A web shell is a malicious script or program uploaded by cybercriminals to a web server, enabling them to remotely control the server through a web-based interface. Once installed, attackers can execute commands, access sensitive data, upload or download files, and even use the compromised server to launch further attacks. Web shells are often disguised as harmless files and can be written in various programming languages like PHP, ASP, or Java. They pose a significant security risk because they allow unauthorized access and persistent control over affected systems, often going undetected by traditional security measures.