This is not a new issue as such but more of the same in line with <a href="https://www.securityfocus.com/archive/1/541884/30/300/threaded">CVE-2017-12410</a> found by Filip Palian.
A a fix was put in place for the original CVE, however it was specific to binaries and not scripts.
The root cause for both issues is allowing a low privileged group excessive permissions to a folder used by a elevated process.
The Kaseya agent (agentmon.exe) runs as SYSTEM by default.
The agent also has a default working folder @ C:\kworking\
It will pull scripts and binaries to this folder and execute them from disk from the controlling web application.
By default the *Authenticated Users* group has all rights to this folder.
Scripts are written to disk however they are not checked for integrity prior to execution.
So a folder can be monitored for script files being dropped and then append malicious code prior to execution.
##Proof of concept##
This PowerShell script will monitor the default working directory.
When a ps1 script drops from a scheduled task or run from the VSA web application it will then append the command "Write-Host 'injected content'" which will run as SYSTEM.
Change the Write-Host command to the code to be executed or update the script to target other script drops such as vb script.
Note: To test you will need to sign up for a trial with VSA to have the ability to deploy an agent & schedule/run scripts
16-06-2019 :: Issue found
18-06-2019 :: security@ emailed requesting steps to disclose
30-06-2019 :: CERT contacted due to non response of vendor from official email address
31-06-2019 :: CERT still unable to contact vendor
07-07-2019 :: CERT makes contact with vendor. Discover security@ address is not monitored by vendor
20-08-2019 :: Vendor confirms receipt of details
27-08-2019 :: Email sent indicating intention to disclose due to lack of response
02-09-2019 :: No response through CERT. Findings published