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.
Dragonfly 2.0 commonly created Web shells on victims' publicly accessible email and web servers, which they used to maintain access to a victim network and download additional malicious files.
|M1042||Disable or Remove Feature or Program||
Consider disabling functions from web technologies such as PHP’s
|M1018||User Account Management||
Enforce the principle of least privilege by limiting privileges of user accounts so only authorized accounts can modify the web directory.
|ID||Data Source||Data Component|
|DS0015||Application Log||Application Log Content|
|DS0029||Network Traffic||Network Traffic Content|
|Network Traffic Flow|
Web shells can be difficult to detect. Unlike other forms of persistent remote access, they do not initiate connections. The portion of the Web shell that is on the server may be small and innocuous looking. The PHP version of the China Chopper Web shell, for example, is the following short payload: 
Nevertheless, detection mechanisms exist. Process monitoring may be used to detect Web servers that perform suspicious actions such as spawning cmd.exe or accessing files that are not in the Web directory.
File monitoring may be used to detect changes to files in the Web directory of a Web server that do not match with updates to the Web server's content and may indicate implantation of a Web shell script.
Log authentication attempts to the server and any unusual traffic patterns to or from the server and internal network.