SANS ISC: Blocking Powershell Connection via Windows Firewall. - SANS Internet Storm Center

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SANS ISC InfoSec Forums

In my last post, I mapped controls to stop a malicious doc calling out via Powershell.  I’m now going to cover how using the Windows firewall can stop the attack chain. Windows firewall can be used to limit the application from making connections. In the attack chain, this means that the user got the malicious document, opened it, the macro ran, and the Powershell script failed to pull down additional malware.

 

If you block all network connections for Powershell, it should look like this

Powershell        All    Yes    Block    No    %SystemRoot%\SysWOW64\WindowsPowerShell\v1.0\powershell.exe    Any    Any    Any    Any    Any    Any    Any    Any    Any    

Powershell2        All    Yes    Block    No    %SystemRoot%\system32\WindowsPowerShell\v1.0\powershell.exe    Any    Any    Any    Any    Any    Any    Any    Any    Any    

To test, I tried downloading Wireshark using PowerShell with the same call the malware used

>cmd /c PowerShell (New-Object System.Net.Webclient).DownloadFile('http://2.na.dl.wireshark.org/win64/Wireshark-win64-2.2.2.exe','%TMP%\tom.exe');

Exception calling "DownloadFile" with "2" argument(s): "Unable to connect to the remote server"

At line:1 char:1

+ (New-Object System.Net.Webclient).DownloadFile('http://2.na.dl.wiresh ...

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException

   + FullyQualifiedErrorId : WebException

If you want to allow local communication for these, then you have to turn on the Default Outgoing Policy and create Allow rules.  The windows firewall always processes the Deny first. A kind of work around is to block specific outbound ports.  So you could block 80,443,and 8080 (see Below). Or better yet, you could block everything except the couple of ports you need (135,139,445).  If you use Powershell just to call another application that then makes the connection, then you should be able to block everything.

 

Powershell2        All    Yes    Block    No    %SystemRoot%\system32\WindowsPowerShell\v1.0\powershell.exe    Any    Any    TCP    Any    443, 80, 8080    Any    Any    Any    Any    

 

This process should work for wscript and cscript also.

--

Tom Webb

@twsecblog

In regard to Tom Webb's posting "Blocking Powershell Connection via Windows Firewall" - there are many versions of windows and personally I'm unable to find any of the firewall interfaces that align with the options shown in Tom's examples. Please consider adding a screenshot or additional information.

Tom,

Glad to see your post, I have been doing a lot of work with Windows Firewall to block malicious behavior and found it to be very effective. Limiting PowerShell is probably the biggest one, but an issue we ran into was that we had PowerShell traffic that needed to be allowed and we wanted to block all other. We didn't have enough data to go full white list with the Windows Firewall, but we ended up using "inverse ranges" to configure a blacklist of what was blocked (essentially all of the IP space that we didn't want to allow) and that allowed us to have approved access while blocking all else for just this application.

If anyone is interested, I started a blog with lots of details. Here is the link: https://limpidwebblog.blogspot.com/2016/10/a-shower-leads-to-powershell-puking.html