Blocking Powershell Connection via Windows Firewall.
Last Updated: 2016-12-18 01:12:22 UTC
by Tom Webb (Version: 1)
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.
Dec 19th 2016
6 years ago
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
Dec 19th 2016
6 years ago