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=======================================================================

Vendor description:
-------------------
"TightGate-Pro is a ReCoB system. ReCoBS stands for Remote-Controlled Browser
System, literally translated 'remote-controlled web browser'. TightGate-Pro
physically separates the web browser execution environment from the workstation.
The system thus shields the internal network from the Internet and reliably
and preventively prevents attacks via the web browser. TightGate-Pro is the
strongest dedicated ReCoBS, because only physical outsourcing on a hardened
system permanently withstands attacks. Local virtualisations, sandboxing
systems or micro-virtualisations do not offer effective protection.
TightGate-Pro is used in public authorities, financial institutions, industrial
companies and critical infrastructures – in short, everywhere where “safe
surfing on the Internet” is indispensable at the workplace and internal
infrastructures must be reliably protected. TightGate-Pro is BSI-certified
according to EAL3+."

Source: https://www.m-privacy.de/en/tightgate-pro-safe-surfing/

Business recommendation:
------------------------
The vendor provides a patch which should be installed immediately.

SEC Consult highly recommends to perform a thorough security review of the product
conducted by security professionals to identify and resolve potential further
security issues.

Vulnerability overview/description:
-----------------------------------
1) Code Execution
Execution of single commands and scripts is possible with the privileges of
the current user. Code execution is possible with any file type on the
server and no specific permissions need to be set for the utilized file.

Vendor response (translated):
"It is intended behavior to execute arbitrary bash scripts. It is not possible
to execute arbitrary programs and libraries. There is no privilege escalation
possible with this vulnerability."

We can confirm that it was not possible to escalate privileges during our test.

2) Access to all Desktops (CVE-2023-47250)
Multiple users are connecting to the same TightGate-Pro server, resulting in
one instance of the X11 window system. Due to insecure permissions of the
X11 sockets it is possible for any user to open arbitrary windows on the
desktop of other users for phishing attacks or installing a keylogger
directly.

Vendor response (translated):
"We acknowledge this issue as a important vulnerability. A fix with full
RSBAC-Jail-Separation and changed Linux-Filesystem permissions is currently
available in the "Prestable" packages:
- mprivacy-tools_2.0.406g
- tightgatevnc_4.1.2~1
- rsbac-policy-tgpro_2.0.159
These can be applied by the admin user "update". The updates will be provided
automatically as Hotfix around 2023-10-24."

3) File Transfer by Abusing the Print function (CVE-2023-47251)
TightGate-Pro allows printing PDF documents on the host system. Documents are
transferred to the host, printed and deleted afterwards. An attacker is able
to control the path of the transferred file and to prevent the automatic
deletion of the file.

Vendor response (translated):
"This is not a severe finding but we already fixed it. The fixes are available
in the packages:
- mprivacy-tools_2.0.406g
- tightgatevnc_4.1.2~1

Now the .spool directly is always scanned for malicious data and the VNC- client
does not transfer files which contain path symbols (e.g. ../)."

4) Outdated Update Server
Based on disclosed version numbers the update server is running outdated software
with known vulnerabilities. The criticality of this issue depends on the
exploitability of these issues.

Vendor response (translated):
"The old version of thttpd is already known. This is not seen as security-relevant.
The access to the updateserver requires a previous registration of a customer-
provided SSH key, which is only available to administrators on the TightGate-Pro
instance. thttpd is isolated on the updateserver and can only *read* files.

Even if an attacker can write malicious updatepackages, these are still secured
by a cryptographic signature and would not be installed on TightGate-Pro instances.
We will eventually replace thttpd with lighthttpd which is still supported."

Proof of concept:
-----------------
1) Code Execution
Code execution is possible using the context menu of any file in the VNC session
of TightGate-Pro. Selecting "Öffnen mit" (Open with) in the context menu of any
file and selecting the "Benutzerdefinierte Befehlszeile" (custom commandline)
section of the menu allows to provide a custom shell command to be executed:

[advisory_ce_open_with.png]
[advisory_ce_custom_command.png]

At this point there are two possible options:
In case the selected file is a bash script typing `/bin/bash` as custom command
will execute the script. For this PoC the following script has been used:

```
#!bin/bash
echo poc >> /home/user/testuser/Desktop/test/PoC2.txt
```

In case any other file is selected a complete command can be used as well.
A possible example is listed below:
`/bin/bash -c "echo poc >> /home/user/testuser/Desktop/test/PoC2.txt"`

According to vendor, arbitrary code execution is not possible as programs and
libraries won't be executed.

