What file extension is commonly used for web shells?

Web shells are often uploaded to servers with various file extensions, but some of the most commonly used ones include:

1. .php: PHP is a popular server-side scripting language, and many web shells are written in PHP.
2. .asp: Used for Active Server Pages, common in ASP.NET environments.
3. .jsp: JavaServer Pages, used in Java web applications.
4. .py: Python scripts can also be used as web shells if the server supports Python.
5. .pl: Perl scripts may be used in environments that support Perl.

These extensions are often chosen because they correspond to server-side languages that can execute code on the server, allowing an attacker to gain control or execute commands. Always ensure proper security measures are in place to prevent unauthorized file uploads.