Web shell

A Web shell is a script that can be uploaded to a web server to enable remote administration of the machine. A web shell can be written in any language that the target web server supports. The most commonly observed web shells are written in languages that are widely supported, such as PHP and ASP. Perl, Ruby, Python, and Unix shell scripts are also used. Once successfully uploaded, an adversary can use the web shell to leverage other exploitation techniques to escalate privileges and to issue commands remotely. These commands are directly linked to the privilege and functionality available to the web server and may include the ability to add, delete, and execute files as well as the ability to run shell commands, further executables, or scripts.
Examples of Web shells.
Web shells such as China Chopper, WSO, C99 and B374K are frequently chosen by adversaries; however these are just a small number of known used web shells.
 &lt;?=`$_GET[1]`?&gt;
Web shell of size 15 bytes.
Delivery tactics.
Web shells can be delivered through a number of web application exploits or configuration weaknesses such as: