What is Web Shell?

A web shell is a script file that an attacker uploads or writes onto a compromised web server, typically written in languages such as PHP, ASP, Python, or Perl. It acts as a backdoor, accepting commands through HTTP GET or POST parameters and executing them on the server's underlying operating system. The output of those commands is then returned in the HTTP response body, often rendered in a simple HTML interface within the attacker's browser. This gives the attacker interactive, remote command-line access without requiring SSH, FTP, or any separate remote-access service.

Once installed, a web shell can be used for a wide range of malicious activities: reading and exfiltrating files (including database credentials and configuration files), uploading additional malware (such as ransomware or keyloggers), pivoting to internal network hosts, and maintaining persistent access even if the original vulnerability is patched. Many web shells are obfuscated to evade signature-based antivirus or web application firewall (WAF) detection. Some, like China Chopper or WSO (Web Shell by oRb), have become well-known families used by advanced persistent threat (APT) groups.

Web shells are a post-exploitation tool. Their presence indicates that a previous vulnerability (such as a file upload flaw, SQL injection, remote file inclusion, or stolen administrative credentials) was already exploited. Detection relies on a combination of file integrity monitoring, behavioral analysis of unusual HTTP requests (especially long POST payloads or cmd parameters), monitoring for unexpected child processes from web server workers, and regular scanning of web-accessible directories. Effective defense requires patching the root vulnerability, changing all secrets, and removing the shell file, which often involves forensic analysis to ensure no additional backdoors remain.