Parsing Windows Firewall Rules

In our last post we discussed how to gather general information about the configuration of a Microsoft Windows Firewall, host based firewall configuration. But what most people are really interested in when doing a firewall audit is how the firewall rules themselves are configured.

One of the challenges of auditing a Microsoft Windows Firewall ruleset is how do you parse all the firewall rules that Microsoft automatically creates for you? It is great that Microsoft automatically configures most of the rules – it helps encourage us to actually leave the firewall turned on. But with all those rules, especially the disabled ones, how does an auditor easily parse through the data? And to make matters worse, the output of commands like NETSH is just text – not a PowerShell object. So that makes it even more difficult and time consuming to parse.

So that got us thinking, what if we could convert the output of a NETSH SHOW command for Microsoft Windows Firewall rules into a PowerShell object that we could more easily parse?

So we did a little digging and found out that Jaap Brasser had already created a basic script to do that at PowerShell.com (http://powershell.com/cs/forums/t/13260.aspx). The following code allows us to take the output from a NETSH command, and convert it to a PowerShell object, so we can more easily parse the ruleset:

$Output = @(netsh advfirewall firewall show rule name=all)

$Object = New-Object -Type PSObject

$Output | Where {$_ -match '^([^:]+):\s*(\S.*)$' } | Foreach -Begin {
$FirstRun = $true
$HashProps = @{}
} -Process {
if (($Matches[1] -eq 'Rule Name') -and (!($FirstRun))) {
New-Object -TypeName PSCustomObject -Property $HashProps

$HashProps = @{}
} $HashProps.$($Matches[1]) = $Matches[2]
$FirstRun = $false
} -End {
New-Object -TypeName PSCustomObject -Property $HashProps}

Now that the firewall rules are a PowerShell object we can use cmdlets like WHERE-OBJECT and SELECT-OBJECT to filter the information. We can perform ad hoc queries and work with the information however we see fit. Enjoy!

Script for Windows Firewall Baseline

Another baseline an auditor or system administrator might want to consider when assessing their systems is a baseline for the general configuration of the Microsoft Windows Firewall. Many organizations are starting to utilize the built in Microsoft Windows Firewall more and more when protecting even their internal systems. The use of a host based firewall should definitely be on the list of things to consider when evaluating the security of a system.

If the system that you are auditing is using a third party firewall product, then you will need to contact that vendor to see how best to automate collecting data about the firewall’s configuration. However if you are using the built in Windows Firewall, then the following script will help by gathering a baseline of the general configuration settings of the firewall:

$Global = (netsh advfirewall show global)
$Profiles = (netsh advfirewall show allprofiles)

$Global | Out-file firewall_config.txt
$Profiles | Out-file -append firewall_config.txt

It should be noted that this script will only work on systems running PowerShell – but you have probably already noticed that theme in our posts. But also please remember that this will only work on systems running the Windows Advanced Firewall that was released with Windows Vista and later systems. This will not work on the earlier, original firewall built into Windows XP.

Parsing Active Directory Groups

In a previous post we shared a PowerShell script that would allow an auditor to parse a list of groups and group members on a Microsoft Windows system as a part of a security assessment or baselining process. The question has come up though – what if someone wants to follow the same process but parse a list of Active Directory groups and their members instead?

It turns out that it is much easier to perform the task against Active Directory than it is to do it against a local Microsoft Windows local Security Accounts Manager (SAM) database. It seems that the native capabilities of the Active Directory PowerShell cmdlets is going to make it easier for us to accomplish this task.

The output requirement for this script will be the same as for the previous. The output should be in a CSV format that could be easily parsed by a spreadsheet program like Microsoft Excel. Also, like the previous script, empty groups should still be listed, but just without any group members listed along with them.

The code we used to parse a list of AD groups and their members is:

Get-ADGroup -Filter * | ForEach-Object { 
    $GroupName = $_.Name 
    $Members = Get-ADGroupMember $_.samaccountname -Recursive | foreach{$_.Name} 
        If ($Members.count -ge 1){
            $Out = $GroupName + "," + [String]::Join(",", $Members)
            $out | Out-File -append ad_group_members.txt
        }
        Else{
            $Out = $GroupName
            $out | Out-File -append ad_group_members.txt  
        }
    } 

Ideally an auditor would combine this script with our previous example and using PowerShell remoting run one script that would parse local and Active Directory groups at the same time and output the data as an easily readable file. But between the two examples, hopefully it will give you enough inspiration to make this usable for you in your audit efforts.

Parsing Local Windows Groups

One step that has become a staple part of any audit of Microsoft Windows systems is a listing of all the local groups on the system. Listing all the groups on a system with all the members of that group can help establish a baseline for the security configuration of a system. Certainly groups like the local Administrators group is a concern during a security audit. Ideally though auditors would have the ability to look at the members of each group on a system and validate that the list of members is appropriate for that system.

