Adversaries may backdoor web servers with web shells to establish persistent access to systems. A Web shell is a Web script that is placed on an openly accessible Web server to allow an adversary to use the Web server as a gateway into a network. A Web shell may provide a set of functions to execute or a command-line interface on the system that hosts the Web server.
In addition to a server-side script, a Web shell may have a client interface program that is used to talk to the Web server (ex: China Chopper Web shell client).(Citation: Lee 2013)
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