does not depend on a particular web site or web application.

There are two different injection points:

-UXSS when user visits a red labelled web site - the payload is rendered in the BlockP…