As with many of the scripts we have recommended, we decided to write a script using PowerShell to pull the information from a local system. We also wanted to make sure that when we pulled the information from a system that we could parse it easily once we were done. Therefore we wrote the script to allow us to pull all the local computer groups from a system, with the members of the group, and then save it as a CSV file for easier parsing with Excel.

Here is the code we used for the script:

$GroupList = Get-WmiObject Win32_Group | ForEach {$_.name}

$ComputerName = "."

ForEach ($GL in $GroupList)

{

    $Computer = [ADSI]("WinNT://" + $ComputerName + ",computer")

    $Group = $Computer.psbase.children.find($GL)

    $Members= $Group.psbase.invoke("Members") | %{$_.GetType().InvokeMember("Name", 'GetProperty', $null, $_, $null)}

        If ($Members.count -ge 1){
            $Out = $GL + "," + [String]::Join(",", $Members)
            $out | Out-File -append local_group_members.csv
        }
        Else{
            $Out = $GL
            $out | Out-File -append local_group_members.csv
        }

}

We ran into a couple issues when writing the script. One was the formatting of the output to a CSV file. Normally we would use an EXPORT-CSV cmdlet to do that, but in this case since we had multiple data sources we had to do some custom formatting. Also since some of the groups were empty, we had to add the IF clause to make sure that even blank groups were reported in our output.

Once the concept of the script starts to make sense to you, it should be an easy next step to consider adding PowerShell remoting capabilities or a computer name list to the script to parse multiple systems at once. You might also consider saving the output from the script in a format (such as HTML) that is easier to read and include in your audit reports.

We hope the script is useful as you audit Microsoft Windows servers or establish security baselines for those systems.

Continuous Monitoring with PowerShell

Welcome back from the holidays! I imagine many of you are just returning from the holidays and are ready get started on those new year’s resolutions. If one of them was to implement continuous monitoring or learn more about scripting, do I have a treat for you! Now that I’m back from some holiday travel myself, I think it’s time to continue our series on automating continuous monitoring and the 20 Critical Controls.

I don’t want these blog posts on an introduction to PowerShell. There are plenty of fine references on that available to you. In talking with Jason Fossen (our resident Windows guru), I have to agree with him that one of the best starter books on the topic is Windows PowerShell in Action, by Bruce Payette. So if you’re looking to get started learning PowerShell, start here, or maybe try some of the Microsoft resources available at the Microsoft Scripting Center.

But let’s say you’ve already made a bit of an investment in coding and you already know what tasks you’d like to perform. For example, maybe you wonder who is a member of your Domain Admins group, so you use Quest’s ActiveRoles AD Management snap in to run the following command:

(Get-QADGroup ‘CN=Domain Admins,CN=Users,DC=auditscripts,DC=com’).members

Or on the other hand, maybe you are concerned about generating a list of user accounts in Active Directory who have their password set to never expire, you’d likely have code such as:

Get-QADUser -passwordneverexpires

Or maybe even you want to run an external binary, like nmap, to scan your machines, you might have a command such as:

Nmap –sS –sV –O –p1-65535 10.1.1.0/24

In any case, the first step is to come up with the code you want to automate. That’s step one.

Next, you don’t just want to run the code, you want the code to be emailed to you on a regular basis, say once a day or once a week. The next step is to use a mailer to email you the results of your script. Now you have a few choices here. One option is to use a third party tool like blat to generate your email. But since we’re using PowerShell, let’s stick with that. Version 2.0 of PowerShell also has some built in mailing capabilities in this regard.

The easiest way to get started is to save the output of the commands you want run to a temporary text file, mail the text file as the body of an email message, and then delete the temporary file. An easy way to do this to get started would be to use the following commands:

$filename = sometextfilewithoutputresultsinit.txt

$smtp = new-object Net.Mail.SmtpClient("mymailserver.auditscripts.com")
$subject="SANS Automated Report - $((Get-Date).ToShortDateString())"
$from="automation@sans.org"

$msg = New-Object system.net.mail.mailmessage
$msg.From = $from
$msg.To.add("automation@auditscripts.com")
$msg.Subject = $subject
$msg.Body = [string]::join("`r`n", (Get-Content $filename))

$smtp.Send($msg)
remove-item $filename

Save your data as an appropriate PS1 file, automate the command to run once in a while using Task Scheduler, and you’re off to the races!

We certainly have more to discuss, but hopefully this inspires some thinking on the matter. I’ll post again soon with some other steps to consider, before we move on the Bash. There’s a lot we can talk about here. Until next time…