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.
|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||Detects|
|DS0015||Application Log||Application Log Content||
Monitor for third-party application logging, messaging, and/or other artifacts that may backdoor web servers with web shells to establish persistent access to systems. Log authentication attempts to the server and any unusual traffic patterns to or from the server and internal network. 
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.
Monitor for changes made to files that may backdoor web servers with web shells to establish persistent access to systems.
|DS0029||Network Traffic||Network Traffic Content||
Monitor and analyze traffic patterns and packet inspection associated to protocol(s) that do not follow the expected protocol standards and traffic flows (e.g extraneous packets that do not belong to established flows, gratuitous or anomalous traffic patterns, anomalous syntax, or structure). Consider correlation with process monitoring and command line to detect anomalous processes execution and command line arguments associated to traffic patterns (e.g. monitor anomalies in use of files that do not normally initiate connections for respective protocol(s)).
|Network Traffic Flow||
Monitor network data for uncommon data flows. Processes utilizing the network that do not normally have network communication or have never been seen before are suspicious.
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: