Windows environments provide a group policy setting which allows a regular user to install a Microsoft Windows Installer Package (MSI) with system privileges. This can be discovered in environments where a standard user wants to install an application which requires system privileges and the administrator would  like to avoid to give temporary local administrator access to a user.

From the security point of view this can be abused by an attacker in order to escalate his privileges to the box to SYSTEM.

Identification

Lets assume that we have already compromised a host inside the network and we have a Meterpreter session.

get-uid-shell-metasploit

Meterpreter Session – Normal user

The easiest method to determine if this issue exist on the host is to query the following registry keys:

reg query HKCU\SOFTWARE\Policies\Microsoft\Windows\Installer /v AlwaysInstallElevated
reg query HKLM\SOFTWARE\Policies\Microsoft\Windows\Installer /v AlwaysInstallElevated
registry-queries-always-install-elevated

Query the registry to identify the issue

Privilege Escalation with Metasploit

The easiest and the fastest way to escalate privileges is via the Metasploit Framework which contains a module that can generate an MSI package with a simple payload that it will be executed as SYSTEM on the target host and it will be removed automatically to prevent the installation of being registered with the operating system.

Metasploit Module - Always-Install-Elevated

Exploitation of Always Install Elevated with Metasploit

Generate MSI Package with PowerSploit

PowerSploit framework contains a script that can discover whether this issue exist on the host by checking the registry entries and another one that can generate an MSI file that will add a user account into the local administrators group.

PowerSploit - Always Install Elevated

PowerSploit – Always Install Elevated

 

User-Add - Adding an Account as Local Admin

Adding an account into Administrators group

The verification that this user has been added into the local administrator group can be done by running the “net localgroup administrators” command from the command prompt.

net-localgroup-administrators

Verification that the “backdoor user has been created

Conclusion

Metasploit Framework can be used as well to generate MSI files however the payload will be executed under the privileges of the user running it which in most of the cases it shouldn’t be the administrator. Therefore the PowerSploit script was the only reliable solution to escalate privileges properly.

In order to mitigate this issue the following settings should be disabled from the GPO:

Computer Configuration\Administrative Templates\Windows Components\Windows Installer
User Configuration\Administrative Templates\Windows Components\Windows Installer
GPO-Always Install With Elevated Privileges

GPO -Always Install With Elevated Privileges Setting

GPO-User - Always Install with elevated privileges

GPO – Always Install with Elevated Privileges Setting