Automatically Create 40 Event Viewer Custom Views

I still find Custom Views useful when troubleshooting on individual workstations, and I’d recently been wondering if it was possible to push them out via GPP or similar. I started creating some views manually, as a test, but it was taking too long.

I’d recently been working on implementing Palantir’s WEF/WEC setup, and wondered whether I could leverage their legwork to automate the creation of these custom views.

The script I came up with took a fraction of the time to write, as opposed to the manual method. It does the following:

  1. Downloads the Palantir ‘windows-event-forwarding’ repo in ZIP format into a temporary folder
  2. Extracts the Event Log query out of each file in the ‘wef-subscriptions’ folder, and
    turns it into an appropriately-named custom Event Viewer view (XML) file in %PROGRAMDATA%\Microsoft\Event Viewer\Views

2017-11-07 16_51_46-Event Viewer

I love how simple PowerShell makes it to work with XML.

The script needs to be run as an admin in order to create the view files in %PROGRAMDATA%, unless you change the output path in the $templateStoragePath variable. It’ll also need to be able to connect to the Internet to download the ZIP file from GitHub.

I’ve started storing my scripts in my PowerShell GitHub repo rather than as Github Gists, and it’s harder to embed them on wordpress.com. View the code via the link below:

https://github.com/dstreefkerk/PowerShell/blob/master/Create-EventViewerCustomViews.ps1

Mitigate commodity malware attacks with Windows Firewall rules

There’s so much that can be done with the built-in Windows tools to prevent commodity malware or ransomware attacks before you even spend a cent on 3rd party tools. All of these things can (and should be) combined to create a good multi-layered strategy:

The last point has been on my to-do list for some time now. I was again reminded of it the other day while watching Sami Laiho’s recent Microsoft Ignite session about PAWs.

A lot of email-delivered malware begins with a macro or via DDE attack, and then attempts to connect to the Internet to pull down more nasties.

Today I came across this great blog post by Branden, in which he describes a handy method to prevent applications from communicating with hosts out on the Internet, while still allowing them to communicate within the internal network.

I set about manually creating a list of outbound firewall rules, including a whole bunch to mitigate the application whitelisting bypasses highlighted by the brilliant Casey Smith here. Doing this via the GUI is painful, and I wouldn’t wish it on anybody:

A listing of outbound firewall rules created in Windows Firewall with Advanced Security

Here’s a screenshot of PowerShell connecting to the web, before putting the firewall rule in place:

A PowerShell prompt, running Invoke-WebRequest to google.com, and showing a successful request

And here’s one taken after I enabled the firewall rule:

But PowerShell can still connect to an internal web server:

A PowerShell prompt, running Invoke-WebRequest against an internal HTTP server. Showing a successful response

There are obviously going to be exceptions to these rules, for example to enable your IT staff to access Azure AD or other cloud-based services via PowerShell, but those things should be done from dedicated administrative hosts anyway. This ruleset is more for the general user population.

When the time came to think about sharing this ruleset here on my blog, I discovered that it’s possible to export the rules from the registry and re-import them elsewhere, however that has its own potential issues.

I instead created the following PowerShell script that will generate all of the appropriate rules using the New-NetFirewallRule cmdlet. It’s also much easier to review this script to see what it does, rather than read a registry export file.

You could extend this script to apply the rules directly to the appropriate GPO by using the -GPOSession parameter on the New-NetFirewallRule cmdlet.

As usual, run at your own risk, and test thoroughly before deploying:

The embedded Github Gist doesn’t show up on mobile devices. Here’s a direct link to the raw script file: https://gist.githubusercontent.com/dstreefkerk/800a9e0a22a6242a28b058be423cf0ba/raw/c2be1189f88fb5ad9acaab708ad985587a576ceb/Create-MitigationFirewallRules.ps1

Automatically drop your Privileged Access Workstation off the network while it’s unattended

“One of my favorite hobbies is hunting sysadmins” – Hacker of Hacking Team’s network

I only periodically log in to my Privileged Access Workstation to carry out administrative tasks. Although I have restrictive policies applied and Windows Firewall locked down, there’s no reason for that machine to be on the network while I’m not actively using it.

