Modern detection engines perform lexical and syntactic analysis on scripts, constructing control flow graphs and data flow graphs to track the propagation of external variables. If an external variable ultimately reaches a command execution function, the script can be flagged as a web shell.
If you suspect a reverse shell:
Disable PHP functions that are not needed for your application, such as exec , shell_exec , system , and passthru , which can be used to execute system commands. Reverse Shell Php
The PHP reverse shell is a perfect microcosm of the cybersecurity arms race. For every defensive measure—disabling exec , filtering egress traffic, scanning files—attackers invent a new evasion: polymorphic code, encrypted tunnels, or leveraging mail() to pop a shell.
The script can be triggered by simply accessing its URL via a web browser or any HTTP client: http://<target-ip>/uploads/shell.php The PHP reverse shell is a perfect microcosm
: It uses PHP functions like proc_open() , system() , or shell_exec() to spawn a shell (such as /bin/sh or /bin/bash on Linux).
(Linux, Windows, or a specific CMS like WordPress) (Linux, Windows, or a specific CMS like WordPress)
Many system administrators harden their php.ini file by disabling execution functions. Look at the disable_functions directive to see if exec , shell_exec , system , or passthru are blocked.
PHP is a server-side scripting language. If an attacker identifies a vulnerability—such as Remote Code Execution (RCE), Unrestricted File Upload, or Local File Inclusion (LFI)—they can execute arbitrary PHP code on the server.
exec : Runs a shell ( /bin/sh ) and redirects its input, output, and error streams ( <&3 >&3 2>&3 ) to the open network socket. The Standalone Script