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SEC Consult Vulnerability Lab Security Advisory < 20240307-0 >
=======================================================================
               title: Local Privilege Escalation via writable files
             product: Checkmk Agent
  vulnerable version: 2.0.0, 2.1.0, 2.2.0
       fixed version: 2.1.0p40, 2.2.0p23, 2.3.0b1, 2.4.0b1
          CVE number: CVE-2024-0670
              impact: high
            homepage: https://checkmk.com
               found: 2023-12-01
                  by: Michael Baer (Office Fürth)
                      SEC Consult Vulnerability Lab

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

                      https://www.sec-consult.com

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

Vendor description:
-------------------
"Checkmk 2.2 has arrived – and is ready to monitor your hybrid IT
infrastructure with new features for monitoring native cloud applications,
OpenShift support, an expanded REST API, UX improvements, enhanced
integrations and over 174 new or reworked checks and agents. Monitor your
cloud assets from top hyperscalers with Checkmk 2.2 in addition to the
powerful monitoring of your on-premises networks and servers."

Source: https://checkmk.com/product/latest-version


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) Local Privilege Escalation via writable files (CVE-2024-0670)
In some cases, the software creates temporary files inside the directory
C:\Windows\Temp that get executed afterwards. An attacker can leverage this
to place write-protected malicious files in the directory beforehand. The files
get executed by Checkmk with SYSTEM privileges allowing attackers to escalate
their privileges.


Proof of concept:
-----------------
1) Local Privilege Escalation via writable files (CVE-2024-0670)
In the first step, the filename that will be used by Checkmk needs to be found.
The application creates temporary files with name cmk_{}_{}_{}.cmd. The
placeholders are replaced with a string, the process id and a counter. The first
string was always 'all' and the counter usually is 0. The process id is not
exactly predictable. However, Windows assigns those numbers in increasing order.
This allows to observe the currently used process ids and define a limited
range of probable ids.

In the second step, the attacker places the malicious binary into the folder
C:\Windows\Temp multiple times. The filenames are constructed using the above
pattern for all different probable ids. After placing the files, the attacker
marks them as read-only. Both can be automated using the following powershell
command. Here, the range of probable ids was determined to be between 10000
and 30000. The file C:\Users\attacker\Desktop\mal.exe is the malicious file.

10000..30000 | foreach {
        copy C:\Users\attacker\Desktop\mal.exe C:\Windows\Temp\cmk_all_${_}_1.cmd;
        Set-ItemProperty -path C:\Windows\Temp\cmk_all_${_}_1.cmd -name IsReadOnly -value $true;
}

For this proof of concept, a binary was created using msfvenom that executes
the command whoami and writes the result to a file. This will allow to verify
the successful execution as the SYSTEM user. The following command was used:

msfvenom -p windows/exec CMD='cmd /c "whoami > C:\abc\file"' -f exe -o mal.exe

It should be noted, that the folder C:\abc has to exist and that the anti-virus
solution must be disabled to execute this particular binary.

The final step is to force Checkmk to write and execute those temporary files.

It was observed that repairing the software is enough. This repair process can
be initiated via the Windows GUI or using the following command. The name
fafda3e.msi will be different on every system. The folder C:\Windows\Installer
can be investigated to find the correct name on a given system.

msiexec /fa C:\Windows\Installer\fafda3e.msi

After the repairing finished, the file written by the malicious binary can be
checked. It was created and contains the string "nt authority\system".
[see figure checkmk_tempfolder.png]


Vulnerable / tested versions:
-----------------------------
The following version has been tested:
* 2.1.0

According to the vendor, the following versions are affected:
* 2.0.0
* 2.1.0
* 2.2.0


Vendor contact timeline:
------------------------
2024-01-15: Contacting vendor through security () checkmk com
2024-01-18: Vendor confirms vulnerability, assigns CVE, and
             prepares a fix
2024-01-26: Providing credits and acknowledging CVSS score.
2024-03-04: Vendor informs us that fixes with Werk #16361
             are available.
2024-03-07: Coordinated release of security advisory.


Solution:
---------
Install the latest version 2.1.0p40 or 2.2.0p23 from the vendor's
download page:

https://checkmk.com/download

More information can be found within the vendor's security advisory:
https://checkmk.com/werk/16361


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

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Twitter: https://twitter.com/sec_consult

EOF Michael Baer / @2024

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