2) Access to all Desktops (CVE-2023-47250)
A normal user without special permissions has read and write access to all X11
sockets stored in the temp folder of the user TightGate-Pro, visible in the
following screenshot:

[advisory_desktop_access_x11_perms.png]

This allows any user for example to open dialogue boxes on the desktop of the
currently connected users as shown in the following screenshots. The command
used is listed below.

`/bin/bash -c 'for i in $(ls /home/tmpdir/tmp510/.X11-unix | cut -b 2-); do
DISPLAY=unix:"$i".0 zenity --password --username & done;'`

[advisory_desktop_access_gui_triggered.png]

As it can be seen in the following screenshot any input to the dialogue boxes
can be read by the attacker.

[advisory_desktop_access_result.png]

3) File Transfer by Abusing the Print function (CVE-2023-47251)
File transfers can be triggered for PDF files which are stored in the
`/home/user/.spool/<username>` directory. By setting a relative path as file
name, the file can be stored in any user directory on the host system.
In case the file name contains Unicode characters, deletion of the file is
not executed after transfer and closing of the print prompt. To store a file
on the user's desktop, the name `..\\..\\..\\..\\..\\Desktop\\Ỻeicar.pdf`
can be used. The transfer can then be triggered by sending the
signal `SIGUSR2` to the `Xtightgatevnc` process:

```
cp eicar.pdf '/home/user/.spool/<username>/..\\..\\..\\..\\..\\Desktop\\Ỻeicar.pdf'
pkill -u $USER --signal SIGUSR2 Xtightgatevnc
```

In addition, this file transfer does not check if any malicious files are
transferred to the host system. The following screenshot shows the warning of
a malware scanner after an eicar testfile was transferred. It is therefore
possible to circumvent the malware scanner of TightGate-Pro which only runs
if the intended way of transfer, namely the TightGate-Schleuse, is used.

[advisory_file_transfer_mal_file.png]

4) Outdated Update Server
Access to the update server is possible with the ssh key stored at
`/etc/cu/id_ed25519` and ssh port forwarding. The ssh key is customized
for each customer. Root access is needed to retrieve the key.
The command used for the forwarding is listed below:

```
ssh -N -L 8000:localhost:85 [email protected] -i id_ed25519 -v
```

Afterwards access is possible at `http:127.0.0.1:8000`.
The server headers return the version of the webserver:
`thttpd/2.25b 29dec2003`

[advisory_outdated_server.png]

This version has in sum four known vulnerabilities (high and medium) listed:
* CVE-2006-1078
* CVE-2006-1079
* CVE-2007-0664
* CVE-2009-4491

Vulnerable / tested versions:
-----------------------------
A TightGate-Pro server with the following package versions was used
for testing:

* tightgatevnc < 4.1.2~1
* rsbac-policy-tgpro < 2.0.159
* mprivacy-tools < 2.0.406g

Vendor contact timeline:
------------------------
2023-10-11: Contacting vendor through [email protected] via GPG
2023-10-13: CEO of m-privacy phones us and thanks us for the advisory,
a developer will send us a written statement next week.
2023-10-16: Received a written statement of their lead developer;
the vulnerabilities #2 (Access to all Desktops) and
#3 (File Transfer by Abusing the Print function) are
confirmed and a fix is available
#1 is seen as a feature not a bug, #4 is claimed to be
prevented by hardening measures on the server, also
thttpd will be replaced by lighthttpd in the future.
2023-10-24: We ask for some clarifications regarding software
versions and advisory publication date.
2023-10-29: Vendor provides software version information and asks us
to publish the advisory after 2023-11-06.
2023-11-22: Public release of security advisory.

Solution:
---------
Install the "Prestable" packages or wait until they are available as hotfix:
* mprivacy-tools_2.0.406g
* tightgatevnc_4.1.2~1
* rsbac-policy-tgpro_2.0.159

Workaround:
-----------
None

Advisory URL:
-------------
https://sec-consult.com/vulnerability-lab/

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SEC Consult Vulnerability Lab
An integrated part of SEC Consult, an Eviden business
Europe | Asia

About SEC Consult Vulnerability Lab
The SEC Consult Vulnerability Lab is an integrated part of SEC Consult, an
Eviden business. It ensures the continued knowledge gain of SEC Consult in the
field of network and application security to stay ahead of the attacker. The
SEC Consult Vulnerability Lab supports high-quality penetration testing and
the evaluation of new offensive and defensive technologies for our customers.
Hence our customers obtain the most current information about vulnerabilities
and valid recommendation about the risk profile of new technologies.

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EOF Daniel Hirschberger, Steven Kurka, Marco Schillinger / @2023