Glossary
Web shell
A script or module planted on a web server that turns HTTP requests into server-side actions such as command execution or file access.
A web shell is code placed on a web-accessible server — typically a PHP, ASPX, JSP or similar file, or a server module — that lets a remote party execute commands, manage files, query databases or tunnel traffic through ordinary HTTP(S) requests.
Web shells are a post-exploitation persistence mechanism tracked by MITRE ATT&CK as T1505.003. They run with the privileges of the web server or application-pool identity and blend into normal web traffic, which makes layered detection — file integrity monitoring, web log analysis and process-tree telemetry — essential.
See What is a web shell? for a full primer.