In an attempt to address this, I created two simple scheduled tasks:

1. Disable all NICs when workstation is locked

2. Enable all NICs when workstation is unlocked

Note that these depend on the correct audit logging being enabled on the machine in question, otherwise these tasks won’t trigger:

It also depends on how you use your PAW. If you regularly log out rather than shut down, you will need to add additional triggers to the tasks to handle the log off/log on events.

Import these tasks into Task Scheduler and use them at your own peril. You may run into issues if you don’t store any cached logons and simultaneously require a domain controller to be accessible at logon.

Quickly uninstall an MSI on multiple computers using WMI

Today I was working on reducing our vulnerability attack surface, and needed to remove Adobe Reader from our servers. It appears that it was installed as part of a VM image, but never maintained afterwards.

Long story short, rather than mess around with ConfigMgr baselines or Applications, I decided to go the direct route. To top it off, PowerShell remoting’s currently playing up. I ended up using WMI via the method that I outlined in my previous post.

Given an array of server names in $servers:

$servers | %{Invoke-WmiMethod -Class Win32_Process -Name Create -ArgumentList 'MsiExec.exe /x "{AC76BA86-7AD7-1033-7B44-AA1000000001}" /norestart /qn' -ComputerName $_}

Trigger a remote GPUpdate without PSRemoting or PSExec

I recently enabled Windows Firewall on an unused server via GPO, but forgot to include the inbound RDP exception. This, of course, kicked me off my RDP session.

Rather than wait ~90 minutes for my revised GPO to take effect, I found that I could trigger a GPUpdate remotely using WMI (WinRM wasn’t enabled, and I didn’t want to use PSExec)

The following command does the trick:

Invoke-WmiMethod -Class Win32_Process -Name Create -ArgumentList "gpupdate.exe" -ComputerName <computername>

View the creation date for AD-integrated DNS records

6 months in to my new job, and I’ve still got a big mess of old static DNS records to clean up from our Active Directory-integrated DNS.

The DNS management console doesn’t show any sort of date information, but I knew that because the data is stored in AD, there should be some sort of created/modified date on each record.

I had a look using ADSIEdit, and sure enough, there were the dates! Here’s a quick one-liner to pull out the records and their created/modified dates:

Get-ChildItem "AD:DC=contoso.com,CN=MicrosoftDNS,CN=System,DC=contoso,DC=com" | Get-ADObject -Properties Created,Modified | Select-Object Name,Created,Modified | Sort-Object -Property Created

Armed with the creation date of each record, I’m in a better position to determine which ones are no longer needed.

PowerShell: Get all unlinked GPOs

I decided to spend some of the quiet time now at the beginning of the new year to clean up Group Policy here at my new job. We had roughly the same number of GPOs as staff!

I’d already removed at least 10-20 GPOs manually, but I wanted a quick way to find all of the un-linked GPOs that were still just sitting around.

I found this great post by Mark Schill from back in 2013 that still does the job.

Mark’s solution, however, pulls out only the display name property. I needed to modify his solution a little, as I wanted to pipe the results to the Remove-GPO cmdlet. Here’s what I did:

Get-GPO -All | Where-Object { $_ | Get-GPOReport -ReportType XML | Select-String -NotMatch "<LinksTo>" }

The above one-liner gets all unlinked GPOs, and returns Microsoft.GroupPolicy.Gpo objects.

It goes without saying that you should be very careful when bulk-deleting anything. I first backed up all of my GPOs and dumped the list into text format to review it. I manually checked the settings on a few of the GPOs in question, and only then, deleted them.

Here’s how I generated the list to review:

$unlinkedGPOs = Get-GPO -All | Where-Object { $_ | Get-GPOReport -ReportType XML | Select-String -NotMatch "<LinksTo>" }
$unlinkedGPOs | Sort-Object -Property DisplayName | Select-Object DisplayName,CreationTime,ModificationTime | Format-Table